Monday, April 18, 1994

                                   9:00 a.m.


Golombek M.*
   Workshop Introduction and Overview




                             Monday, April 18, 1994
                            HOW WILL WE GET THERE?
                             MISSION INTRODUCTION
                               9:30 - 10:30 a.m.


Spear A.*
   Project Status

Cook R.*
   Mission Design and Flight System

Dias W.*
   Rover and Surface Scenarios




                             Monday, April 18, 1994
              WHAT MEASUREMENTS WILL WE MAKE AFTER WE GET THERE?
                            INSTRUMENT DESCRIPTIONS
                               10:40 - 12:00 noon


Smith P. H.*
   Imager for Mars Pathfinder (IMP)

The IMP camera is a near surface remote sensing experiment with many
capabilities beyond those normally associated with an imager. The camera
is fully pointable in both elevation and azimuth with a protected,
stowed position looking straight down. Stereo separation is provided
with two, left and right, optical paths; each path has a 12-position
filter wheel. The two light paths converge onto a single CCD detector
which divides its 512 x 256 active pixels evenly between them. The CCD
is a frame transfer device that can transfer a frame in 0.5 msec
avoiding the need for a shutter. Because the detector has a high quantum
efficiency (QE) and our filters are relatively broad (40 nm FWHM), the
camera optics are stopped down to f/18 giving a large depth of field;
objects between 0.6 m and infinity are in focus, no active focussing is
available. A jack-in-the-box mast elevates the camera about 75 cm above
its stowed position on top of the lander electronics housing; the camera
is fully functional in its stowed position so that pictures taken of the
same object in each position can be compared to give accurate ranging
information. The camera is designed, built and tested at Martin
Marietta. Laboratory testing of flight-like CCDs has been done at the
Max Planck Institute for Aeronomy in Lindau, Germany under the direction
of co-investigator Dr. H. Uwe Keller, who is providing the focal plane
array, the pre-amp board, and the CCD readout electronics with a 12-bit
ADC. The important specifications for the IMP camera from the point of
view of the scientists using the camera are given in the table below.
For comparison the same quantities are also provided from the Viking
camera system.

Science Objectives. The primary function of the camera, strongly tied to
Mission success, is to take a color panorama of the surrounding terrain.
IMP requires approximately 120 images to give a complete downward
hemisphere from the deployed position. The local horizon would be about
3 km away on a flat plain, so that one can hope to have some information
over a 28 sq km area. At the horizon a pixel covers 3 m but the
resolution improves at closer distances, just outside the lander edge a
pixel is 1.6 mm. Therefore, IMP provides the geologist, and everyone
else too, a view of the local morphology with millimeter to meter scale
resolution over a broad area. Accurate ranging to local features is
obtained with the stereo separation, at 5 m distance we can locate a
rock to 1-2 cm by cross-correlating edge features. In addition to the
general morphology of the locale, IMP has a large complement of
specially chosen filters to aid in both the identification of the
mineral types and their degree of weathering. Dr. Robert Singer will
present both the filters and the scientific goals for this part of the
IMP experiment in the next talk.

The IMP team plans to study the atmosphere in several ways. Our baseline
filter set includes 3 solar filters for viewing the solar disk. One of
these filters is specially designed to be centered on the deepest lobe
of the 935 nm water absorption band, an adjoining continuum filter
provides calibration reference. The ratio of the two filters is obtained
many times during the course of the day, especially when the sun is low
in the sky and the absorptive path is longest. Although the ratio is
estimated to be only 0.98-0.99 for nominal water vapor values, we have
gone to great pains to minimize systematic errors and it is anticipated
that the water vapor mixing ratio can be obtained to within 20%.

An additional solar filter at 425 nm has been added for studying the
optical depth of the atmospheric dust. The wavelength baseline when
combined with the 925 nm continuum filter is a factor of two, sizes can
be accurately obtained for dust particles less than 1.4 micrometers in
diameter by comparing the ratio of optical depths in the two colors to
Mie scattering models. The non-solar filters can also be used for
studying the scattering of sunlight from the sky. In this way, the phase
function of the dust can be determined and the sizes of larger particles
can be estimated. These experiments can be continued into the night by
observing Phobos. Other experiments to learn about the atmospheric dust
can be easily imagined.

Not only can IMP observe the dust in the sky, we can trap the magnetic
portion of that dust onto a series of magnetic targets of varying
strength. Dr. Jens Martin Knudsen of the University of Copenhagen in
Denmark has developed a special set of targets for the Pathfinder
Mission and has shown in the laboratory the usefulness of imaging these
targets with the IMP spectral filters to identify which magnetic mineral
he has captured. In addition to the spectral information the magnetic
strength of the material can also be determined by seeing which targets
have trapped the dust. We currently plan for two sets of targets at
different heights: close to the surface and about 0.5 m above the
surface.

A final aspect of IMP related to atmospheric studies is the wind sock
experiment. Dr. Ronald Greeley of Arizona State University is developing
and testing small telltales to be place at varying heights between the
surface and 0.5 m on one of the antennae. By imaging these targets both
the direction and velocity of the wind can be estimated. Calibration is
being done at the Ames Martian Wind Tunnel where the pressure and wind
speeds can be simulated; of course, the gravity must be scaled. By
including wind socks at several heights the local aerodynamic roughness
of the terrain can be determined and the winds can then be accurately
extrapolated above the lander site. Viking landers had wind measurements
at 1.6 m above the surface only, extrapolation to other heights was very
uncertain.


Singer R. B.*
   Surface Science Capabilities from IMP Spectral Imaging

The Imager for Mars Pathfinder (IMP) originally had a single 12-position
filter wheel for one of its two "eyes." Originally 8, and then 9 of
these filters were optimized for surface science, and 3 narrow-band
filters for atmospheric science. Because of some design revisions we
will now have filter wheels on both sides. The wheels for right and left
eyes are identical, 12 filter positions each, and rigidly linked to the
same rotation shaft. There are now 13 surface filters, in addition to 5
for atmospheric observations. Please refer to Table 1 for details of all
the filter positions. Figure 1 shows approximate gaussian bandpasses for
the 13 surface filters.

Geologic Science The geology or surface filters are targeted for
specific science objectives, and are therefore not necessarily uniforrn
in bandwidth or spacing. A major capability of the geology bandpasses is
to differentiate (and in many cases identify) most types of crystalline
ferric oxide and oxyhydroxide from each other and from poorly
crystalline or nanophase ferric oxides such as found in Mars-analog
palagonites [1-4]. This provides knowledge of phases indicative of
different environments and modes of alteration. With high signal/noise
we can also study some subtle differences due to mixtures and coatings
of weathered minerals. An equally important capability is to
characterize unaltered crustal material (dark materials). In most cases
we can estimate pyroxene Fe^2+ "1 micrometer"' band positions well
enough to estimate the composition and mineralogy. Most spectrally-
observed dark regions on Mars show pyroxene bands centered from about
0.92 to 0.98 micrometers [5,6]. Where Fe^2+ band minima occur slightly
longward of 1.0, however, such as for very high Ca pyroxenes as well as
for olivine, we are limited by the silicon detector spectral range. If
we observe any dark materials with IMP which do not display a "1-
micrometer" band our interpretations will have to rely on inferences
based on the shape and slope of the spectrum shortward of 1.0
micrometers.

The original set of 8 geology filters was augmented to 9 last fall with
the addition of a 0.48 micrometer filter. This improved discrimination
particularly among crystalline hematite (alpha Fe(sub)20(sub)3),
crystalline goethite (alpha FeOOH), and nanophase or poorly-crystalline
ferric oxide. This distinction is important for interpreting alteration
histories, and is also a significant benefit for the magnetic properties
experiment. These 9 bandpasses did a good job, but still left some
spectral gaps. With the current set of 13 bandpasses we have extended
the short wavelength coverage to 0.40 micrometers, filled the most
significant spectral gaps, and provided an extra channel in the
important Fe^2+ and Fe^3+ region between 0.89 and 1.0 micrometers. While
still not a complete spectrometer, IMP is quite powerful for determining
surface mineralogy, considerably more so than Viking.

The geology filters are arranged from the violet to 0.75 micrometers in
one eye, and from 0.75 micrometers to the IR in the other. This is to
avoid the risk of choppiness or jitter in spectral data which can occur
when measurements from different detectors are interleaved. (Both
Galileo NIMS and Phobos2 ISM have had such problems.) Because IMP has a
small number of relatively broad bandpasses, it is especially important
that we can trust our calibration of contiguous channels.

Overlap between the two eyes is provided at 0.75 miconmeters, a region
where Mars-like materials generally lack absorption bands and have high
reflectance. This is also a region of good signal/noise and high
spectral contrast among surface materials, and so will be used for
obtaining stereo and nominal monochrome images.

Condensates. IMP is also sensitive to condensates on the surface. There
are striking differences in both albedo and color between frost and
martian soils and rocks, making IMP sensitive to even thin or patchy
condensate deposits. This is tme for both H20 and CO2 frosts. Shortward
of 1 micrometer the strongest H20 ice band is an overtone centered near
0.95 micrometers. This band can vary in depth from as much as 10% for
coarse-grained frost (400-2000 microns) to as little as 2% for fine-
grained frost (50 microns) [7]. This water ice band is broad enough that
for an optically thick frost layer it should be detectable and well
defined by the 5 bandpasses from 0.86 to 1.0 micrometers. For an
optically thin frost layer the 0.95 micrometer band can be difficult to
see, depending on the substrate, even though the effect on visual slope
and albedo is still large [8].

True Color Imaging. Accurate visual color rendition is important to the
mission, for public distribution as well as science. Human color vision
is a complicated and apparently not fully understood topic. After
extensive research we've concluded that there is no single set of 3
bandpasses, which is accepted as "best" at reproducing the colors that
most people see most of the time. (MIPS at JPL has apparently reached a
similar conclusion.) One common published system uses primaries of 436,
546, and 700nm [e.g. 9], while another standardizes on 444, 526, and
645nm [10]. We propose to use the IMP bandpasses at 440, 530, and 670
for standard "true color" imaging.

References: [1] Singer R. B. (1982) JGR, 87, 10159-10168. [2] Sherman D.
M. and Waite T. D. (1985) Am. Mineral., 70, 1262-1269. [3] Morris R. V.
et al. (1989) JGR, 90, 3126-3144. [4] Burns R. G. (1993) RGA, 3-29,
Cambridge. [5] Singer R. B. and McSween H. Y. Jr. (1993) in Resources of
Near-Earth Space, 709-736, Univ. of Arizona. [6] Mustard J. F. et al.
(1993) JGR, 98, 3387-3400. [7] Clark R. N. (1981) JGR, 86, 3087-3096.
[8] Clark R. N. (1981) JGR, 86, 3074-3086. [9] Guild (1931) Phil. Trans.
Royal Soc. (London), A230, 149. [10] Stiles W. S. and Burch J. M. (1959)
Optica Acta 6, 1.


Rieder R.*   Wanke H.   Economou T.
   Alpha Proton X-Ray Spectrometer

Mars Pathfinder will carry an Alpha Proton X-Ray Spectrometer (APX) for
the determination of the elemental chemical composition of martian rocks
and soils. The instrument will measure the concentration of all major
and some minor elements, including C, N, and O, at levels above
typically 1%. The method employed consists of bombarding a sample of
50mm diameter with Alpha-particles from a radioactive source (50 mCi of
Curium 244) and measuring (1) backscattered Alpha-particles (Rutherford
Backscatter = RBS mode), (2) protons from A( ,p)B reactions (Proton
mode) and (3) characteristic X-Rays emitted from the sample (X-Ray
mode). In RBS mode all elements with atomic mass greater than 4 are
registered, thus permitting to normalize results to 100% concentration.
This feature permits accurate quantitative analysis independent (within
limits) of the actual measurement geometry. Data obtained from Proton
and X-Ray modes are used to enhance selectivity of the RBS mode for the
rock forming elements Mg, Al, and Si and for heavier elements (K and Ca,
Fe-group): Resolving power in the RBS mode is determined by the energy
spread of the Alpha-source and the range of backscatter angles observed
by the detectors. These parameters in turn determine the number of
backscattered Alpha-particles per unit time. In the present design the
use of Proton and X-Ray data permits to trade in selectivity for
sensitivity.

The instrument has a long standing space heritage, going back to the
days of Surveyors V, VI, and VII (1968/69) and Phobos (1988). The
present design is the result of an endeavor to reduce mass and power
consumption (Surveyor: 10kg/10W, Phobos: 2.7kg/2.5W, this instrument:
0.6kg/0.3W); four instruments are scheduled to fly on the Russian Mars-
94 mission: 2 on penetrators (without X-Ray mode) and 2 on small
stations (including the X-Ray mode, using "room temperature" mercuric
iodide detectors provided by the University of Chicago). These are
currently being calibrated and prepared for integration.

The instrument for Mars Pathfinder will be a duplicate of the
instruments for the Mars-94 small stations with but minor changes. It
consists of a sensor head, incorporating the Alpha-sources, a telescope
of silicon detectors (35 and 700 m thick) for the detection of Alpha-
particles and Protons and a mercuric iodide X-Ray detector with
preamplifier, and an electronics box (80 x 70 x 60mm) containing a
microcontroler-based multichannel spectrometer. The sensor head will be
mounted on the rear of the Mars Pathfinder Microrover on a deployment
mechanism, that permits to place the sensor in contact with sample
surfaces inclined at any angle from horizontal to vertical, thus
permitting to measure composition of soil and rock sample. The
electronic box will be contained in the microrovers "warm" container
and will communicate with the microrover control system through a
standard RS 232 serial interface.


Seiff A.
   Atmosphere Structure and Meteorology Instrument for Mars Pathfinder

The Mesur Science Definition Team recommended that all Mesur probes
including Pathfinder carry an ASI

MET experiment, in order that no opportunity be lost to characterize the
atmosphere of Mars in passing through it. The experiment was thus
included on Pathfinder from the start (Feb., 1992), but on an
essentially non-interference basis--it was to make no unusual demands on
the spacecraft. A Science Advisory Team appointed by NASA Headquarters
in Sept., 93 first met on Nov. 3 to initiate formal science
participation, and the level of activity has since been high. The
instrument passed its Preliminary Design Review on Feb. 28.

The structure of the atmosphere is measured during entry and descent;
and meteorological parameters, pressure, temperature, and wind velocity,
are collected during the mission lifetime after landing. The structure
experiment has two phases. During high speed entry, from 160 km to near
8 km (where the parachute is deployed), accelerometers define the
density structure. In the parachute descent, atmospheric pressure and
temperature are measured until airbags are deployed ~150 m above
touchdown. Entry phase pressures are obtained by integrating measured
densities assuming hydrostatic equilibrium (the technique used on the
Viking and Pioneer Venus missions and to be used on the Galileo Probe);
the equation of state then yields temperatures. The sensors employed are
guidance-quality accelerometers, Tavis and Vaisalla pressure sensors,
and chromel-constantan thermocouples with platinum resistance
thermometers at their cold junctions.

Constraints imposed do not allow the descent phase sensors to project
outside the Lander envelope, nor are the accelerometers in an optimum
configuration about the c.g.. The Science Advisory Team (SAT) is
exploring the effects of these limitations, but they should not prevent
the acquisition of valuable data.

The meteorology measurements were originally limited to pressure and
temperature, but were extended to include winds because of the apparent
simplicity of the hardware. The measurement resolution will be 256 x
better than that of Viking, which was resolution limited. Temperature
measurements at 3 elevations above the surface, and wind measurements at
2 heights (if affordable within available Lander resources), will define
profiles not available from the Viking instrument. Rapid sampling for 5
minutes per hour will define both diurnal and seasonal variations, and
turbulence. Consideration is being given to sampling over selected one-
hour intervals for better definition of fluctuations. Pressure sensors
are shared with the ASI. Temperature sensors are chromel-constantan
thermocouples of 75 mm wire diameter. Wind is sensed from the convective
heat loss of heated wires. Sensors have been designed and evaluated
analytically. They will be evaluated experimentally and refined if
necessary from tests at Mars surface conditions in the Mars Wind Tunnel
at Ames Research Center (operated by Arizona State University).

To move them away from thermal influence of the Lander electronics, the
temperature and baseline wind sensor are mounted on the whip antenna
which communicates with the Rover, about 1 m away from the Lander core.
The wind sensor and primary temperature sensors are about 0.5 m above
the surface; two other temperature sensors are 0.25 m and 0.125 m above
the surface. The second wind sensor is proposed to be mounted on the low
gain antenna about 1 m above the surface. Pressure sensors are in the
Warm Electronics Box in the Lander core. The temperature profiles will
differentiate between stable and convective near-surface conditions, and
define atmospheric heating rate. Wind profiles will likewise
discriminate stable from unstable conditions, and define near-surface
shear, as well as provide a valuable input to boundary layer models.

The greatest concern we have for the descent phase results from the
restriction against external deployment of the temperature sensor (for
reasons of air bag safety). The sensor must sample atmosphere flowing
through the Lander rather than around it, at velocities well below the
descent velocity. This slows sensor response time, e.g., to ~4 sec if
the internal velocity (yet to be established) is 1 m

s. For the entry phase, the major problem is correction of measured
accelerations for angular inputs at off-c.g. locations. For landed
meteorology, the major concern is the design of the wind sensor to work
sensitively in the low density atmosphere and define wind directions.
Problems of thermal contamination are also inevitably present.









                             Monday, April 18, 1994
                        WHY SHOULD WE GO TO THAT SITE?
                       GENERAL LANDING SITE PERSPECTIVES
                                1:15 - 3:10 p.m.


Moore H. J.*
   A Perspective of Landing-Site Selection

The Viking `75 Project began examining the problems of landing two
spacecraft on Mars immediately after project authorization in 1969. This
examination resulted in the Viking-Mars Engineering Model [1], which
addresses the interplanetary, near-Mars (>60 km), atmospheric (<60 km),
and surface environments and astrodynamical data.

During the Mariner 9 Mission, a Viking Data Analysis Team examined
images and other data in near-real time, assessed Earth-based radar echo
data, and prepared terrain maps with the intent of identifying potential
landing sites [2]. No sites were identified because of uncertainties in
image interpretation engendered by a hazy atmosphere, conflicting
elevations from different sources, and other factors.

A Viking Landing Site Working Group was convened in early 1972 to
identify site-selection criteria compatible with landing safety, system
capabilities, and science objectives [3]. Among numerous criteria were
low elevation (for parachute performance), large separations of site
pairs (for communications), and a "warm and wet" environment (favorable
for life).

Eleven landing sites between 30 degrees N and 30 degrees S were selected
and considered by the Landing Site Working Group [4,5]. Later, seven
sites from about 43 degrees to 73 degrees N were considered because of
their relative abundance of water vapor [5]. Still later, four
equatorial sites were added because of existing radar data on them and
their accessibility to future radar observations. Most of the sites were
rejected for various reasons.

Four landing sites were approved by NASA Headquarters: (1) Chryse (prime
A1; 19 degrees N., 34 degrees W.), (2) Tritonis Lacus (back-up A2; 20
degrees N., 252 degrees W.), (3) Cydonia (prime B1; 43 degrres N., 11
degrees W.), and (4) Alba (back-up B2; 43 degrees N., 110 degrees W.).
The northern B sites replaced earlier southern sites (Apollinares and
Memnonia) because the B sites were thought to have higher atmospheric
water contents. Two equatorial sites were retained because of their
radar signatures: (1) Capri (C1; 6 degrees S., 43 degrees W.) and (2)
Meridiani Sinus (C2; 5 degrees S., 5 degrees W.).

For Mission Operations, the Landing Site Working Group was augmented by
the Viking Flight Team and renamed the Landing Site Staff [3]. This
latter group was responsible for Site Certification when the first
orbiters instruments could observe the prime site (A1) and on-going
radar observations could be analyzed; its responsibilities included
certification of the second landing site. Certification criteria were
much the same as those for selection: (1) landing ellipse size, (2)
elevation, (3) surface temperatures, (4) geology, (5) surface roughness
(slopes), (6) protuberances (rocks), (7) "soil" properties (bulk
density, etc.), (8) radar reflectivity, (9) density-temperature profile
of atmosphere, (10) atmospheric composition, (11) dust storms, and (12)
winds.

There was no landing at any pre-selected site. Plans to land the first
spacecraft at the initial Chryse site on July 4, 1976, were discarded
because the surface, which appeared to be smooth and nearly featureless
in hazy Mariner 9 images, appeared extremely rough, complicated, and
eroded (and probably rocky) in the Viking images [6-8]. Arecibo quasi-
specular radar echoes at 12.6 cm from the vicinity of the site suggested
a rough surface (RMS slopes near 5 degrees-7 degrees), but near-average
reflectivity [9]. Small signal-to-noise ratios of Goldstone echoes (3.5-
cm wavelength) from the site were particularly worrisome because they
contrasted with large signal-to-noise ratios from Tritonis Lacus [9],
and scenarios to explain the small ratios were all unfavorable. Other
criteria appeared to be satisfied.

Viking 1 then began a search for a new site to the northwest of the
original site based on images and Arecibo quasi-specular radar
observations [6,9]. A priori selection and certification of the final
site were satisfying and defensible, because the Project could say (1)
there is evidence for abundant soillike materials in the images, (2) the
rms slopes (4.5 degrees-5.5 degrees) are like those of lunar maria where
Surveyors had landed, and (3) the reflectivity (0.07) is average for
Mars [6-9]. The Viking Project made a sincere effort to find a safe
landing site and was rewarded with a successful landing.

After the first lander demonstrated Viking's capabilities for entry,
descent, and landing, almost everyone wanted to explore to the north
where atmospheric water vapor abundances were high [3,10]. A new
northern site, Utopia Planitia (B3), was added, and orbiter temperature
observations replaced the radar as a tool to assess surface material
properties. Both the Cydonia (B1) and Alba (B2) sites appeared
unexpectedly rough; again, Mariner 9 images taken through hazy skies had
suggested smooth and mantled surfaces. B-1 was rejected because large
areas appeared rough and eroded; extensive "mantles" and "dune fields"
were not found. B-3 was chosen over a western extension of B-2 because
of the operational complexity that would be introduced; the modest
difference in water-vapor abundance and inferred thicknesses and extents
of "mantles" and "dunes" did not warrant the increased risk engendered
by the increased operational complexity [3,10]. Thermal inertia at the
B3 site was judged to be about the same as that of the Lander 1 site,
but it was not possible to distinguish between a surface of sand and a
surface like that around Lander 1 [10]. The B2 site had a lower thermal
inertia than the B3 site [10]. Lander 2 was a success, but those
expecting to see extensive mantling deposits or abundant sand dunes were
surprised by the rocky scene.

The problems that now confront Mars Pathfinder are much the same as
those that confronted Viking, but more and better information exists
today. Like Viking, Mars Pathfinder must select a landing site
compatible with lander and rover designs as evidenced by available data
(Viking images, radar and thermal observations, albedo and color
observations, visible-infrared spectra, etc.). Most regions at low
elevations probably contain favorable sites, but some sites at low
elevations with weak quasi-specular echoes and low thermal inertias may
be unfavorable [11].

References: [1] Anon. (1974) NASA, Langley Res. Ctr., Viking 75 Project
Doc. M 75-125-3, 337, 1 plate. [2] Anon. (1972) NASA, Langley Res. Ctr.,
Viking 75 Project Doc. M 75-144-0, 190, 8 maps. [3] Masursky H. and
Crabill N. L. (1981) NASA SP-429, 34. [4] Masursky H. and Strobell M. H.
(1976) Astrogeol., 59, 76-431, 73. [5] Masursky H. and Strobell M. H.
(1976) Astrogeol., 60, 76-432, 46. [6] Masursky H. and Crabill N. L.
(1976) Science, 193, 809-812. [7] Young R. S. (1976) Am. Scientist, 64,
620-627. [8] Moore H. J. et al. (1987) USGS Prof. Paper, 1389, 222. [9]
Tyler G. L. et al. (1976) Science, 193, 812-815. [10] Masursky H. and
Crabill N. L. (1976) Science, 194, 62-68. [11] Moore H. J. and Jakosky
B. M. (1989) Icarus, 81, 164-184.


Greeley R.*   Kuzmin R.
   Strategy for Selecting Mars Pathfinder Landing Sites

Many feasibility studies have been undertaken for martian roving
vehicles. Most studies assumed rovers that would be capable of
traversing tens, hundreds, or even thousands of kilometers over diverse
terrains. Such capabilities are scientifically desirable but
operationally unrealistic with current budget limitations. Instead,
attention must focus on rovers traversing less than a few hundred meters
and involving a relatively limited scientific payload. Consequently, a
strategy for Pathfinder site selection must be developed that is
fundamentally different from most previous considerations. At least two
approaches can be identified.

In one approach, the objective is to select a site representing a key
geologic unit on Mars, i.e., a unit that is widespread, easily
recognized, and used frequently as a datum in various investigations. An
example is a site on Lunae Planum (20 degrees N, 61 degrees W; +1 km
elevation). This site is on Hesperian-age ridged plains, a unit that is
widespread on Mars and serves as a key datum for geologic mapping. This
material is of very high priority for a future sample-return in order to
obtain an absolute age for the base of the Hesperian System. Although
ridged plains are inferred to be volcanic and interpreted to be basaltic
lava flows, this interpretation is based on analogy with lunar mare
units and is open to question. Compositional measurements and
observations of rocks at the site via a rover would address the origin
of ridged plains and contribute substantially to understanding martian
history. For example, should ridged plains not be basalts or other
igneous rocks, the interpretation of the volcanic evolution on Mars
would be very different from current models. The disadvantage to the
approach of landing on a homogeneous unit, such as the ridged plains, is
that the measurements would be primarily for a single rock type (but of
known geologic context) and would not address questions of compositional
diversity on Mars.

The second approach is to select a site that potentially affords access
to a wide variety of rock types. Because rover range is limited, rocks
from a variety of sources must be assembled in a small area for
sampling. Sedimentary deposits, such as channel deltas, derived from
sources of various ages and rock types potentially affords this
opportunity. For example, a site in southeast Chryse Planitia (19.3
degrees N, 35 degrees W; -1.5 to -1.0 km elevation) is on outwash plains
from Ares, Tiu, Shalbatana, and Simud Valles. Headwind regions for these
channels include assemblages of ancient crust (Noachian plateau
material) and Hesperian ridged plains, as well as modern aeolian
deposits indicated by local wind streaks. This general approach is
demonstrated in Death Valley, where landing site studies were conducted,
simulating Mars. A randomly-located "touch down" was made on the Furnace
Creek alluvial fan. Within a 1 m radius of the landing site, samples of
rock included basalt, rhyolite, diorite, quartzite, limestone, and
siltstone; within a 2 m radius, additional rocks included sedimentary
breccia, carbonate siltstone, and gabbro. All of these rocks were
transported from the surrounding mountains. Although Death Valley is not
a complete analog to Mars, the area shows that alluvial fans and river
mouths may be good sites to collect a wide variety of rocks. The
disadvantage of this approach on Mars is that the geological context of
the rocks in the deposit is not known, and the compositions of the
potential contributing source units must be inferred.

Regardless of the approach taken in site selection, the Pathfinder site
should include aeolian deposits and provisions should be made to obtain
measurements on soils. It is important to note the fundamental
difference between dust (known to exist on Mars) and sand (suggested to
exist). Martian dust is <10 micrometers in diameter and is settled from
suspension. The dust is probably derived from a wide variety of sources
and is thoroughly mixed through repeated cycles of global dust storms.
As such, dust represents a global "homogenization." In contrast, sand is
deposited from transport in saltation and reflects mostly local and
regional sources upwind from the site. Sand grains are probably a few
hundred micrometers in diameter or larger. Wind streak orientations and
general circulation models of the atmosphere provide clues to the
sources for sand. In addition to sand and dust, soils may include
material derived from local weathering. Thus, it is desirable to be able
to handle and analyze all three potential components of martian soil,
dust, sand, and locally weathered material.

Tests conducted in March 1994 at Amboy lava field in the Mojave Desert
with the Russian Marsokhod rover provide insight into the scientific use
and operation of small rovers. The range was <100 m and the imaging
system was limited in resolution. "Descent" images (a series of
progressively higher resolution images from orbital scales down to ~20
cm/pixel) were available for planning the science tests and rover
operations. Initial results indicate (1) without the context provided by
the descent images, the geologic setting of the site would have been
difficult or impossible to determine (Pathfinder, for example, will not
have descent imaging), (2) the low height (~1 m) of the stereo camera on
the rover gives a different perspective of the terrain than is obtained
from standing in the field. (3) the stereo imaging system developed for
navigation by the rover was inadequate for most science analyses, and
(4) the use of a simulated hand lense (x10) and microscope (x100) was
extremely valuable for analysis of sand, dust, and rock samples.

Based on these considerations, a recommended approach for selecting the
Mars Pathfinder landing site is to identify a deltaic deposit composed
of sediments derived from sources of various ages and geologic units,
and which shows evidence of aeolian activity. The site should be located
as close as possible to the part of the outwash where rapid deposition
occurred (as at the mouth of a channel), because the likelihood of
"sorting" by size and composition increases with distance, decreasing
the probability of heterogeneity. In addition, it is recommended that
field operation tests be conducted to gain experience and insight into
conducting science with Pathfinder.



Farmer J. D.*   Des Marais D. J.
   Exobiology Site Priorities for Mars Pathfinder

Although present martian surface conditions appear unfavorable for life
as we know it [1], there is compelling geological evidence that the
climate of early Mars was much more Earth-like, with a denser atmosphere
and abundant surface water [2]. Three key post-Viking discoveries
mandate a more rigorous search for a martian biosphere. First, 3.5
billion year old fossils indicate that our own biosphere might be almost
as old as Earth itself [3]. Second, climates on both Earth and Mars have
evolved through time. Third, the range of Earth's habitable environments
is greater than previously known. Extremes for terrestrial life include
polar deserts, deep subsurface aquifers, and hydrothermal systems, and
high salinity ponds and lakes. The fact that life developed on the Earth
within the first billion years of its history makes it quite plausible
that life may have also developed on Mars [4]. If life did develop on
Mars it undoubtedly left behind a fossil record. Such a fossil record is
likely to be more accessible than either subsurface environments that
may harbor life, or scattered "oases" that may be present at the
surface. Consequently, the post-Viking approach of Mars Exobiology has
shifted focus to search for evidence of an ancient Martian biosphere.
This has led to the emergence of a new subdiscipline of paleontology,
herein termed "exopaleontology" [5], which deals with the exploration
for fossils on other planets and whose core concepts derive from Earth-
based Precambrian paleontology, microbial ecology, and sedimentology
[6,7].

By analogy with the Precambrian record of the Earth, an early martian
biosphere is likely to have been microbial. The types of
micropaleontological information we could expect to find on Mars include
microbial body and trace fossils, biostratification structures, (e.g.,
stromatolites) and biomolecular fossils. Based on what we have learned
from the Precambrian record on Earth, the best preservation of
microorganisms as fossils occurs when they are rapidly entombed by
aqueous minerals while the organisms are still viable, or at least prior
to cellular degradation [6,7]. For long-term preservation (i.e.,
billions of years) organic materials must be incorporated into or
replaced by fine-grained, stable phases (e.g., silica, phosphate, or
carbonate). Terrestrial microfossils were preserved in this way, being
permineralized in siliceous sediments (cherts) associated with ancient
volcanic terrains in Australia and South Africa [8,9].

The above observations lie at the core of the proposed exploration
strategy to search for a fossil record on Mars. Terrestrial environments
where high rates of aqueous mineral precipitation and microbial activity
coincide include subaerial thermal springs and shallow hydrothermal
systems, sub-lacustrine springs and evaporitic lakes, subsurface soils
where "hardpans" (e.g., calcretes, silcretes) form, vadose zone karst
deposits and silcretes associated with karst paleosols, and high
latitude frozen soils or ground ice [6,7].

Subaerial thermal spring deposits are key targets for a fossil record on
Mars [10] because high rates of mineral precipitation may occur together
with microbial activity. Volcanic terrains are widespread on Mars and
some possess outflow channels that are likely to have formed by spring
sapping [11]. The association of such features with potential heat
sources, such as volcanic cones or thermokarst features, indicates the
possibility for past hydrothermal activity on Mars. Within the landing
site constraints for Mars Pathfinder, a number of potential exploration
targets meet the basic requirements for hydrothermal activity and
associated mineralization, based on analysis of Viking images. These
include thermokarst features and areas possibly affected by hydrothermal
processes, including (1) the head reaches of small channels in the Ares
and Tiu Vallis outflow systems, which originate from areas of chaos and
(b) the floors of chasmata, such as Echus Chasma. Target deposits in
such areas include the common subaerial spring minerals, silica and
carbonate, as well as hydrothermal alteration halos associated with
shallow igneous intrusives (including dike swarms) where hydrous clays
may have been formed through hydrothermal alteration of host rocks.

Reliable identification of aqueous mineral phases requires that we
incorporate rover-based techniques for in situ compositional analysis
that provide structural information, in addition to elemental abundance.
Target minerals for Exopaleontology have characteristic signatures in
the near- and mid-infrared that should be detectable using rover-based
spectroscopy. Future landed missions should incorporate such approaches,
along with fiber optic-based visible- and UV-microscopy, as standard
payload exploration tools for Exopaleontology. Even if life never
developed on Mars, aqueous mineral deposits still hold great interest as
potential sources of information about the nature and abundance of the
precursor organic molecules available in the early solar system. Fluid
inclusions incorporated into aqueous minerals during their
crystallization provide valuable samples of primary liquid and vapor
phases, and potentially, also microorganisms and biomolecules [12].
Although more research is needed, rover-based spectral analysis may
provide a sensitive, in situ method for distinguishing fluid inclusion-
rich mineral deposits from "dry" rocks [13].

Through Earth-based analog studies, we have also been investigating the
paleontology and sedimentology of subaqueous spring deposits formed over
a range of temperatures, as potential targets for Mars Exopaleontology.
Rates of mineral precipitation within such environments are often high
enough to entomb associated microbial mat communities, and deposits
formed at lower temperatures have the advantage of preserving a higher
proportion of organic matter [14]. Thus, in contrast to subaerial
thermal spring deposits, tufas and evaporites often contain abundant
microbial fossils and organic matter. Sublacustrine springs are common
in many water-rich volcanic settings, particularly in association with
crater and caldera lakes [7]. Sediments deposited in such settings are
frequently heavily mineralized and are important exploration targets for
many types of economic ore deposits. Some of our finest examples of
excellent preservation in the terrestrial fossil record are found in
such facies (15).

In pluvial lake basins in western North America, subaqueous spring
deposits and sedimentary cements are commonly found along the distal
margins of fan delta deposits within mixing zones where fresh ground
water encounters alkaline lake water [16,17]. In such terminal lake
settings, evaporites are commonly deposited in basinward locations
during lake low stands. Evaporite minerals frequently entrap
halobacteria and organic matter within fluid inclusions during
crystallization. Although the long-term viability of salt-entrapped
organisms is debatable [18], entombed organics appear to survive for
long periods of geologic time. Consequently, "evaporites" are regarded
to be prime targets for Mars Exopaleontology for reasons outlined above.
The major disadvantage is that, in the presence of an active
hydrological system, evaporites tend to have short crustal residence
times and are easily lost from the stratigraphic record by dissolution.
Thus, terrestrial evaporites are quite rare in Precambrian sequences.
However, this may not apply to Mars. Given the early decline of a
martian hydrological cycle involving liquid water, it is possible that
Archean-aged evaporites have survived there.

Potential targets on Mars for subaqueous spring deposits, sedimentary
cements, and evaporites are ancient terminal lake basins where
hydrological systems could have endured for some time under arid
conditions [19,20]. Potential targets for the Mars Pathfinder mission
include channeled impact craters and areas of deranged drainage
associated with outflows in NW Arabia and Xanthe Terra, where water may
have ponded temporarily to form lakes. The major uncertainty of such
targets is their comparatively younger age and the potentially short
duration of hydrological activity, compared to older paleolake basins
found in the southern hemisphere. However, it has been suggested that
cycles of catastrophic flooding associated with Tharsis volcanism may
have sustained a large body of water, Oceanus Borealis, in the northern
plains area until quite late in martian history [21,22]. Although
problematic, the shoreline areas of the proposed northern ocean, (e.g.,
along the Isidis impact basin and the plains of Elysium, Chryse, and
Amazonis) provide potential targets for a Mars Pathfinder mission aimed
at exploring for carbonates or other potentially fossiliferous marine
deposits. Carbonates and evaporites possess characteristic spectral
signatures in the near-IR [23] and should be detectable using rover-
based spectroscopy and other methods for in situ mineralogical analysis.

Many terrestrial soils are known to preserve microbial fossils and
biogenic fabrics within the mineralized subzones of soils, such as
calcretes, silcretes, or other types of "hard-pans" [24]. For example,
the oldest terrestrial microbiota are preserved in silcretes associated
with 1.7 Ga karst. Viking biology experiments indicate that surface
soils on Mars are highly-oxidizing and destructive to organic compounds.
However, mineralized soil horizons could protect fossil organic matter
from oxidation and should not be overlooked as potential targets for
Exopaleontology. At the Viking Lander 2 site, soils showed the
development of duricrust suggesting cementation [25], and sulfate and
carbonate minerals are inferred to be present in the martian regolith
based on elemental analysis by x-ray fluoresence. Although Viking
conclusively demonstrated the absence of organic compounds in the soils
analyzed, the presence of cements in martian surface sediments suggests
a possibility for hard-pan mineralization that could afford protection
to organic materials against oxidation. The best places to explore for
mineralized paleosols are deflational areas where wind erosion may have
stripped away surface sediments, exposing indurated zones formed at
depth. Such sites are widespread within the potential landing area for
Mars Pathfinder.

References: [1] Klein H. P. (1992) Orig. Life Evol. Biosphere 21, 255-
261. [2] Pollack J. B. et al. (1987) Icarus, 71, 203-224. [3] Oberbeck
V. R. and Fogleman G. (1989) Orig. Life Evol. Biosphere, 19, 549-560.
[4] McKay C. P. and Stoker C. R. (1989) Rev. Geophys., 27, 189-214. [5]
Farmer J. D. and Des Marais D. J. (1994) Geol. Soc. of Amer., Abstracts
with Prog. [6] Farmer J. D. and Des Marais D. J. (1993) Case for Mars V,
33-34. [7] Farmer J. D. and Des Marais D. J. (1994) LPSC 25. [8] Awramik
S. M. et al. (1983) Science, 20, 357-374. [9] Walsh M. M. and Lowe D. R.
(1985) Nature, 314, 530-532. [10] Walter M. R. and Des Marais D. J.
(1993) Icarus, 101, 129-143. [11] Carr M. H. (1981) The Surface of Mars,
Yale Univ., 232 pp. [12] Bargar K. E. et al. (1985) Geology, 13, 483-
486. [13] Gaffey S. J. (1989) in Amer. Chem. Soc. Sympos., 415, 94-116,
(L. M. Coyne et al., eds.), 94-116. [14] Farmer J. D. and Des Marais D.
J. (1994) in Microbial Mats. Structure, Development, and Environmental
Significance, Springer-Verlag, (L. J. Stal and P. Caumette, eds.). [15]
Rolfe W. D. I. et al. (1990) GSA Spec. Paper, 244, 13-24. [16] Blevins
M. L. et al. (1987) Los Angeles Dept. of Water and Power, Unpub. Rpt.
(March 1987). [17] Rogers D. B. and Dreiss S. J. (1993) GSA, 25, 6, 183.
[18] Rothchild L. J. (1990) Icarus, 88, 246-260. [19] Farmer J. D. et
al. (1994) in Mars Landing Site Catalog, NASA Ref. Publ. (R. Greeley,
ed.), 124. [20] Farmer J. D. et al. (1994) Advances in Space Res. 13,
14. [21] Parker T. J. et al. (1989) Icarus, 82, 111-145. [22] Baker et
al. (1991) Nature, 352, 589-594. [23] Crowley J. K. (1991) JGR, 96,
(B10), 16231-16240. [24] Jones B. and Kahle C. F. (1985) J. Sed. Pet.,
56, 217-227. [25] Moore H. J. et al (1987) USGS Prof. Paper, 1389, 22
pp. [26] Zvyagintsev D. G et al. (l990) Mikrobiologiya, 59, 491-498.
[27] Khlebnikova et al.(l990) Mikrobiologiya, 59, 148-155; Squyres S. W.
and Carr M. H. (1986) Science, 231, 249-252.


Seiff A.   Haberle R.   Murphy J.
   Landing Site Considerations for Atmosphere Structure and Meteorology

The goal of the ASI/MET experiments is to extend our knowledge of Mars
atmosphere structure and meteorology over that established by the Viking
mission. The two in situ soundings of Mars atmosphere by Vikings 1 and 2
were highly similar, but radio occultations and infrared soundings have
shown large variability in atmosphere structure on Mars with latitude,
season, and terrain elevation [1,2]. It would be of great interest to
obtain an in-situ sounding showing strong contrast in thermal structure
with the Viking profiles. These would be expected to occur in the winter
season, in the southern hemisphere, or at polar latitudes. These options
are ruled out by Pathfinder Mission constraints, which place the entry
in low, northern latitudes in mid-summer, with small seasonal difference
from the two Viking landers, and small latitude difference from Viking
1. The Pathfinder arrival date and latitude correspond to a seasonally
equivalent Earth date of Aug 15, compared to June 21 for Viking 1 and
July 17 Viking 2, not a striking difference.

Within the Pathfinder constraints, the best possibilities for extending
observations to other conditions are: (1) to maximize latitude contrast
with Viking 1 by moving toward the equator; (2) to study the influence
of prominent terrain features on structure and the general circulation;
or (3) to examine the effect of dark albedo features on the overlying
structure, since radiative equilibrium with the surface controls the
temperature structure to first order [3]. Landing sites satisfying the
above criteria are discussed below.

Near-equatorial sites, from 0 to 10 degrees N latitude, are available in
the region from 235 degrees W to 150 degrees W, with the eastern end in
Elysium Planitia. This terrain is of mixed albedo, variegated light and
dark. It would thus present a terrain contrast with Viking 1, in
addition to a significant latitude difference. This region is south of
Cerberus Rupes.

To study the influence of a prominent terrain feature, what better to
choose than Olympus Mons? A site in Amazonis Planitia due east of
Olympus at 150 degrees W, 18 degrees N has an elevation of -1.5 km. This
terrain will certainly affect the atmospheric circulation below 20 km,
and radiation effects from the inclined terrain could also influence the
thermal structure. The descending terrain at this site will have a slope
wind signature. It is an interesting site.

Another terrain influence possibility occurs in Isidis Planitia,
directly east of Syrtis Major which rises to an elevation of 4 km within
a few hundred km of a possible landing site at - 2 km, an ideal location
for the study of slope winds as well as terrain influence.

Within Isidis farther from Syrtis Major, the primary goal for atmosphere
structure would simply be to look for temporal change from 1976 and the
effect of the small seasonal change. Centered at 270 degrees W, there is
a smooth, bland region with a large area below -2 km (>10 degrees
diameter) in which the terrain appears to be very similar to that in
Chryse on the USGS maps. Other sites apparently similar to Chryse lie
east of Chryse Planitia extending as far as 20 degrees W longitude. A
generally interesting target in this region is the upper region of Ares
Vallis, which looks like a broad, dark-albedo flood plain, and therefore
presents a difference. The Tui Valles region at about 13 degrees N and
33 degrees W also has some interesting characteristics. It is at the
required low elevation, is a valley centered on a river channel in a
region of albedo contrasts.

Two members of the Science Advisory Team who worked extensively on
analysis of Viking data favored continuation of the Viking 1 data set as
close as possible to the Viking 1 landing site. This would no doubt be
valuable. However, it can be argued that the exploration of other sites
is more likely to lead to major advancements in understanding of Mars
meteorology.

To satisfy the third objective, a site within the unnamed but prominent
dark albedo feature about 350 km wide, which sweeps across Elysium
Planitia from northeast to southwest is suggested. At 15 degrees N,
where the center of this feature is at 245 degrees W, terrain elevation
is -1 km. The width of this feature nearly matches the horizontal region
sampled by the structure experiment, so that sampling entirely within
the vertical region overlying this feature is possible.

Assigning priorities to these options, we suggest, from the standpoint
of atmosphere structure, the following sites: (1) Amazonis Planitia, 18
degrees N, 150 degrees W, elevation -1.5 km. (2) Isidis Planitia, 13
degrees N, 278 degrees W, elevation -2 km. (3) Ares Vallis, 16 degrees
N, 32 degrees W, elevation -2 km. (4) Cerberus Rupes region in Elysium
Planitia, 5 degrees N, 190 degrees to 197 degrees W, elevation -2 km.
(5) Dark feature in Elysium Planitia, 15 degrees N, 246 degrees W,
elevation -2 km.

By and large, site selection factors for atmosphere structure, to define
the effects of latitude, terrain, and soil temperature, are also
important to landed meteorology. Viking established that slope winds are
dominant in summer and their study would be continued at sites 1, 2, and
3. To further examine the pressure fluctuations associated with
traveling baroclinic disturbances, seen at the two Viking sites, a site
at midlatitudes is preferred [4]. The 5 sites we list are not optimum
from this standpoint, but are well suited to monitoring such tropical
phenomena as thermal tides, Kelvin waves, normal modes, and Hadley
circulations.

Winds at the several landing sites have been examined using the Ames GCM
[5]. Ten-day average winds at the two sites near the large terrain
obstacles are ~20 m/s with extremes of 5 and 35 m/s, much larger than
mean winds at the two Viking sites. Certainly, a key objective of the
Pathfinder experiment will be to see if these predicted winds are
verified by measurements. If so, it will establish the validity of the
GCM not only for understanding Mars circulation, but also as a tool for
future mission design.

References: [1] Kliore A. (1977) JGR, 78, 4331-4343 [2] Zurek R. W. et
al. (1992) in Mars, Chapter 26, Univ. Arizona Press. [3] Seiff A. and
Kirk D.B. (1977) JGR, 82, 4364-4378. [4] Barnes J. (1994) this volume.
[5] Pollack J. et al. (1990) JGR, 95, 1447-1474.






                             Monday, April 18, 1994
         WHERE ARE WE?  WHAT WILL IT LOOK LIKE?  HOW SAFE WILL IT BE?
             WHAT WILL THE WEATHER BE?  WHAT WAS THE WEATHER LIKE?
                                3:30 - 5:30 p.m


Keller H. U.*
   Observations by the Mars '94 Orbiter and Possible Correlations with Mars
   Pathfinder

The Mars `94 spacecraft will still be operational when MESUR Pathfinder
begins its observations. While it will probably not be possible to
detect the lander directly the terrain including the landing error
ellipse can be covered in high resolution (10 m) in various color bands.
The stereo capability of the high resolution camera will provide a 3-
dimensional terrain map. The landing site of Pathfinder could possibly
be chosen so that correlated observations of IMP and the remote sensing
instruments on board Mars `94 may be possible. We will discuss this
scenario based on the presently adopted Mars `94 orbit and resulting
enhancements stemming from correlations of data obtained by both
spacecraft.


Betts B. H.*
   Implications of High-Spatial-Resolution Thermal Infrared (Termoskan) Data
   for Mars Landing Site Selection

Thermal infrared observations of Mars from spacecraft provide physical
information about the upper thermal skin depth of the surface, which is
on the order of a few centimeters in depth and thus very significant for
lander site selection. The Termoskan instrument onboard the Soviet
Phobos 88 spacecraft acquired the highest spatial resolution thermal
infrared data obtained for Mars, ranging in resolution from 300 m to 3
km per pixel [1-3]. It simultaneously obtained broad-band reflected
solar flux data. Although the 6 degrees N to 30 degrees S Termoskan
coverage only slightly overlaps the nominal Mars Pathfinder target
range, the implications of Termoskan data for that overlap region and
the extrapolations that can be made to other regions give important
clues for optimal landing site selection.

For example, Termoskan highlighted two types of features that would
yield high lander science return: thermally distinct ejecta blankets and
channels. Both types of features are rare examples (on Mars) where
morphology correlates strongly with thermal inertia. This indicates that
evidence of the processes that formed these morphologic features likely
still exists at the surface. Thermally distinct ejecta blankets (Fig. 1)
are not significantly mantled by aeolian material, and material ejected
from depth should be exposed at the surface [4]. In addition, their
unmantled surfaces should still contain morphologic clues to the exact
process that formed the uniquely martian fluidized ejecta blankets.
Thermally distinctive channel floors (e.g., Fig. 2) probably have
material exposed from various stratigraphic layers and locations. In
addition, the possibility that flat channel floors owe their enhanced
inertias to water related processing (bonding) of fines makes these
sites intriguing [5].

Where coverage exists (Fig. 3), Termoskan can also be used to assess
thermal inertias and the degree of thermal inertia homogeneity. These
are important for lander safety considerations as well as science. Those
interested in particular regions are encouraged to contact the author
for more details.

References: [1] Selivanov A. S. et al. (1989) Nature, 341, 593-595. [2]
Murray B. C. et al. (1991) Planet. Space Sci., 39, 237-265. [3] Betts B.
H. (1993) Ph.D. thesis, Caltech. [4] Betts B. H. and Murray B. C. (1993)
JGR, 98, 11043-11059. [5] Betts B. H. and Murray B. C. (1994) JGR, 99,
1983-1986. [6] Witbeck et al. (1991) USGS map, I-2010. [7] Scott D. H.
and Tanaka K. L. (1986) USGS map, I-1802A.

Fig. 1, which appears here in the hard copy, shows ejecta blankets
distinct in the thermal infrared (EDITHs): Termoskan thermal infrared
image. North is top. A small part of Valles Marineris appears at top
right. Time of day is near local noon. Darker areas are cooler, lighter,
are warmer. Note the thermally distinct ejecta blankets, which appear as
bright or dark rings surrounding craters (examples denoted by arrows).
EDITH boundaries usually closely match fluidized ejecta termini. White
lines are geologic map boundaries (from [6,7]). Throughout the data,
almost all EDITHs observed are on Hesperian aged terrains with almost
none on the older Noachian units, presumably due to a lack of
distinctive layering in Noachian terrains (see [4] for more
information). EDITHs are excellent locations for future landers because
of relatively dust free surface exposures of material excavated from
depth.

Fig. 2, which appears here in the hard copy, shows (Top) Termoskan
thermal and (bottom) visible images centered approximately upon 1 degree
S, 39 degrees W. North is top. In all thermal images shown here, darker
is cooler. Shalbatana, Simud, and Tiu Valles all continue for several
hundred kilometers north of this image. Note the cool and generally
uniform floors of all channels except the eastern (and rough floored)
end of Ravi Vallis. Note also that the thermal boundaries closely match
the boundaries of the channel floors and depart significantly from
albedo boundaries seen in the visible image. Note also the dark,
presumably aeolian deposits localized within the southern portions of
Shalbatana Vallis and the southwestern portion of Hydraotes Chaos and
spreading onto the surrounding plains in both cases. Buttes, including
the large labeled one in the northeast of the image, within the channels
appear similar in temperature and appearance to the surrounding plains,
not the channels. We favor nonaeolian explanations of the overall
channel inertia enhancements based primarily upon the channel floors
thermal homogeneity and the strong correlation of thermal boundaries
with floor boundaries. See [5] for more information.

Fig. 3, which appears here in the hard copy, shows coverage of
Termoskan's four panoramas (boxed regions) overlaid on a simplified
geologic map of Mars from Barlow and Bradley [1990]. Note that on this
simplified map, ridged plains are not split into Noachian and Hesperian
ages. Note also that regions near the outer edges of each panorama are
badly foreshortened because they were observed near the limb.


Slade M. A.*   Jurgens R. F.
   Goldstone Radar Contributions to Mars Pathfinder Landing Safety

Goldstone radar can provide topography "profiles," statistical surface
roughness, and radar images within a few degrees of the subearth point.
Goldstone/Very Large Array bistatic radar observations can image the
whole disk of Mars with integration times on the order of 10 minutes
before pixel smearing occurs. Data from all of these radar techniques
can be useful for observing the local surface conditions relating to
landing safety issues for Mars Pathfinder. Topographic profiles will be
presented from the 1978 opposition (subradar latitude ~10 degrees N),
and the 1980, 1982 oppositions (subradar latitudes ~20-22 degrees N) at
13 cm wavelength with a radar "footprint" of ~8 km (long.) by 80 km
(lat.). The 1992-1993 Opposition (subradar latitudes ~4-1 degrees N) has
both Goldstone/VLA images and topographic profiles at 3.5 cm wavelength
(many of the latter have yet to be reduced).

During the 1995 Opposition, additional opportunities exist for obtaining
the data types described above at latitudes between 17 degrees N to 22
degrees N. (See Fig. 1 below). Upgrades to the radar system at Goldstone
since 1982 will permit higher accuracy for the same distance with a
reduced footprint size at 3.5 cm. Since the Arecibo radar will still be
in the midst of their upgrade for this upcoming opposition (which starts
~November 1994, with closest approach in Feb. 1995), the Goldstone radar
will be the only source of refined radar landing site information before
the Mars Pathfinder landing.


Barnes J.*
   Meteorological Observations of Synoptic Disturbances:  Sensitivity to
   Latitude

The Mars Pathfinder MET experiment will make pressure, temperature, and
wind measurments on the surface of Mars. The Viking Lander Meteorology
Experiment measurements were marked by the presence of variations
associated with synoptic weather disturbances throughout the fall and
winter seasons. These variations were characterized by periods in the
broad range of about 2-10 days, and were most prominent at the
midlatitude (48 degrees N) Viking Lander 2 site. The synoptic
disturbances were observed to essentially disappear during the summer
season. At the subtropical (22.5 degrees N) Viking Lander 1 site,
variations with similar periodicities were seen, but the amplitudes of
these were reduced in comparison to those at Lander 2 by factors of 2-3
or more. The identification of the weather variations has been helped
greatly by numerical simulations of the Mars atmospheric circulation
performed with various models.

These models show that the winter midlatitudes are the center of
activity for traveling disturbances of planetary scale, disturbances
that have their fundamental origin in the baroclinic instability of the
wintertime Mars atmospheric circulation. The numerical studies are
consistent with the Viking observations in that the disturbances decay
in amplitude towards lower latitudes; direct comparisons of the models
with the Viking data are quite favorable, though the models seem to
produce larger amplitudes in the subtropics than were seen at the Lander
1 site. If Mars Pathfinder is able to survive for 2-3 months, then it
will observe the transition from the very quiescent summer season into
the much more active winter season. The further north it is located, the
more clearly will it be able to detect the signatures of the midlatitude
weather systems. The basic mission constraint of a low elevation landing
site should favor the observation of the weather disturbances: the model
simulations show that the weather activity is enhanced in the subtropics
in the three low regions of the northern hemisphere.

This is at least partly due to the presence of "standing eddies" in the
circulation, which are forced by the topography. A landing site close to
15 degrees N should allow measurement of the weather disturbances, along
with observations of the thermal tides, slope winds, and the relatively
steady winds associated with the general circulation--the "trade winds"
of Mars. Model simulations show that the latter can be very strong in
certain locations, especially near the western edges of low elevation
regions. A landing site near 15 degrees N would be significantly further
equatorward than the Viking Lander 1 site, and thus would provide more
of a view of tropical circulation processes. There could be some
"surprises" in such observations.


Forget F.*   Hourdin F.   Talagrand O.
   Mars Pathfinder Meteorological Observations on the Basis of Results of an
   Atmospheric Global Circulation Model

The Mars Pathfinder Meteorological Package (ASI/MET) will measure the
local pressure, temperature, and winds at its future landing site,
somewhere between the latitudes 0 degrees N and 30 degrees N.

Comparable measurements have already been obtained at the surface of
Mars by the Viking Landers at 22 degrees N (VL1) and 48 degrees N (VL2),
providing many useful informations on the martian atmosphere. In
particular, the pressure measurements contain very instructive
information on the global atmospheric circulation. The large amplitude
seasonal oscillations of the pressure are due to the variations of the
atmospheric mass (which results from condensation-sublimation of a
substantial fraction of the atmospheric carbon dioxide in the polar
caps), but also to internal latitudinal mass redistribution associated
to the atmospheric circulation.

The more rapid oscillations of the surface pressure, with periods of 2
to 5 sols, are signatures of the transient planetary waves, which are
present, at least in the northern hemisphere, during autumn and winter.

At the Laboratoire de Meteorologie Dynamique (LMD), we have analyzed and
simulate these measurements with a martian atmospheric global
circulation model, which was the first GCM to simulate the martian
atmospheric circulation over more than one year [1,2]. The model is able
to reproduce rather accurately many observed features of the martian
atmosphere including the long and short period oscillations of the
surface pressure observed by the Viking landers (Fig.1).

Both the annual pressure cycle and the characteristic of the rapid
oscillations have been shown to be highly variable with the location on
the planet. For instance, simulated surface pressure obtained in the
middle latitudes of the southern hemisphere look very different than the
Viking landers measurements because of the effect of an opposite
meteorological seasonal component. From this particular point of view,
much could be learned from a future lander located in the southern
hemisphere. As we were able to simulate the surface pressure variation
from any point on the planet, we have used the LMD GCM to investigate
the climatological properties of the different possible landing sites.
Figure 2 shows the surface pressure as simulated at three different
points in Isidis Planitia. As in the other possible landing areas, the
amplitude of the transient eddies is found to decrease with latitude.
Longitudinal differences between the areas below 0 km are small, except
that the VL1 site in Chryse Planitia seems to be surprisingly more
active than any other possible landing site at the same latitude. The
seasonal meteorological component of the annual pressure cycle is
minimum at the equator, thus, the local pressure oscillations should
reflect the oscillation of the planetary averaged surface pressure,
providing a more accurate estimation of the total atmospheric mass than
with the Viking data. Such a location near the equator would extend the
latidunal coverage of the Viking Landers. It should also be interesting
to observe the behavior of the atmospheric waves and local winds, where
the Coriolis force is negligible. Therefore, from a meteorological point
of view, we think that a landing site located near or at the equator
would be an interesting choice.

References: [1] Talagrand et al. (1991) BAAS, 23, 1217. [2] Hourdin et
al. (1993) J. Atmos. Sci., 50, 3625-3640.


Zent A. P.*
   Climatological Targets for Mars Pathfinder

Major Climatological Questions. Did Mars have a wet, warm climate early
in its history? There is evidence that water flowed across the martian
surface during the Noachian [1], and a hydrologic cycle was probably
required [2]. However, surface temperatures early in martian history are
predicted to have been too low for liquid water [3]. An atmospheric
greenhouse, with C02 as the major constituent, has been postulated as a
mechanism to raise surface temperatures. The subsequent fate of that C02
remains a puzzle; 2 to 5 bars would have been required, equivalent to a
global layer of calcite 46 to 115 m thick. Although bulk carbonates have
recently been reported in martian meteorites [4], it is important to
search for in situ martian carbonates.

Did the discharge of martian outflow channels produced a large ocean in
the northern plains, and Hesparian and Amazonian periods of clement
climate? It has been hypothesized [5] that return of C02 to the
atmosphere could have occurred during the creation of the outflow
channels, and that subsequent higher surface temperatures could have
permitted a global hydrologic cycle that was responsible for formation
of a vast Austral ice sheet. The outflow would have formed a northern
ocean that would eventually have re-precipitated the C02 into
carbonates, thereby ending the warm, wet periods.

Is chemical weathering proceeding at present on Mars? The reactive
nature of regolith materials suggests either that reactive oxidants are
present in the soil [6], or more likely, that heterogeneous chemistry is
taking place between surface materials and photochemically produced
oxidizing compounds in the atmosphere [7].

Target Considerations. The ability to look into the past means the
ability to look down the sedimentary sequence. The ideal landing site is
one in which sedimentary units are exposed. Ideally, a mixture of
clastic and chemical sediments will be present; decimeter-scale coherent
igneous rocks would provide the opportunity to examine chemical
weathering processes. A near-shore deposit, where local channels show
evidence of having dissected units of a variety of ages, would be ideal.

We search for depressions within the allowable latitude and elevation
domain, into which channels or valleys clearly flow, and which show no
obvious signs of subsequent deposition, such as wrinkle ridges, or the
high albedo/low thermal inertia signatures of thick dust deposits.

Target materials include carbonates, nitrates, sulfates, halides,
phosphates, clays, and Fe-oxides. Much of the original C02 inventory
expected to be locked up in carbonates somewhere in the martian
regolith. Although small amounts of carbonate have been detected in
airborne dust [8] no significant in situ deposits or coherent carbonate
rocks have been identified. Nitrates are important because nitrogen is
an element of major biological significance and it has not been
identified in the martian soil. Moreover, some models of the Viking
Labeled Release Experiment [9] require significant nitrate deposits. The
presence of sulphates, particularly in the absence of carbonates and
nitrates, would constitute support for the hypothesis [10] that
reactions between S-rich volcanic aerosols and precipitates may have
displaced C02 and NO(sub)x back into the atmosphere. Halides are not
predicted to form under any circumstances [11], so indications that they
exist would be an important constraint on martian geochemistry.

Available Measurements and Strategies. The APXS imager may be able to
identify depositional environments; if trenching can be done with the
tires, even to a very limited depth, additional information may be
gained. Small scale stratigraphy can be very revealing. During Marsokhod
rover tests in the Mohave in March 1994, the presence of well-rounded,
high-sphericity pebbles at 15 cm was conclusive evidence of flooding.
Resolution of 1 mm should reveal evidence of fluvial transport, if any,
in clastic sediments.

Measurements of atmophilic elements in sediments may shed light on the
climatic conditions that obtained during their deposition. Coherent
rocks of any probable evaporite could probably be identified with a
combination of APXS and IMP imagery. Conversely, if evaporites are
present only in the fines, unique identification is problematic,
although APXS data may detect constituent elements. Mass balance
calculations and geochemical considerations [12] may permit
identification of evaporites. Examination of any crusts should be a high
priority, using both APXS and IMP data.

An important test for contemporaneous heterogeneous chemistry can be
carried out with any rock larger than the saltation mean free path
length. Weathering reactions should produce a rind, which mantles
underlying material from subsequent alteration until accumulated unit-
cell mismatches and physical abrasion cause spallation. If heterogeneous
weathering is occurring, the windward and leeward sides of rocks may
exhibit compositional gradients in their surfaces, detectable by the
APXS. Comparison of the two sides of any large rock should be considered
a high priority.

Target Area: Four areas fit within the elevation and latitude
constraints: Chryse, Elysium, Amazonis, and Isidis. There is geomorphic
evidence that all have supported standing water. In some sense, it would
be difficult to pick a landing site which had no hope of teaching us
about the climatic history of Mars.

The southeast Elysium basin (3 degrees N; 184.5 degrees W) provides an
optimal target in which a variety of materials may be accessible in a
near-shore environment [13]. The albedo of the region is moderately low,
and the thermal inertia indicative of moderate rock coverage or some
consolidation of fines, arguing that the site has not been covered with
aeolian dust deposits.

The orientation of the landing ellipse is parallel to the inferred
shoreline, which is the circum-global highland-lowland scarp. The
probability of landing in a near-shore paleo-environment, in which small
but coherent fragments of highlands materials might be deposited, is
increased where the paleo-shore lies along the long axis of the landing
ellipse.

References: [1] Masursky H. et al. (1977) JGR, 82, 4016-4038. [2]
Goldspiel J. M. and Squyres S. W. (1991) Icarus, 89, 392-410. [3]
Pollack J. B. (1979) Icarus, 37, 479-553. [4] Mittlefehldt D. W. (1994)
Meteoritics, 29, 214-221. [5] Baker et al. (1991) Nature, 352, 589-594.
[6] Oyama V. and Berdahl B. (1979) JGR, 82, 4669-4676. [7] Zent A. P.
and McKay C. P. (1994) Icarus, 108, in press. [8] Pollack J. B. et al.
(1990) JGR, 95, 14595-14627. [9] Plumb R. C. et al. (1989) Nature, 338,
633-635. [10] Clark B. C. et al. (1979) J. Molec. Evol., 14, 91-102.
[11] Plumlee G. S. et al. (1993) LPI Tech. Rpt. 93-06, 41-42. [12]
Toulmin P. et al. (1977) IGR, 82, 4625-4634. [13] Scott D. H. and
Chapman M. G. (1991) Proc. LPSC 21, 669-677.






                             Monday, April 18, 1994
                        POSTER DISCUSSION AND RECEPTION
                                5:30 - 7:30 p.m.


Kuzmin R.*   Landheim R.   Greeley R.
   Potential Landing Sites for Mars Pathfinder

The last successful landing on Mars occurred in 1976 with the Viking
mission. In the ensuing years, much has been learned about Mars and the
characteristics of its surface. In addition to a better understanding of
the geological evolution of Mars, new techniques for processing
available data have emerged, new data have been acquired, and the
engineering approaches for placing spacecraft on the surface have
evolved. Selection of the Mars Pathfinder landing site must take these
issues into account, along with mission constraints. In addition,
consideration should be given to complementary sites chosen for the
Russian Mars-94/96 lander. The Mars-94 mission will establish a network
of two small stations and two penetrators (Table 1) in Arcadia Planitia.
Sedimentary and volcanic deposits are characteristic of the northern and
southern regions, respectively.

An advantage of Mars Pathfinder is the rover for sampling surface
materials over a range of tens of meters. However, engineering
constraints and the limited scientific payload of this mission require
new approaches for landing site selection (see Greeley and Kuzmin, this
issue). One approach is to select sites exhibiting a wide variety of
rocks near the lander (e.g., Arago crater, Site 12). An alternative
approach is to select sites in which the regional geology consists of a
single rock type representing a key datum for the geological study of
Mars, and is uniformly distributed within the landing ellipse. Examples
of this approach include (1) landing sites on rocks of Hesperian age,
e.g., Ridged Plains (Site 5), (2) sites that contain sedimentary
deposits of Amazonian age with sharply distinct individual surface
morphology, e.g., deposits of the Medusae Fossae Formation (Sites 3 and
4), and (3) young volcanic deposits, e.g., Marti Vallis (Sites 6 and 7).

Based on these approaches and consideration of landing safety, 12 sites
were selected for Mars Pathfinder (Table 1). Of these landing sites, six
sites (Sites 1, 6, 7, 8, 9, and 10) are consistent with the nominal
mission requirements. Three additional sites (Sites 4, 5, and 12) can be
considered if elevation constraints are increased to 2 km. Three other
sites (Sites 2, 3, and 11) are located between 0 and 1 km. Six of the
sites (Sites 2, 3, 4, 6, 7, and 12) are included in the area occupied by
surface Unit 1 [2]. Another three sites (Sites 5, 8, and 11) are located
within Unit 3, and the remaining three sites (Sites 1, 9, and 10) are
located in the boundary zone between Units 2 and 3. From the 12 proposed
sites, nine sites (Sites 2, 3, 4, 5, 6, 7, 8, 11, and 12) have a rock
abundance of 3-8%. Three other sites (Sites 1, 9, and 10) have a rock
abundance of 8-15%. All selected sites are in regions with different
surface roughness characteristics (meters to tens of meters scale)
expressed as RMS slope values. From the 12 sites, only one site (Site 3)
is characterized by the highest RMS slope value (10 degrees-15 degrees),
but exhibits the lowest values of thermal inertia (<3 x 10^-3
cal/cm^2s^1/2K) and rock abundance (<6%). The remaining eleven sites
have RMS values <8 degrees.

Under nominal elevation constraints, especially with regard to Mars
Pathfinder, we propose the Ares-Tiu Valles and Maja Valles delta areas
(Sites 1 and 8), and Marti Vallis (Sites 6 and 7) as high priority
targets. If the maximum elevation constraints are increased to 2 km, the
more favorable sites are the Ares-Tiu Vallis delta area (Site 1), Kasei
Vallis bend area (Site 2), Medusae Fossae (Sites 3 and 4), and Lunae
Planum (Site 5).

References: [1] Kuzmin R. D. (1993) NAGW-2102, Subgrant No. 93-102SG.
[2] Christensen P. R. and Moore H. J. (1992) in Mars (H. Kieffer et al.
eds.), 686-729.


Christensen P. R.   Edgett K. S.
   Physical Properties (Particle Size, Rock Abundance) from Thermal Infrared
   Remote Observations: Implications for Mars Landing Sites

Critical to the assessment of potential sites for the 1997 Pathfinder
landing is estimation of general physical properties of the martian
surface. Surface properties have been studied using a variety of
spacecraft and Earth-based remote sensing observations [1,2], plus in
situ studies at the Viking lander sites [2,3]. Because of their value in
identifying landing hazards and defining scientific objectives, we focus
this discussion on thermal inertia and rock abundance derived from
middle infrared (6-30 microns) observations. Used in conjunction with
other datasets, particularly albedo and Viking orbiter images, thermal
inertia and rock abundance provide clues about the properties of
potential Mars landing sites.

Here we discuss the combined albedo [4], thermal inertia [2,5], and rock
abundance [6], results (derived from Viking Infrared Thermal Mapper
(IRTM) data collected 1976-1980) for regions that fit the Pathfinder
landing constraints: areas below ~0 km elevation between 0 degrees and
30 degrees N latitude. Lately there has been considerable discussion of
the uncertainty in thermal inertia derived under a relatively dusty
martian atmosphere [7-11]. In particular, Hayashi et al. [8] suggest
that the thermal inertias, which we describe below are 50-100 (units of
J m^-2 s^-0.5 K^-1, hereafter referred to as "units") too high for
regions with moderate and high inertias (> 300 units) and 0-50 units
high for regions of low inertia (< 300 units). However, our
interpretation of physical properties is general and accounts for
uncertainty due to modeling of suspended dust.

Thermal inertia is related to average particle size of an assumed
smooth, homogeneous surface to depths of 2-10 cm [12]. Rock abundance is
derived from multi-wavelength observations to resolve surface materials
into fine (sub cm-scale) and rocky (approx. 10 cm) components, based on
the fact that temperature of rocks and fines can differ by up to 60 K at
night [6]. Low rock abundances generally indicate areas with dust or
sand deposits, while areas of high rock abundance are commonly outflow
channel deposits and/or regions deflated by wind [2,5,6].

Christensen and Moore (Fig. 11, [2]) identified four physical units that
describe the general variation in surface properties on Mars. The data
products used in this analysis include a 0.5 degrees/bin resolution
thermal inertia map [5], a 1 degree/bin resolution Viking-era albedo map
[4], and the 1 degree/bin rock abundance map [6].

Unit 1 is defined by low thermal inertia (40-150 units), high albedo
(0.26-0.40), and low rock abundance (< 5%). Unit 1 surfaces are
interpreted as being mantled by dust up to 1 m thick. Most of these
surfaces are in the high-elevation Tharsis, Arabia, and Elysium regions;
however two regions lower than 0 km elevation between 0 degrees and 30
degrees N have similar deposits: Amazonis Planitia and Elysium Basin
(150 degrees W-210 degrees W).

Unit 2 is characterized by high thermal inertia (300-850 units), low
albedo (0.1-0.2), with rock abundances high but variable. Southern
Acidalia and Oxia Palus (0 degrees-60 degrees W) fit this description,
and are considered to be regions of active sand transport and rocky lag
deposits. Other Unit 2 surfaces include Syrtis Major (elevation > 0 km)
and Cerberus (elevation < 0 km), which have lower rock abundances (< 7%)
and are probably more sandy and less rocky than Acidalia.

Unit 3 surfaces have moderate thermal inertias (150-350 units), average
albedos (0.15-0.25), and moderate to low rock abundances. Parts of
Western Arabia near Oxia Palus and parts of Xanthe Terra and Lunae
Planum fit this description. These have been interpreted as possible
surface exposures of indurated dust/soil deposits similar to the crusted
materials seen a few cm beneath the surface at the Viking lander sites.

Unit 4 has moderate-to-high inertias (210-380 units), a relatively high
albedo (0.25 to 0.30), and a high rock abundance (> 7%). This unit
includes the two Viking lander sites [13]. The Viking sites have
elements of all the above Mars surface deposit types (dust, rocks,
crust) except the low albedo, sandy material of Unit 2 [2]. Much of
Chryse Planitia and parts of Isidis Planitia and Elysium Planitia (210
degrees W-250 degrees W) can be described as possible Unit 4 surfaces.

Finally, there is some interest in landing sites in or at the mouths of
outflow channels. Henry and Zimbelman [14] and Betts and Murray [15]
have provided IRTM and Phobos 2 Termoskan evidence (respectively) that
channel floors tend to have enhanced thermal inertias probably related,
in part, to the presence of blocky material on the channel floors. Henry
and Zimbelman saw a general "downstream" decrease in thermal inertia in
Ares Vallis, consistent with a decrease in clast size down the channel.
Surfaces at the mouths of major outflow channels, however, have enhanced
rock abundances [6].

References: [1] Kieffer H. H. et al. (1992) Mars, Univ. Ariz. (L. J.
Martin et al. eds.), 34-70; (R. A. Simpson et al. eds.), 652-685; (L. A.
Soderblom ed.), 557-593. [2] Christensen P. R. and Moore H. J. (1992) in
Mars, Univ. Ariz. Press., 686-729. [3] Moore H. J. et al. (1987) USGS
Prof. Paper, 1389. [4] Pleskot L. K. and Miner E. D. (1981) Icarus, 45,
447-467. [5] Christensen P. R. and Malin M. C. (1993) LPSC XXIV, 285-
286. [6] Christensen P. R. (1986) Icarus, 68, 217-238. [7] Haberle R. M.
and Jakosky B. M. (1991) Icarus, 90, 187-204. [8] Hayashi J. N. et al.
(1994) submitted to JGR. [9] Bridges N. T. (1994) GRL, in press. [10]
Edgett K. S. and Christensen P. R. (1994) JGR, 99, 1997-2018. [11] Paige
D. A. et al. (1994) JGR, in press. [12] Kieffer H. H. et al. (1973) JGR,
78, 4291-4312. [13] Jakosky B. M. and Christensen (1986) Icarus, 66,
125-133. [14] Henry L. Y. and Zimbelman J. R. (1988) LPSC XIX, 479-480.
[15] Betts B. H. and Murray B. C. (1994) JGR, 99, 1983-1996.





                            Tuesday, April 19, 1994
    WHAT LONGITUDE ARE WE GOING TO? (i.e., WHERE DO WE TARGET THE ROCKET?)
                    WHAT WILL WE LEARN AFTER WE GET THERE?
                      25 DEGREES W-55 DEGREES W LONGITUDE
                               8:30 - 10:10 a.m.


Craddock R. A.*
   Rationale for a Mars Pathfinder Mission to Chryse Planitia (20 Degrees N, 40
   Degrees W) and the Viking 1 Lander

Presently the landing site for Mars Pathfinder will be constrained to
latitudes between 0 degrees and 30 degrees N so that the lander and
rover solar arrays can generate the maximum possible power and to
facilitate communication with Earth. The reference elevation of the site
must also be below 0 km so that the descent parachute, a Viking
derivative, has sufficient time to open and slow the lander to the
correct terminal velocity. Although Mars has as much land surface area
as the continental crust of the Earth, such engineering constraints
immediately limit the number of possible landing sites to only three
broad regions: Amazonis, Chryse, and Isidis Planitiae. Of these, both
Chryse and Isidis Planitiae stand out as the sites offering the most
information to address several broad scientific topics.

An immediate reaction to proposing Chryse Planitia as a potential
landing site is, "Why go back to an area previously explored by the
Viking 1 lander?" However, this question answers itself. Viking 1 landed
successfully, proving that it is safe and providing us with valuable
ground-truth observations of the martian surface. For example, Viking
Lander 1 data have provided information useful in determining the
physical properties of the martian surface materials [1]. Observations
such as these have undoubtingly been incorporated into the Mars
Pathfinder spacecraft and rover design, making them well equipped to
successfully operate in the Chryse Planitia environment. We simply don't
know with any level of certainty what the hazards may be in the other
areas. The extensive photographic coverage of Chryse Planitia by the
Viking Orbiters and Earth-based radar observations has provided 100 m
resolution topography in the vicinity of the Viking 1 lander [2].
Analysis of these data and lander photographs indicate that Chryse
Planitia may be unique in that features >50 km away from the lander
(such as the rims of Lexington and Yorktown craters) are visible over
the horizon [3]. This type of information could potentially aid in
roving vehicle navigation. However, the most important use of the Viking
orbiter data will be in simply determining the location of the Mars
Pathfinder spacecraft on the surface. These same data were useful in
determining the location of the Viking 1 lander to within ~50 m [4].

Ideally a landing site should include access to as many different
geologic units as possible. In addition to the materials debouched into
the Chryse basin by the large martian channel complex [5], the Hesperian
age ridged plains covering much of region [6] represent the single most
important geologic unit needed for age-dating materials on Mars.
Composing ~3% of the total Mars surface area [7], the ridged plains are
fairly widespread in comparison to other geologic units and, more
importantly, are the Hesperian epoch referent [8]. Because the Hesperian
epoch represents the interval of time immediately following the period
of heavy bombardment (~3.8 Ga; [9]), an absolute age determined from a
ridged plain sample would allow estimates of the post-heavy bombardment
impact flux on Mars to be calibrated. It may then be possible to
determine the absolute ages of every younger geologic unit on Mars based
on crater statistics. Potential Hesperian ridged plains outcrops
identified in Viking 1 lander images may represent the only known
bedrock exposures on Mars. Mars Pathfinder rover analyses of these
materials could provide data to support the hypothesis that these are
bedrock materials, which could be crucial for future sample return
missions. In addition, materials washed down from the highlands may be
present in Chryse Planitia. Although the absolute ages of these
materials almost certainly correspond to the period of heavy
bombardment, analysis of their composition could provide some insight
into the early geologic history of Mars. Also, the distribution of the
materials in Chryse Planitia as determined by a long rover traverse may
be indicative of the channel formation mechanism. For example,
catastrophic flooding would lead to a Bouma sequence deposit in the
Chryse basin [10]; in liquefaction, an accretionary lobe in the
debouching area results in larger particles dropping out first with
smaller particles being transported greater distances [11]. The Mars
Pathfinder Meteorology Package (MET) would almost certainly augment the
data obtained from the Viking Meteorology Experiment. The Viking
Meteorology Experiment was capable of providing information at only one
height, which is insufficient for determining the boundary layer profile
in Chryse Planitia. However, because the MET will provide information
from multiple heights, profiles from the Viking data may be derived.

Because of the likelihood of running water debouching into Chryse
Planitia in the past, the Viking 1 landing site was considered an ideal
place to look for complex organic molecules [12]. Although the Viking
biological experiments did not identify the presence of organic life
[13], controversy still exists as to the meaning of the Labeled-Release
Experiment [14]. A landing in Chryse Planitia would make it possible to
investigate the composition of the same soil samples investigated by the
Viking 1 lander. Rocks seen in lander images could also be analyzed,
answering questions concerning their compositional and erosional
properties. Depending on exact Mars Pathfinder landing site and the
accuracy of rover navigation, it may be possible to examine the Viking 1
lander 1 itself! In situ erosional analysis of Lander 1 could allow the
current martian weathering rate and aeolian deposition to be determined.
Such information could also serve as a valuable aid in developing future
martian spacecraft materials. Alternatively, it may be possible to
navigate from the lander to the crater caused by the jettisoned Viking
aeroshell. Ejecta from this fresh crater would represent Chryse
stratigraphy to a depth of ~1 m, providing additional information on the
nature of the surface materials observed by the Viking 1 lander.
Although crater ejecta is frequently suggested as material that should
be sampled by a spacecraft during a traverse, such stops are rarely
justified. Crater ejecta, especially the outer ejecta blanket, typically
has the same composition as the surrounding rock. It is the traverse up
to the crater rim crest where material at depth is gradually exposed;
the deepest material being exposed directly at the rim crest, typically
from a depth equivalent to 1/10th the crater diameter. However, a simple
examination of crater ejecta from a larger diameter crater could
potentially provide some valuable information. "Rampart" [15] or
"fluidized ejecta" craters [16] have been suggested as forming from the
incorporation of volatile material from depth [15] or from the
interaction of the ejecta curtain with a thin atmosphere during
emplacement [17]. The derived volatile content and/or sediment
distribution from a rampart crater (e.g., Yorktown, a 7.9-km-diameter,
~45 km northwest of the Viking 1 landing site) could provide clues as to
which formation mechanism is the most viable.

References: [1] Moore H. J. et al. (1987) USGS. Prof. Paper, 1389, 222.
[2] USGS. Misc. Invest. Series Map, I-1059 (1977) USGS, Denver. [3]
Craddock R. A. and Zimbelman J. R. (1989) LPSC XX, 193-194. [4] Morris
E. C. and Jones K. L. (1980) Icarus, 44, 217-222. [5] Craddock R. A. et
al. (1993) LPSC XXIV, 335-336. [6] Scott D. H. and Tanaka K. L. (1986)
USGS Misc. Invest. Series Map I-1802A, USGS, Denver. [7] Watters T. R.
(1988) MEVTV-LPI, 63-65. [8] Tanaka K. L.(1986) JGR, 91, E139-E158. [9]
Hartmann W. K. et al. (1981) In Basaltic Volcanism on the Terrestrial
Planets, Pergamon, New York. [10] Komar P. D. (1980) Icarus, 42, 317-
329. [11] Nummedal D. and D. B. Prior (1981) Icarus, 45, 77-86. [12]
Masursky H. and Crabill N. L. (1976) Science, 193, 809-812. [13] Klein
H. P. (1977) JGR, 82, 4677-4680. [14] Levin G. V. and Straat P. A.
(1977) JGR, 82, 4663-4667. [15] Carr M. H. et al. (1977) JGR, 82, 4055-
4066. [16] Mouginis-Mark P. J. (1979) JGR, 84, 8011-8022. [17] Schultz
P. H. and Gault D. E. (1981) Third International Colloquium on Mars,
226-228.




Crumpler L. S.*
   Chryse Planitia (25 Degrees N, 45 Degrees W) as a Mars Pathfinder Landing
   Site:  The Imperative of Building on Previous Groundtruth

Introduction. Based on consideration of the geological characteristics
of Chryse Planitia, the requirements for Mars Pathfinder landing sites,
the nature of the mission, the scale of the observations to be made, and
the need to build outward from previous experience, a new mission to
Chryse Planitia offers several advantages that are difficult to ignore
and offers a low gamble-high return mission scenario. Considering the
need to assure a successful mission, and to assure the continued health
of planetary exploration, the reasons for a new mission to Chryse
Planitia are compelling.

Results of 1:500,000 Mapping. Based on recent geologic mapping [1,2],
Chryse Planitia is the result of the following generalized sequence of
erosional and depositional events: (1) an early impact basin [3]
centered at 32.5 degrees N/35.5 degrees W [4]; (2) basin resurfaced
during emplacement of an unspecified material, possibly lavas; (2)
geologic unit on which the Viking Lander is situated was buried to a
depth of several hundred meters by an early mare-type ridged plains
surface and superposed impact craters; (3) sediments shed from the
highlands formed a uniform unit over much of the southern and
southeastem basin; (4) subsequent impact cratering and regional mare-
type ridge formation; (5) catastrophic outflows of water from Maja
Valles to the west and Kasei Valles to the northwest scoured and incised
all pre-existing units and resulted in additional deposition in the
basin interior; and (6) the Viking Landing site was resurfaced at least
in part by the deposition of sediments carned by Maja Valles and Kasei
Valles.

Operational Benefits of Chryse Planitia. All potential landing sites
within Chryse Planitia satisfy the primary operational requirements for
the Mars Pathfinder mission: (1) low Latitude = 22 degrres N +- and 46.5
degrees W +- 5 degrees; (2) altitude below 0 km = -1 to -3 km below
datum; (3)1anding ellipse free of large scale hazards = Chryse Planitia
has some of the largest expanses of low hazard terrain on Mars. A
critical advantage is that the engineering requirements for landing in
Chryse are well-known at lander scales. The block size distribution is
known and predictable: blocks >= 1m account for <=4% of the surface
area. Known meteorological conditions during pre-dawn landing of Mars
pathfinder indicate very low winds at the surface and aloft. And high
resolution regional images exist for local landing site selection. Based
on my experience on the Viking Lander 2 site selection effort, landing
ellipse are difficult to situate at large scales and these additional
considerations are likely to be overriding during f1nal landing
decisions.

Science Benefits of Chryse Planitia. Different scale features require
different path lengths in order to accumulate adequate "truth" (N) about
that feature; the cumulative rate is an inverse exponential (to some
constant, n) of traverse length (R) such that ( N~ ((R)exp(1/n))/C). The
proposed Mars Pathfinder rover capabilities are excellent for addressing
many of the small-scale mineralogical and lithological questions raised
by initial Viking Lander study, but regional lithologic questions cannot
be addressed with the same rover capabilities. The probable diverse
lithologies of blocks at small scale in the Chryse basin deposited from
outflow events in Maja and Kasei take advantage of the ability of Mars
Pathfinder rover to make small scale mineralogical investigations, while
increasing the information content of small scale investigations during
short traverse lengths.

Knowing the detailed questions that will be asked greatly increase the
ability to design a successful experimental test: Viking Lander 1 raised
detailed questions that Mars Pathfinder may be designed and engineered
specifically to address. For example, what are the dense, coarse, and
pitted lithologies?; What is the origin of the block distribution?
Impact? Outwash? In situ weathering? What is the grain size of the
surface fines-sand or silt? What is the microstratigraphy of the fines?
Is there a stable substrate in Chryse at lander scales of observation?
Other regional geological issues remain--what is the geological unit of
central Chryse, volcanic or sedimentary? Is there micro-scale evidence
for standing water or water-derived precipitates? If these questions
cannot be answered in Chryse with carefully designed experiments and
some prior knowledge, then they will be more difficult elsewhere where
the questions are unknown.

Conclusions and Site Recommendations. It is tempting to set the sights
for future prospects on many of the additional interesting areas defined
on Mars since the Viking mission and expand the database for ground
truth to new and different terrains. This desire should be balanced with
the critical need for success in planetary exploration in general and
avoidance of an inconclusive mission result in particular. An important
goal should be learning how to operate on Mars and addressing answerable
questions. A firm start on the latter can be attained by a new mission
to Chryse Planitia in the region specified, and in which the existing
ground truth of the Viking Lander 1 is used to constrain the engineering
choices and to design appropriate instrument goals to fully utilize the
lander limitations and capabilities. Three proposed sites are indicated
on Fig. 1 along with the currently defined landing ellipses. Site E1 is
most likely to be similar to VL-1; site E2 is less well-constrained and
is likely to differ from VL-1 in some respects; and site E3 is likely to
be similar to VL-1, but incorporates slightly less hazardous, but
slightly different terrain.

References: [1] Crumpler L. S. et al. (1994) USGS, 1:1M scale map. [2]
Craddock, R. A. et al. (1994) JGR, submitted. [3] Schultz R. A. and Frey
H. V. (1990) JGR, 95, 14175. [4]Stockman S and Frey J. (1993) EOS, 74,
199.

Fig. 1, which appears here in the hard copy, shows a geologic map of the
MTM 25047 and 20047 photomosaic sheets in the Viking Lander 1 region of
Chryse Planitia. Map units: Hr2=Upper ridged plains; Hr1= Lower ridged
plains; Hsp = Smooth plains; Hcrp = Channel and ridge plateau; Hchp=
Channel plains; Hip = Incised plains. See Crumpler et al. [1] for more
detailed description and interpretation of units. Numbered landing
ellipses are 100 km x 200 km and indicate recommended sites: E1 =
safest; E2 = less safe; E3 =intermediate level of saftey based on degree
of similarity to VL-1 site.


Rice J. W.*
   Maja Valles and the Chryse Outflow Complex Sites (19 Degrees N, 53 Degrees
   W; 15 Degrees N, 35 Degrees W)

Maja Valles Region. This candidate landing site is located at 19 degrees
N; 53.5 degrees W near the mouth of a major outflow channel, Maja
Valles, and two "valley network" channel systems, Maumee, and Vedra
Valles. This region has been mapped in detail by Rice and De Hon and is
in press as a USGS 1:500,000 scale geologic map. The advantages to this
site are the following: Two distinct channel forms (outflow and
dendritic valley network) in one location. These channels were formed by
different processes. The outflow channels are believed to have formed by
catastrophic release of water and the valley networks by surface runoff
and or sapping. The ideal landing site, if it could be pinpointed, would
be on the fan delta complex located at the terminus of the three
channels (Maja, Maumee, and Vedra Valles). The fan delta complex would
be a fairly smooth surface with shallow slopes.

Water was impounded behind the wrinkle ridge system, Xanthe Scopulus,
forming a temporal lake. This paleolake bed would also present itself as
a safe landing site, perhaps similar to playas. Once the wrinkle ridges
were breached the water flowed north-eastward in the direction of the
Viking 1 lander, some 350 km away.

Objectives to be analyzed in this region are (1) origin and
paleohydrology of outflow and valley network channels, fan delta complex
composition (this deposit located in this area is one of the few
deposits identified at the mouths of any channels on the planet), (2)
analysis of any paleolake sediments (carbonates, evaporites). Another
advantage to this area will be any blocks and boulders that were plucked
out and carried along the 1600 km course of Maja Valles. These samples
would provide a virtual grab bag of lithologies. For example, the oldest
mappable rock unit (Nb, Noachian basement material) and the Hesperian
ridged plains (Hr) are cut by Maja Valles before it empties into Chryse.
It can be argued that we will not know their exact location, which is
true, but it will provide us with information about the variety of rock
types on Mars by only landing in one site. Other questions to be
investigated in the area are the origin of wrinkle ridges by viewing
ridge walls that were incised by the outflow, streamlined islands/bars;
are they erosional or depositional, and if the location permits view
channel wall stratigraphy, fan delta stratigraphy, and perhaps send the
rover up a channel mouth near the end of its mission.

This site is below the 0-km elevation datum, within the latitude
restrictions (19 degrees North), and all of the objectives stated above
are within the 150 km landing error ellipse. This region is also imaged
at resolutions of 40 to 50 m/p.

The Chrsye Outflow Complex Region (Ares, Tiu, Mawrth, Simud, and
Shalbatana Valles). The overall philosphy and objectives described above
for the Maja Valles Region apply here as well. The primary objectives
here would be outflow channel dynamics (paleohydrology) of five
different channel systems. One question to be answered might be are all
outflow channels of the same origin and type. They probaly are all
somewhat different in terms of duration, age, source, and perhaps even
origin. The grab bag philosophy of various rock types being deposited
near channel mouths would apply here also. The site is located at 15
degrees N; 35 degrees W. However, the longitudinal coordinate can be
relaxed or slid farther to either side of 35 degrees W. Sliding the
ellipse farther to the east would allow investigations of Mawrth Valles.
The region near the mouth of Mawrth Valles would be of interest because
this area contains material that appears to have been dissected thus
exposing the stratigraphy of what may possibly be deltaic sediments.

This site is located at 16 degrees N; 177 degrees W on the floodplains
of Marte Valles, which is perhaps the youngest channel system on Mars.
However, the coordinates for the landing site are flexible. Moving the
site more to the southwest would allow investigations of possible
lacustrine sediments. The channel extends for about 3000 km from
southeastern Elysium Planitia into western Amazonis Planitia. This
system appears to originate within the knobby cratered material around
Cerberus Rupes, a set of en echelon fractures that extend for more than
1000 km. Crater counts indicate that this system is Amazonian in age.
This channel may have also acted as a spillway between paleolakes
located in Elysium and Amazonis Planitia. The young age of this channel
warrants investigation because of climatic implications for fluvial
activity in recent geologic time. The paucity of craters makes this an
excellent site in terms of safety requirements. Detailed work by Tanaka
and Scott indicate that embayed craters larger than 1 km diameter appear
embayed by the channeled plains unit, suggesting that it is only tens of
meters thick. This material contributed to the resurfacing of the
northern lowlands of the planet. Some of the objectives stated
previously for the Maja Valles Region would also apply to this site
(grab bag of rock types etc.). This site is below the 0 km datum,
located at 16 degrees N, and has the young channeled plains, bars,
terraces, and streamlined albedo patterns located within the 150 km
landing error ellipse. Resolution coverage in some areas is as high as
13m/p.



De Hon R. A.*
   A Highland Sample Strategy for Pathfinder (29 Degrees N, 20 Degrees W)

Mission Constraints: Potential landing sites are confined to latitudes
between 0 degrees and 30 degrees N and surfaces below 0 km elevation.
The landing ellipse is 100 by 200 km oriented N74 degrees E. The
constraints essentially eliminate the slopes of Elysium Mons, Olympus
Mons, Tharsis Ridge, Lunae Plaunum, all of the southern highlands, and
almost all of the Noachian material of Arabia Terra. Those areas that
remain as potential landing sites are chiefly lowland plains of Amazonis
Chryse, Isidis, amd Elysium Planitiae.

Siting Strategy: With only two previous Viking landing sites in widely
separate locations, almost any landing will provide new data. Viking 1
landed on Chryse Planitia on a surface that is presumed to be Hesperian
ridged plains material, which is interpreted to consist of volcanic
flows [1]. Viking 2 landed on knobby material in Utopia Planitia, which
is interpreted as aeolian and volcanic materials [2]. The large landing
ellipse precludes a finely targeted sampling strategy. Several site
selection strategies are equally valid. Presumably, Noachian materials
are more representative of the geochemical character of the planet than
the volumetrically less important surfical flows and sedimentary
materials.

Any attempt to sample highland material further constrains the possible
landing sites by eliminating areas of Hesperian or Amazonian lavas and
sediments. Materials of possible Noachian age are primarily located
above 0 km elevation and at southern latitudes except for minor
occurrences in Arabia Terra and a narrow zone along the southern edge of
Elysium Planitia.

One possible sampling strategy is to sample materials within those few
"highland" terrains that extend to low elevations. Minor occurrences of
Noachian materials are exposed at low elevations as outliers flanked by
younger material within Nepenthes Mensas and Aeolis Mensas, but such
areas are rugged and unsuitable for a safe landing. However, parts of
western Arabia Terra extend to acceptable elevations and are reasonably
smooth.

A second strategy is to sample materials at the mouth of an outflow
channel that drains from the highlands. Channels may terminate in
deltas, alluvial fans, or sheet deposits. For simplicity these deposits
will be refered to as "fans." Fans provide materials from a large
sampling area, and dissected fans offer the advantage of providing a
vertical section of exposed stratification that records the history of
the outflow.

On first appraisal, any fan at the mouth of a channel draining from
highlands offers a potential target, but not all fans offer equal
opportunities. Many outflow channels have ponded along their length;
hence, any sediment carried by the discharge at the mouth is derived
only from the last site of ponding [3]. Further, although catastrophic
outflows are characterized by high sediment load and large caliper, the
last sediments deposited during waning discharge are generally fine-
grained.

Potential Landing Sites: Potential landing sites include outflow channel
material at the edge of Chryse Planitia and highland materials bordering
southern Amazonis Planitia. The Circum-Chryse channels include Kasei
Valles into northwest Chryse Planitia; Bahram Vallis, Vedra Valles,
Maumee Valles, and Maja Valles into western Chryse; Shalbatana Vallis
into southwest Chryse; channels draining from Capri and Eos Chasmata
into southern Chryse (Simud. Tiu, and Ares Valles); and Mawrth Vallis
into northeast Chryse Planitia. With so many large outflow systems
terminating within Chryse basin, it is probable that any landing site
within the basin (including Viking 1 landing site) will be blanketed by
sediments from catastrophic outflows or outflow sediments reworked by
aeolian activity. Channels in southern Chryse Planitia, originating
within Vallis Marineris or chaotic terrain south of Chryse Planitia,
traverse Noachian material before entering the basin, but they do not
provide readily identifiable fans.

Best Bets: Mawrth Vallis of the Oxia Palus region cuts Noachian cratered
plateau material, which is interpreted to be largely impact breccia of
ancient crust [4]. The plateau surface bordering the lower reaches of
the channel, below 0 and -1 km elevation, is one of the few places on
Mars where typical highland material can be found below 0 km elevation.
Three landing sites are feasible. One potential site is at the mouth of
the channel (29 degrees N; 21 degrees W); an alternate site is on the
plateau surface adjacent to the valley (28 degrees N; 18 degrees W); and
a third site is south of Mawrth Vallis and east of Ares Vallis (2
degrees N; 2 degrees W). The highland site adjacent to Mawrth Vallis is
more likely to contain less surfical cover than the site east of Ares
Vallis. If not covered by surfical material, highland sites are likely
to consist of highly commuted materials; they would provide an estimate
of the geochemical character of the homogenized early crust.

The mouth of Maja Canyon (18 degrees N; 50 degrees W), with remnant fan
material cut by late stage discharges [5,6], offers the best channel
mouth target. The chief constituents here are likely to be detritus from
Noachian material of the Xanthe Terra region carried by outflow that
spilled onto Chryse Planitia following ponding behind a barrier massif
of Noachian basement material.

Pitfalls and Predictions: The large landing ellipse and low resolution
of Viking images do not allow assurance that the landing site will
contain any particular anticipated material. Extremely localized
deposits of young materials are possible in the highlands, and mature,
winnowed sediments are possible in the plains. Interpretation of
chemical anlyses of fan materials without corresponding petrologic
comparison will be challenging.

In all probability, the final choice of a landing site will be a level
lowland within the planatiae. A sedimentary surface of an essentially
mono-minerallic, aeolian (sand or loess-like) material or a lacustrine
deposit should not be a surprise.

References: [1] Scott D. H. and Tanaka K. L. (1986) USGS Misc. Inves.
Series Map I-1802-A. [2] Greeley R. and Guest J. (1987) USGS Misc.
Inves. Series Map I-1802-B. [3] De Hon R. A. and Pani E. A. (1993) JGR,
98, 9129-9138. [4] Wilhelms D. E. (1976) USGS Misc. Inves. Series Map I-
895. [5] Rice J. W. (1994) This volume. [6] Rice J. W. and De Hon R. A.
(1994) USGS Misc. Inves. Series Map I-2432.
PATHFINDER HIGHLAND STRATEGY:  R.A. De
Hon




Treiman A.*   Murchie S.
   Melas Chasma: A Mars Pathfinder View of Valles Marineris (10 Degrees S, 73
   Degrees W)

A Mars Pathfinder landing site in Melas Chasma (Valles Marineris) would
yield significant science return, but is outside present mission
constraints. In Melas Chasma, Mars Pathfinder could investigate
minimally altered basaltic material, sedimentary deposits, chemical
weathering, tectonic features, the highlands crust, equatorial weather,
and Valles mists. Critical issues include (1) nature and origin of the
Valles interior layered deposits, important for understanding water as a
sedimentary and chernical agent, and for the past existence of
environments favorable for life; (2) compositions of little-altered
basaltic sands, important for understanding magma genesis and weathering
on Mars, and the martian meteorites; and (3) structure and composition
of the highland crust, important for understanding Mars early history.
Data from Melas Chasma would provide ground truth calibration of remote-
sensing data sets, including Phobos ISM.

Mission Constraints. I: In the first workshop circular, the landing site
was to be "roughly between the equator and 30 degrees N" with a
"landing...uncertainty of roughly 150 km." In the final circular, the
landing site is restricted to "0 degrees N and 30 degrees N ... within a
100-km x 200-km ellipse along a N74E axis around the targeted site...."
No hazard-free nominal site in Melas Chasma satisfies these later
criteria. However, a hazard-free restricted sites to 85 x 170 km at the
same orientation can be accommodated (Fig. 1).

The Site. The proposed landing site is at 9.75S 72.75W in Melas Chasma,
the widest portion of Valles Marineris [1-3]. The restricted site (Fig.
1) is a flat, smooth surface, -2 to +1/2 km in elevation, mapped as
Younger Massive Material [1,2]. This surface is probably composed of
basaltic sand, very slightly hydrated and oxidized. A thermal inertia of
8-10 x 10^-3 cal cm^-2sec^-l/2K^-1 [4] and block abundances of 5-10% [5]
suggest sand with scattered blocks (fewer than at VL1) and little dust.
A surface of slightly altered basalt or basaltic glass [6] is suggested
by its dark (albedo 0.18-0.2) and slightly reddish color [4,7]; its
abundance of high-Ca pyroxene [8]; and the presence of H(sub)20 but
little structural OH [7].

North and east in the restricted site are Rough Floor Material and
Landslide Material [1], the smooth distal tongues of landslides. In the
nominal site, but not the restricted site, are mesas of layered
material, probably volcanic or lacustrine sediments [1,9]. Mesa
elevations are to 2.5 km, and some are bounded by cliffs.

The Surface. Imagery of the landing surface will help elucidate recent
surface-atmosphere interactions (wind) and past geological processes in
Valles Marineris (sedimentary or volcanic deposition, erosion by wind
and water). Characterization of the landing surface will provide ground-
truth calibration for remotely sensed data from Viking color and IRTM,
Phobos 2 ISM, and Earth-based spectroscopy and radar.

Sand, rocks, and dust should be accessible. The sand has little adhering
dust [6], so IMP and APXS analyses of sand will include little dust
component. Data on the sand, if basaltic, will help explain martian
magma genesis and volcanic processes, provide tests of the origins of
martian meteorites (via element abundance ratios [10]); and provide
clues to aqueous alteration processes (especially from IMP spectra).
Rocks on the landing surface probably represent local types, including
basalt, sediment (layered material), and highlands material from Chasma
walls. Chemical and spectral data on rocks will be important in
elucidating the geologic history of the Valles Marineris area, and will
be relevant to all sedimentary, highlands, and volcanic terrains on
Mars. There will likely be local concentrations of dust for analysis.

The Scene. The IMP will have spectacular views of Chasma walls to the N
(~5.5 degrees vert. angle, 101 IMP pixels, 60 m/pixel) and of mesas of
layered material to the SW (~1.5 degrees vert. angle). Spectra from IMP
will help reveal the mineralogies and compositions of highlands crust
(in Chasma walls); Lunae/Syria Planum resurfacing units; layering at
tops of the Chasma wall; and the sedimentary Layered Material. IMP and
synthetic stereo imagery will help clarify structures, material
properties, and slope processes of the Chasma walls; tectonic structures
in and around Melas; and stratigraphic, depositional, and exobiological
implications of layered Valles fill.

The Atmosphere. Meteorological data from Melas Chasma would be the first
from an equatorial site, but local effects could be significant. Valles
mists could be studied directly, and the Chasma wall and mesas could
provide some calibration for airmass optical depths as a function of
elevation, at least to the wall heights.

Mission Constraints II. To investigate Melas Chasma requires landing at
10 degrees S, entailing decreases of: @10-15% photovoltaic power (vs. 15
degrees N), and ~1 hr/day line-of-sight with Earth (vs. 0 degrees N)
[11]. To maintain safety, a landing ellipse with an aspect ratio of 2:1
and elongation on N74E must be < ~170 x 85 km. Ellipses of 100 x 200 km
aligned between E-W and ~S30 degrees E can be accommodated in Melas
Chasma with no elevation above about 1 km.

References. [1] Witbeck et al. (1991) USGS I-2010. [2] Puelvast and
Masson (1993) Earth, Moon, Planets, 61, 219. [3] Lucchitta et al. (1994)
JGR, 99, 3787. [4] Palluconi and Kieffer (1981) Icarus, 45, 415. [5]
Christensen (1986) Icarus, 68, 217. [6] Murchie and Mustard (1994) LPS
XXV, 955. [7] Murchie et al. (1993) Icarus, 105, 454. [8] Erard et al.
(1991) Proc. LPSC 21, 437. Murchie et al. (1993) LPS XXIV, 1039. [9]
Nedell et al. (1987) Icarus, 70, 409. Komatsu et al. (1993) JGR, 98,
11105. [10] Treiman et al. (1986) GCA, 50, 1071. Lindstrom et al. (1994)
LPS XXV, 797. [11] From The Astronomical Almanac (1994) USGPO.

Fig. 1, which appears here in the hard copy, shows nominal and
restricted landing ellipses (100 x 200 km and 75 x 150 km) proposed for
Melas Chasma. Scene is 8 to 12S, 67.5-80W; ellipses centered near 9.75S
72.75W.






                            Tuesday, April 19, 1994
    WHAT LONGITUDE ARE WE GOING TO? (i.e., WHERE DO WE TARGET THE ROCKET?)
                    WHAT WILL WE LEARN AFTER WE GET THERE?
                     145 DEGREES W-200 DEGREES W LONGITUDE
                               10:30 - 12:10 p.m.


Parker T. J.*
   Scientific Rationale for Selecting Northern Eumenides Dorsum (9-11 Degrees N
   Latitude, 159-162 Longitude) as a Potential Mars Pathfinder Landing Site

The proposed site is the northernmost occurrence of the Medusae Fossae
Formation (MFF), and lies at or below the -2km contour. The MFF is the
famous radar "stealth" deposit that extends from south of Olympus Mons
westward across southern Amazonis Planitia to southern Elysium Planitia.
The MFF appears to be composed of some kind of wind-eroded friable
material, the origin of which is very problematic. It appears to be a
radar-absorbing material [1], whereas Mars south polar layered deposits
appear bright in the same scenes. Synthetic aperture radar images of
young terrestrial ash deposits in the Andes also appear relatively
bright. The MFFs radar signature appears to require a uniformly fine-
grained material (on the order of dust-size to fine sand-size) at least
several meters thick, in order not to transmit reflections off
underlying terrain or internal reflective horizons. A number of very
different hypotheses have been proposed over the years to explain this
formation. It has been interpreted as either a large, wind-blown
volcanic ash deposit [2] or other wind-blown material trapped along the
escarpment between highlands and lowlands [3,4]; ancient polar layered
deposits [5] requiring a massive change in the planets rotation axis;
and finally as carbonate platform deposits [6] or banks of low-density
volcanic material deposited in an ocean [7].

Accumulation of tens to thousands of meters of unwelded, friable ash
blankets, necessary to avoid formation of internal reflectors, would
seem to require a large number of discrete, relatively thin deposits.
The radar signature, therefore, seems inconsistent with the volcanic ash
and polar layered material interpretations. The "stealth" requirement
may be met by an uncemented sand or loess material, thus supporting the
suggestion eolian hypothesis. It might also be met by inferring
chemically precipitated, but largely uncemented carbonates. If correct,
this last model would have important implications with regard to search
strategies for fossil organic materials or the environments that might
be conducive to their development. In 1991, I suggested that the surface
morphology of the MFF is comparable to terrestrial carbonate platform
deposits [6]. The best modern analogs would be oolitic deposits, such as
found over large regions of the Bahama Banks. To fit the observed
morphologies and radar signature, a process akin to inorganic oolite
precipitation and transport by oceanic currents or agitation by waves
[8,9], with little cementation, was proposed. An oolitic grain size is
necessary both to provide the "stealth" radar signature and to allow the
development of sand wave-like bedforms visible in some high-resolution
images of the deposit. A largely uncemented state is also needed to
explain the radar signature, requiring relatively rapid deposition and
little to no subsequent cementation or diagenesis.

This requirement probably can be met because ocean transgressions on
Mars were likely short-lived and separated by long periods with
temperatures below freezing, thus preventing dissolution and cementation
through rain or groundwater migration within the deposit. Finally,
carbonate precipitation would have taken place fairly rapidly when
liquid water was present due to the planets high atmospheric CO2
content. Surface science that can address the chemical/mineralogical
composition and physical properties of this material would be very
important to understanding a relatively recent (Amazonian) volcanic or
paleoclimatic process that resulted in a 2.7 million km^2 deposit along
the martian equator. Based on assumptions about the average thickness of
this deposit, this area corresponds to a total volume of material on the
order of 27 thousand-2.7 million km^3. The proposed Pathfinder landing
site lies on a relatively smooth, "unmodified" portion of the MFF, more
than 100km away from its northern and western edges that exhibit
evidence of eolian etching in the form of closely-spaced yardangs. There
are no large craters or steep slopes within a few hundred kilometers of
the landing site.

References: [1] Forsythe R. D. and Zimbelman J. R. (1990) LPS XXI, 383-
384. [2] Scott D. H. and Tanaka K. L. (1982) JGR, 87, 1179-1190. [3] Lee
S. W. et al. (1982) JGR, 87, 10025-10041. [4] Thomas P. (1982) JGR, 87,
9999-10008. [5] Schultz P. H. and Lutz-Garihan A. B. (1988) Icarus, 73,
91-141. [6] Parker T. J. (1991) LPS XXII, 1029-1030. [7] Mouginis-Mark
(1993) LPS XXIV, 1021-1022. [8] Halley R. B. et al. (1983) Bank Margin
Environment, AAPG, 464-506. [9] Bathurst R. G. C. (1971) Carbonate
Sediments and Their Diagenesis, 7, Elsevier.


Barlow N. G.*
   Mars Pathfinder and the Exploration of Southern Amazonis Planitia (0 Degrees
   N, 162 Degrees W)

The southern region of Amazonis Planitia provides a variety of target
terrains for a roving vehicle such as the Mars Pathfinder Mission. A
landing site is proposed at 4 degrees N latitude 162 degrees W
longitude. This area has a reference altitude of between 0 and -1 km and
consists of relatively smooth Amazonian-aged deposits within the entire
100 x 200 km landing ellipse. The proposed landing site is within the
Upper Member Medusae Fossae Formation deposits (Amu) and near the
boundary with Middle Member Medusae Fossae Formation deposits (Amm) and
Member 1 Arcadia Formation plains (Aa(sub)1). Slightly further afield
are 107-km-diameter Nicholson crater, its ejecta deposits, and knobby
terrain of proposed Hesperian age (HNu) [1]. Depending on the exact
landing site of the spacecraft and the traverse distance of the rover,
these materials also may be sampled.

Regional Geologic Setting. The Medusae Fossae Formation consists of a
series of fine-grained, layered deposits of enigmatic origin generally
within the area 12 degrees N-11 degrres S and 127 degrees-190 degrres W.
The fine-grained nature of the material is revealed through low thermal
inertia values [2,3], little to no radar return [4], greater than
expected crater depth-diameter ratios for fresh impact craters [5], and
the presence of eolian erosional features such as yardangs [6]. The
origin of this material remains controversial-theories include
ignimbrite deposits from explosive volcanic eruptions [7], ancient polar
deposits, which have ended up in their present location as a result of
extensive polar wander [8], an exhumed chemical boundary layer caused by
a subregolith paleowater table [4], or simply thick deposits of eolian
emplaced debris [1]. Analysis of the chemical composition of the
material may help to resolve the origin of this mysterious and unique
martian terrain.

The proposed landing site lies within the Upper Member of the Medusae
Fossae Formation, a discontinuous region of deposits, which tend to be
smooth and flat to gently rolling. In some locations, this material has
been sculpted by eolian processes into ridges and grooves, which may
allow direct observation of different layers within the material. To the
west lies the Middle Member of the Medusae Fossae Formation, which is
similar to the Upper Member except for appearing rougher and more deeply
eroded. The rover likely will have difficulty traversing this terrain
and therefore sampling of only the outlying regions is desired for
comparison with the Upper Member.

To the northwest of the proposed landing site is the Member 1 Arcadia
Formation plains. These plains are characterized by smooth, flat
topography occasionally interrupted by knobs and hills of presumed
Hesperian or Noachian aged material. Mare-type wrinkle ridges are
common, suggesting that these plains are of volcanic origin. Since this
area is located to the southwest of Olympus Mons, the volcanism of the
region is likely related to volcanism of the Tharsis region. The Member
1 plains are the oldest unit of the Arcadia Formation and are
stratigraphically similar in age to portions of Alba Patera and the
Olympus Mons aureole [1,9].

Approximately 200 km southwest of the proposed landing site is the 107-
km-diameter crater Nicholson. Although relatively fresh in appearance,
Nicholson is partially embayed by the Medusae Fossae deposits and
therefore appears to be intermediate in age between the Member 1 Arcadia
formation on which it is superposed and the Upper and Middle Members of
the Medusae Fossae Formation. The ejecta blanket of the crater is still
preserved although slightly reworked. Analysis of this ejected material
should provide information about changes in target composition with
depth in this vicinity.

Information From Mars Pathfinder Rover. The instruments aboard the Mars
Pathfinder Rover can help address several questions regarding the
terrain in this region. Among these questions are the following: (1)
What are the chemical composition and mineralogy of the different
geologic units at the landing site and within the traverse distance of
the rover? (2) Are there regional variations in chemical
composition/mineralogy within the same stratigraphic unit? (3) What is
the magnetic susceptibility of the material at the lander site? (4) What
is the ratio of fine-grained to rocky material at each location? (5)
What is the composition of the dust, which likely will accumulate on the
rover during its traverse? (6) What is the appearance of different
geologic features from surface level and what can the resolution of the
imaging system reveal about layering in, erosion of, and possible origin
of these features? (7) What is the trafficability of the different units
traversed by the rover?

The camera systems and the APXS sensor will provide the answers to most
of these questions. The ability of the APXS sensor to analyze both soil
and rocks should provide a much better understanding of the materials
composing the martian surface in this region. Analysis of exposed layers
within ridges, grooves, and hills by the APXS and multispectral
capabilities of the imaging system can provide information about
chemical and mineralogic variations within the near-surface region. This
information will provide constraints on the potential origin(s) of the
features studied.

This particular landing site was selected primarily to address the
question of the composition and possible origin of the Medusae Fossae
Formation Deposits. These deposits appear to be a unique landform on
Mars and have intrigued a large number of investigators. Why are the
deposits concentrated in this region of the planet? The crater density
and superposition relationship to surrounding terrain suggests a young
age for this material. What process or processes occurred to create this
material in relatively recent time? Do these deposits imply anything
about possible environmental changes for Mars? It is hoped that the
instruments aboard the Mars Pathfinder lander and rover can provide new
constraints on the theories advanced about this enigmatic region of
Mars.

References: [1] Scott D. H. and Tanaka K. L. (1986) USGS I-1802-A. [2]
Kieffer H. H. et al. (1977) JGR, 82, 4249-4291. [3] Zimbelman J. R. and
Kieffer H. H. (1979) JGR, 84, 8239-8251. [4] Forsythe R. D. and
Zimbelman J. R. (1980) LPSC XXI, 383-384. [5] Barlow N. G. (1993) LPSC
XXIV, 61-62. [6] Ward A. W. (1979) JGR, 84, 8147-8166. [7] Scott D. H.
and Kanaka K. L. (1982) JGR, 87, 1179-1190. [8] Schultz P. H. and Lutz
A. B. (1988) Icarus, 72, 91-141. [9] Tanaka K. L. (1986) Proc. LPSC
17th, in JGR, 91, E139-E158.


Golombek M. P.*
   Pathfinder Landing Sites at Candidate SNC Impact Ejection Sites (18 Degrees
   N, 164 Degrees W)

If Mars Pathfinder were able to land at a site on Mars from which the
SNC meteorites were ejected by impact, the Pathfinder mission would
essentially represent a very inexpensive sample return mission. If this
were possible, a particularly significant benefit to Mars science would
be having a radiometric age date on a sample from a known location on
Mars, which would enable a more precise assignment of absolute ages to
the crater/stratigraphic time scale for Mars. Providing such a date
would substantially improve our interpretation of the absolute age of
virtually all events in the geological, climatological, and atmospheric
evolution of Mars. This abstract evaluates the possibility of landing at
potential SNC ejection sites and the ability of Pathfinder to identify
the landing site as the place from which a SNC meteorite came.
Unfortunately, although considerable information could be gained from
Pathfinder that might support the hypothesis that the SNC meteorites
have indeed come from Mars, it is likely not possible to uniquely
identify a site on Mars as being a SNC meteorite ejection site.

Shergottites, Nakhlites, and Chassigny meteorites (SNC) meteorites are
unique mafic to ultramafic meteorites with young crystallization ages
that are believed to have been ejected from the martian surface by
impact and traveled to Earth [1,2]. Recent interpretations suggest that
the Shergottites have different crystallization ages and cosmic ray
exposure times from Chassigny and the Nakhlites (180 m.y. and <2.5 m.y.
vs. 1.3 b.y., and 11 m.y., respectively), implying different impact
ejection events on Mars [see 3 and references therein]. The young ages
of these meteorites and crater-absolute age time scales [4] related to
martian stratigraphy [5] limit their place of origin on Mars to Upper
Amazonian (Shergottites) and Middle or Early Amazonian (Chassigny and
the Nakhlites) volcanics on Mars. Tharsis is the only area on Mars that
has regionally extensive lava flows of Middle and Upper Amazonian age
with fresh impact craters larger than 10 km diameter, required to eject
the rocks from Mars [6]. Nine fresh (young) impact craters greater than
10 km diameter have been identified on Amazonian volcanics around the
Tharsis region [6]. Of these, craters 1 and 2 are below 2 km elevation
and within 10 degrees latitude of 15 degrees N. In addition, 2 other
craters in Middle Amazonian lava flows of Amazonis Planitia, northwest
of Olympus Mons, are possible SNC craters that are between 20 degrees N
and 30 degrees N latitude and below 0 km elevation. Geologic units in
which these craters are found could be visited by Pathfinder. These 4
sites are described below.

Crater 1 is 11.6 km in diameter, located at 10.8 degrees N, 135.2
degrees W (1.5 km elevation) on Upper Amazonian lava flows (Unit Aop, 7)
around Olympus Mons. This unit is composed of some of the youngest lava
flows on Mars with crater densities suggesting ages of less than 250 or
700 m.y. (depending on crater-absolute age time scale, 4), which makes
the crater a candidate ejection site for the Shergottites. A 200 km by
100 km landing ellipse would easily fit in this unit. From the crater,
the Olympus Rupes scarp is about 1 degree above the horizon and Olympus
Mons is about 1.5 degrees above the horizon, which would register on 15
and 26 pixels, respectively, in the Imager for Mars Pathfinder (IMP).
Landing on unit Aop directly adjacent to Olympus Rupes would result in 9
degrees of scarp above the horizon (or ~160 IMP pixels). As a result,
imaging of a large scarp (and any exposed stratigraphy) and volcano (and
clouds referenced to an altitude) should be possible at this landing
site, provided they are not obscured by local obstacles or topography.

Crater 2 is a 29.2 km diameter oblique impact crater located at 24.8
degrees N, 142.1 degrees W (0 km elevation) on Upper Amazonian Olympus
Mons aureole material (unit Ae, 7). This unit is also very young,
although the origin of the aureole material is quite uncertain. Landing
on unit Ae directly adjacent to Olympus Rupes (100 km away due to
landing uncertainty) would result in ~5 degrees of scarp above the
horizon (or 85 IMP pixels), although Olympus Mons would not be in view.

Two other craters 26 km and 28 km in diameter located at 29.5 degrees N,
153 degrees W and 23.5 degrees N, 152 degrees W (elevations between -1
km and -3 km), respectively, are located in Middle Amazonian lava flows
(unit Ae3, 7) in Amazonis Planitia, northwest of Olympus Mons. These
craters, originally proposed for the SNC meteorites by Jones [8], were
dismissed by Mouginis-Mark [6] due to their mantling by smooth plains of
apparent windblown origin. Nevertheless, geological relations nearby
indicate this smooth material is underlain by lava flows, so that
impacts into this unit by these two fairly large craters could have
easily excavated underlying lavas.

Pathfinder is equipped with three instruments that could help identify
the rock types near the landing site. The alpha proton x-ray
spectrometer (APXS) will determine the elemental abundances of most
light elements, except hydrogen. This instrument, mounted on the rover,
will measure the composition of rocks and surface materials surrounding
the lander. In addition, the cameras on the rover will take millimeter-
scale images of every APXS measurement site, so that when combined with
the spectral images from the lander IMP, the basic rock type and its
mineralogy should be decipherable. For the most part this data should be
enough to determine if the rocks at the Pathfinder landing site are
consistent with SNC mineralogy; i.e., are the rocks mafic to ultramafic
cumulates or fine-grained lavas? If the answer is affirmative, the
observation significantly strengthens the interpretation that the SNC
meteorites do, in fact, come from Mars. Unfortunately, this does not by
itself establish that the SNC meteorites came from the Pathfinder
landing site. Establishing this may be difficult if not impossible for a
remotely operated lander on Mars. The kinds of tests required might
include minor and trace element chemistry, as well as oxygen and carbon
isotopes, and it is not clear that these measurements, by themselves,
uniquely identify that the SNC meteorites came from a particular site as
opposed to coming from Mars in general. In addition, most lava flow
fields are heterogeneous on a local scale, exhibiting a variety of
mineralogies in close proximity. Thus, landing on a flow that has a
mineralogy closely matching that of a SNC meteorite would be
serendipitous.

Geologic units that contain four potential impact craters from which SNC
meteorites could have been ejected from Mars are accessible to the Mars
Pathfinder lander. Determining that SNC meteorites came from a
particular spot on Mars raises the intriguing possibility of using
Pathfinder as a sample return mission and providing a radiometric age
for the considerably uncertain martian crater-age time scale. Pathfinder
instruments are capable of determining if the rock type at the landing
site is similar to that of one or more of the SNC meteorites, which
would strengthen the hypothesis that the SNC meteorites did, in fact,
come from Mars. Unfortunately, instrument observations from Pathfinder
(or any remotely operated landed vehicle) are probably not capable of
determining if the geologic unit sampled by the lander is definitively
the unit from which a SNC meteorite came from as opposed to Mars in
general or perhaps a particular region on Mars.

References: [1] Wood C. A. and Ashwal L. D. (1981) Proc. LPSC 12B, 1359-
1375. [2] McSween H. Y. (1985) Rev. Geophys., 23, 391-416. [3] Treiman
A. H. (1994) LPSC XXV, 1413-1414. [4] Hartmann W. K. et al. (1981) in
Basaltic Volcanism on the Terrestrial Planets, Pergamon, 1049-1127. (G.
Neukum and D. U. Wise (1976) Science, 194, 1381. [5] Tanaka K. L. (1986)
Proc. LPSC 17th, JGR, 91, E139-E158. [6] Mouginis-Mark P. J. et al.
(1992) JGR, 97, 10213-10225. [7] Scott D. H. and Tanaka K. L. (1986)
Geologic Map of the Western Equatorial Region of Mars, USGS Map I-1802A.
{8] Jones J. H. (1985) LPSC XVI, 408-409.


Rice J. W.*
   Marta Valles Channel System in the Cerberus Rupes Region (16 Degrees N, 177
   Degrees W)

Plescia J. B.*
   Cerberus Plains (5 Degrees N, 190 Degrees W): A Most Excellent Pathfinder
   Landing Site

Introduction: The Cerberus Plains in southeastern Elysium and western
Amazonis cover >10^5 km^2; extending an east-west distance of ~3000 km
and a north-south distance of up to 700 km near 195 degrees. Crater
numbers are 89 +- 15 craters >1 km/10^6 km^2, similar to values obtained
by [2,3], indicating a stratigraphic age of Upper Amazonian and an
absolute age of 200-500 Ma [1]. The material forming the surface is
referred to as the Cerberus Formation. The units origin is
controversial; two ideas have been postulated, fluvial [4,1] and
volcanic [5]. Regardless of which interpretation is correct, the
Cerberus Plains is an important candidate for a Pathfinder lander
because it represents the youngest major geologic event (be it fluvial
or volcanic) on Mars.

Geology: The unit exhibits lobate albedo patterns and embayment
relations with older terrane. These patterns suggest flow eastward
across Cerberus, then northeastward through the knobby terrane into
Amazonis (exploiting a series of older channels carved into knobby
terrane and ridged plains). Albedo patterns in the east are regionally
organized into bands up to 40 km wide; in the west, albedo patterns are
complex and intricate with digitate boundaries. Small-scale surface
texture is variable. Near 19 degrees N, 174 degrees W, where the unit
fills a channel, the floor appears smooth whereas the surrounding
terrane has significant texture. The southern margin exhibits pressure
ridges, flow fronts, and flowage around obstacles.

The morphology of the Cerberus Plains is interpreted to indicate that it
is an example of flood basalt volcanism (e.g., Deccan Traps, Columbia
Plateau); the morphology of western part indicates plains style
volcanism (e.g., Snake River Plains). Terrestrial flood basalt provinces
[6,7] are characterized by flows 5-45 m thick extending over large areas
having little relief. Eruption rates are very high with fissure vents
tens to hundreds of km long in zones several km wide. Six low shields
have been identified in the western plains. Some of the Cerberus shields
are elongate having elliptical vents; others are more symmetric.

Pathfinder Mission Implications: The Cerberus Formation occurs between
longitudes 165 degrees and 220 degrees and latitudes 5 degrees S and 30
degrees N, although the material does not completely cover this area.
The largest expanse occurs at 180 degrees-210 degrees W and 5 degrees S-
10 degrees N. Thus, the area of exposure is within the Pathfinder
constraints (0 degrees-30 degrees N). Elevations [8] are at altitudes <-
1 km; a northeast trending band from 5 degrees N, 197 degrees W toward
10 degrees N, 180 degrees W has elevations <- 2 km. These altitudes are
within the Pathfinder range (<0 km). A 100 km x 200 km ellipse along a
N74 degrees E trend is easily found within the unit, a target for the
center of the landing ellipse is 6 degrees N, 183 degrees W, a location
ensuring landing within in the unit. The Cerberus region has low thermal
inertia [9] (<4 x 10^-3 cal cm^-2 s^-1/2 K^-1) interpreted to indicate a
low rock fraction exposed at the surface [10], <10%. This suggests the
area would be relatively safe for landing, but still offer the potential
for finding exposed rock.

Possible Scientific Implications: The first order question to be
resolved is whether the Cerberus Formation is of volcanic or fluvial
origin. This alternative is testable with both imaging and elemental
data. A volcanic flood basalt terrain should show a level, possibly
slightly rolling surface; flow fronts and pressure ridges may be
present. Rock analysis, both spectral and elemental, should show a
relatively uniform composition. A fluvial environment should show
channels and a scoured surface, evidence of erosion should be abundant
at all scales. Since debris on the surface would be from many sources,
significant heterogeneity would be expected in the spectral and
elemental analysis of the rocks.

It can be postulated that the Cerberus Plains are the source for some of
the SNC meteorites, specifically Shergottites, on the basis of age and
volcanic style. Shergotty, Zagami, ALHA 77005, and EETA 79001 have ages
of 160-180 Ma [11,12]. Only the Cerberus Formation is of sufficient size
and age to be a statistically significant source region. Major element
chemistry for the Shergottites is SiO2 at 43-51%, FeO at 18-20%, A1203
at 3-9%, MgO at 9-28%, and CaO at 3-11%. The apx unit will provide key
elemental data at the % level. Shergottites are dominated by pigeonite
(~26-40%), augite (11-37%), and plagioclase-maskelynite (10-29%). The
presence of these minerals may be detectable by the filters in the
imaging system, depending on the choice of band passes. These two
instruments should provide sufficient data to determine whether the
Cerberus Formation is the unit from which the Shergottites were derived.

The interpretation that Cerberus Plains results from flood volcanism
late in martian history carries implications for martian thermal
history. Although central vent volcanism has been recognized as
occurring late, flood volcanism has not. Flood volcanism in the period
<700 Ma indicates that, at least in the Elysium region, sufficient heat
remained to generate large volumes of low viscosity lavas.

References: [1] Tanaka K. L. (1986) LPSC 17th, JGR, 91, E139-E158. [2]
Carr M. and Clow G. (1981) Icarus, 48, 91-117. [3] Scott D. H. and
Tanaka K. L. (1986) USGS Misc. Inv. Map 1-1802A. [4] Tanaka K. L. and
Scott D. H. (1986) LPS XVII, 865-866. [5] Plescia J. (1990) Icarus, 88,
465-490. [6] Greeley R. (1976) LPSC VII, 2747-2759. [7] Greeley R.
(1982) JGR, 87, 2705-2712. [8] USGS (1991) USGS Misc.Inv. Map 1-2160.
[9] Christensen P. R. (1986) JGR, 91, 3533-3545. [10] Christensen P. R.
(1986) Icarus, 68, 217-238. [11] McSween H. (1985) Rev. Geophys., 23,
391-416. [12] Jones J. (1986) GCA, 38, 517-531.






                            Tuesday, April 19, 1994
    WHAT LONGITUDE ARE WE GOING TO? (i.e., WHERE DO WE TARGET THE ROCKET?)
                    WHAT WILL WE LEARN AFTER WE GET THERE?
               145 DEGREES W-200 DEGREES W LONGITUDE, CONTINUED
                                1:30 - 2:30 p.m.


Brakenridge G. R.*
   A Mars Pathfinder Landing on a Recently Drained Ephemeral Sea: Cerberus
   Plains (2 Degrees S, 196 Degrees W)

Along a 500 km-wide belt extending between 202 degrees and 180 degrees W
and lying astride the martian equator, moderately low albedo, uncratered
smooth plains exhibit low thermal inertia and potentially favorable
conditions for the preservation of near-surface ice. The Cerberus Plains
occupy a topographic trough as much as 2 km below the planetary datum
[1,2], and the denser atmosphere at these altitudes would also favor
long residence times for near-surface ice once emplaced [3]. The plains
have previously been interpreted as the result of young (Late Amazonian)
low viscosity lava flows [4] or similarly youthful fluvial deposition
[5,6]. However, the plains are also included in maps of possibly
extensive martian paleoseas or paleolakes [7]; Scott, 1991 #133]. Ice
emplaced as such seas dissipated could still be preserved under thin (a
few tens of cm) sedimentary cover [8].In any case, and if a sea once
existed, aqueous-born interstitial cementation, probably including
hydrated iron oxides and sulfate minerals, would have been favored and
is now susceptible to investigation by the Pathfinder alpha proton x-ray
spectrometer and multispectral imager.

There is interesting supporting evidence indicating an aqueous origin
for the Cerberus Plains. On Viking Orbiter high resolution images, some
near-shoreline portions of the plains exhibit intersecting, very low-
relief linear or curvilinear ridges that may define ridge-interior,
polygon-shaped, angular-to-rounded ice cakes and ice flows [9]. Lead-
and pressure ridge-like forms can be mapped, although local relief is
very low. The shelf ice-like pattern outlines flows that are similar in
size to those that occur on Earth, and the general fragmental character
is quite different from the smooth surface morphology imaged at Viking
resolution on unmantled plains confidently known to have formed by lava
flows. Finally, a suite of landforms elsewhere considered to be coastal
in origin [10] occur along the southern margin of the plains: these are
compatible with a marine or lacustral model but not with a lava flow
origin. Such landforms include peninsulas and bays, spits, strandlines,
and stepped massifs, and all are consistent with a maximum sea level
reaching to ~-1000 m altitude.

For example, at 3 degrees S, 197 degrees W, the dark albedo, low thermal
inertia plains unit embays and overlaps the knobby terrains to the south
along or very close to the -1000 m contour. Four hundred km to the
northeast, the "sea floor" plain reaches to below -2000 m, implying
maximum stage water depths of at least 1000 m. In the deep region, two
isolated massifs (Hibes Montes) extend to above -1000 m altitude, and
both exhibit topographic steps at that altitude: these may be wave-cut
or other coastal features. In contrast, if lava extrusions were instead
centered in this deepest part of the basin and formed the Cerberus
Plains [4], these lavas must have flowed uphill and at relatively steep
gradients to reach the southern margin of the plains. Either the
topography as now mapped is greatly in error (and there is no trough),
or water is the more likely fluid to have formed the embayment features
along the southern margin.

A 180 km-wide outflow channel typical in its molphology but unusual in
its youthfulness (it too is uncratered) extends from the Cerberus Plain
trough northeastward to a "spillway" at 24 degrees N, 172 degrees W. The
spillway lies at -1000 m altitude and some 1100 km from the Hibes Montes
islands. In agreement with [4], streamlined interchannel islands
indicate fluid flow to the northeast, from Cerberus and into Amazonis
Planitia and the deeper (-3000 m altitude) basin therein. This could not
have occurred unless fluid levels reached over the spillway; again, the
basin must have once filled to ca -1000 m altitude, and this too
suggests water and not lava as the fluid involved. The Cerberus Sea
probably formed in much the same manner as did the outflow channels, but
the surface discharge occurred within a topographic basin, and the basin
itself was first filled before overtopping the lowest spillway and
discharging excess water and ice into Amazonia Planitia. Slow filling,
perhaps under a perennial ice cover, could instead have occurred if a
global groundwater system exists [11] or if regional geothermal sources
such as recently present at Elysium or Orcus Patera stimulated large
scale hydrothermal circulation [7] and water discharge along faults and
fractures (in this case, at Cerberus Rupes). Whether filling was slow or
rapid, much evidence indicates that an ice-covered sea recently existed
at the location of the present-day Cerberus Plains, and this poses
unique opportunities for a Pathfinder landing that would investigate the
sedimentary and soil geochemical traces of the planets water cycle.

At the suggested landing location, shelf ice may still exist, and be
frozen together into extensive grounded composite flows and thinly
mantled by cemented low thermal inertial eolian deposits. Alternatively,
sediment-laden and perhaps mantled shelf ice existed here late in Mars
history and has since sublimed or melted. In either event, the present
sedimentary cover is resistant to wind erosion and thus probably
cemented. There exists here the uncertain possibility of detecting near
surface ice, but the probable opportunity to analyze in detail
chemically-cemented fine sediment and thus learn much about interstitial
water characteristics.

References: [1] USGS, Misc. Inv. Map Series I-2118 (1991). [2] USGS,
Misc. Inv. Map Series I-2127, (1991). [3] Mellon M. T. and Jakosky B. M.
(1993) JGR, 98, 3345-3364. [4] Plescia J. B. (1990) Icarus, 88, 465-490.
[5] Greeley R. and Guest J. E. (1987) USGS Map I-1802B. [6] Tanaka K. L.
(1986) JGR, 91, E139-E158. [7] Baker V. R. et al. (1991) Nature, 352,
589-594. [8] Paige D. A. (1992) Nature, 356, 43-45. [9] Brakenridge G.
R. (1993) LPSC. [10] Parker T. J. et al. (1993) JGR, 98, 11061-11078.
[11] Clifford S. M. (1993) JGR, 98,10973-11016.


Edgett K. S.*   Singer R. B.   Geissler P. E.
   Opportunity to Sample Something Different: The Dark, Unweathered, Mafic
   Sands of Cerberus (13 Degrees N, 200 Degrees W) and the Pathfinder 1997 Mars
   Landing

Dark Material Critical to Understanding Mars Surface: A very important
surface component, typically described as "dark gray material" [1], was
not seen at the Viking lander sites, but is common to all low albedo
regions on Mars. Dark material likely includes unaltered mafic volcanic
and/or crustal rock and soil not coated by dust, weathering rinds, or
varnish [2].

A Pathfinder landing in Cerberus (9 degrees N - 16 degrees N, 194
degrees W - 215 degrees W) will guarantee examination of materials that
are distinctly different from the two Viking lander sites. In situ study
of dark material will provide vital ground truth for orbiter-based
observations like those anticipated from Mars 94/96 and Mars Global
Surveyor.

Surface Properties and Regional Context: The Cerberus region is (a) not
as rocky as the Viking sites, (b) not blanketed by dust, and (c) offers
sampling of a range of rock and soil types including lava flows of
different ages, ancient crustal rock, dark sand, bright dust, and
possible fluviolacustrine materials. Cerberus lies between 0 and -1 km
elevation (+- 1 km; USGS 1991 topography), and is large enough to meet
our main objective within the landing ellipse constraints. Cerberus is
an active aeolian environment but major dune fields are absent and
activity is not as vigorous as in Syrtis Major [3]. The landing will
occur in Northern Summer, a period when predicted winds are not at their
strongest in the region [4].

The dark surfaces have low albedos (< 0.15), and intermediate thermal
inertias [5] (300-400 J m^-2 s^-0.5 K^-1), which indicate that much of
this material is sand (100-1000 microns). Rock abundance [6] varies from
~12% in the east, where the surface has many knobs and mesas, to ~1% in
the west, with average over most of Cerberus ~7%. (Viking 1 rock
abundance was ~10% and Viking 2 was ~20% [5]).

The dark soil of Cerberus is superposed on several different geomorphic
features. Central and western Cerberus is underlain by Early Amazonian
volcanic flows originating on the Elysium Rise [7]. Cerberus is bounded
in the south and east by interpreted paleolake deposits [7,8] and very
recent (0 to 700 Ma) lava flows [9]. Eastern Cerberus includes a smooth
surface of Late Amazonian fluvial, lacustrine, and/or volcanic deposits
[7-9], and Noachian(?) mesas and knobs of the Tartarus Colles [7].

Dark Material and Source: Dark sand has been transported through and
deposited within Cerberus [3,10,11]. Lateral variation in sand deposit
thickness is probable, with sand filling low areas on and between lava
flows. Dark material has blown from east to west over the lava flows of
southern Elysium, perhaps making Cerberus similar to the Amboy volcanic
field of Southern California. Amboy has lava flows overlain by windblown
sand stripped from an upwind dry lake [12].

Dark material might be eroded from sediments of the proposed lacustrine
basin [8] east of Cerberus. Alternatively, the sand may have a volcanic
source, perhaps pyroclastic material from the Elysium volcanoes or from
eruptions along the Cerberus Rupes fractures. Dark material appears to
emanate from some Cerberus Rupes fractures ([11], Viking image 883A09).
Do the fractures expose a layer of dark material, which is now eroding
[10,11]? Did dark material from the east fill the fractures, and now is
being deflated? Or were the fractures the sites of pyroclastic
eruptions? One fracture seen in the upper left corner of Viking frame
385S23 has a dark, semi-elliptical mantle deposit similar to pyroclastic
deposits on the Moon and Io.

Pathfinder Science: Cerberus is large enough that several landing sites
could be chosen. Although any landing in Cerberus will satisfy our main
objective, we suggest a site near 13 degrees N, 200.5 degrees W (see
Viking frame 883A06), because it has both dark material and proximity to
the three major geomorphic units in the region (lava flows, possible
lake sediments, and knobs of ancient crust). In addition, at this site
there is a dark lobe, which appears to emanate from a Cerberus Rupes
fracture (Viking frames #883A04-06) that might be one of the youngest,
unaltered lava flows on Mars. Although not reporting on this specific
flow, Plescia [9] proposed that the Cerberus Rupes were the source of
some very young lava flows (<700 Ma).

The main objective for a Cerberus landing will be to determine the
composition and physical properties of dark soils thought to be derived
from unaltered primary igneous crustal material. Characterization of the
size, shape, and mineralogy of dark grains 0.1 mm to 10 cm will allow
assessment of sediment maturity. Is martian sand formed of ancient,
resistant mineral grains or fresher, easily altered material? The answer
to this question will place constraints on chemical weathering and
aeolian abrasion rates. Bright wind streaks in the lee of craters in
Cerberus suggest that dust might also be available for sampling,
particularly in the lee of meter-scale obstacles. Finally, the presence
of sand or granule wind ripples would provide insight into the nature of
surface-atmosphere interactions on Mars.

References: [1] Arvidson R. E. et al. (1989) JGR, 94, 1573-1587. [2]
Singer R. B. et al. (1979) JGR, 84, 8415-8426. [3] Lee S. W. (1986) LPI
TR 87-01, 71-72. [4] Greeley R. et al. (1993) JGR, 98, 3183-3196. [5]
Christensen P. R. and Moore H. J. (1992) In Mars, Univ. Ariz., 686-729.
[6] Christensen P. R. (1986) Icarus, 68, 217-238. [7] Tanaka K. L. et
al. (1992) USGS Map I-2147. [8] Scott D. H. and Chapman M. G. (1991)
Proc. LPS, 21, 669-677. [9] Plescia J. B. (1990) Icarus, 88, 65-490.
[10] Head J. N. et al. (1991) BAAS, 23, 1176. [11] Head J. N. et al.
(1992) LPSC XXIII, 509-510. [12] Greeley R. and Iversen J. D. (1978) In
Aeolian Features Southern California: A Comparative Planetary Geology
Guidebook, NASA, 23-52.


Murchie S.*   Treiman A.
   Tartarus Colles: A Sampling of the Martian Highlands (12 Degrees N, 198
   Degrees W)

Several of the most fundamental issues about the geology of Mars can be
addressed using information on composition and structure of the plateau
plains ("highlands") that cover approximately half the planet [1,2]. The
units comprising the highlands are interpreted as a mixture of volcanic,
fluvial, lacustrine, and impact ejecta deposits. A more precise
inventory of differing of igneous and sedimentary lithologies in
highland rock units would not only lead to a better understanding of how
the plateau plains formed, but would also clarify the nature of the
surface environment during the first 800 Myr of martian history.
Structural features including bedforms, joints, and small faults that
are unresolved from orbit record a history of the emplacement and
deformation of the highlands. In addition, weathering products present
in this very ancient terrain represent a mineralogic record of past
climate, and of the pathways by which bedrock is altered chemically [3].
Their similarity or dissimilarlity to bright soils observed
spectroscopically and in situ at the Viking Lander sites [4-6] will be
evidence for the relative roles of regional sources and global eolian
transport in producing the widespread cover of "dust."

Unfortunately these issues are difficult to address in the plateau
plains proper, because bedrock is covered by mobile sand and weathering
products, which dominate both surface composition and remotely
measurable spectral properties [5]. However the "Tartarus Colles" site
(Fig. 1), located at 11.41 degrees N, 197.69 degrees W at an elevation
of -1 km, provides an excellent opportunity to address the highland
geology within the mission constraints of Mars Pathfinder. The site is
mapped as unit HNu [7], and consists of knobby remnants of deeply eroded
highlands. It contains rolling hills, but lacks steep escarpments and
massifs common in most highland remnants, and is free of large channels
that would have removed colluvium from eroded upper portions of the
stratigraphic column. These characteristics indicate that a variety of
bedrock types from thoughout the Noachian-Hesperian stratigraphic column
may remain at the site.

Six characteristics of the site indicate that Mars Pathfinder can
successfully be used here to address the fundamental issues outlined
above:

(1) The Provenance of the Site is Known. This occurrence of unit HNu
completely encompasses the landing ellipse, so that the geologic context
of the landing site would be known independently of refinements in
lander location.

(2) The Site Contains Locally Derived Material. The knobby morphology of
the site, the lack of channels, and a measured block abundance of ~10-
15% [5] are all consistent with the presence of dm-sized rock fragments
derived from within several km of their present locations.

(3) The Exposed Unit is of Global Import. The highlands bedrock
accessible here contains a record of early martian history absent from
the younger Northern Plains Assemblage [1,2], which dominates most
locations within the elevation and latitude range intended for Mars
Pathfinder. Comparable exposures do occur in walls of outflow channels,
the walls of Valles Marineris, and walls and massifs of large craters
and basins, but these sites generally are characterized by very rough
topography and/or they form targets much smaller than the Mars
Pathfinder landing ellipse.

(4) The Site Contains Nearly Unaltered Material. The presence of
relatively unaltered material is critical to an accurate compositional
determination of the substrate. Visible color of the landing ellipse is
dominated by "dark gray" materials, which are shown by NIR spectroscopic
studies to consist of relatively unaltered, basaltic particles [4,5]. In
addition the thermal inertia of the site is ~8 x 10^-3, consistent with
abundant sand [5]. Saltating sand may have partially abraded weathered
rinds from locally derived blocks.

(5) The Site Also Contains Weathering Products. Albedo patterns at the
site reveal the presence of segregated patches of bright red dust.
Furthermore the ancient origin of the block cover is consistent with
substantial chemical alteration of at least portions of exposed rock
particles.

(6) The Site Contains Evidence to Address Tractable Questions. The major
issues about highland geology outlined above can be summarized in three
questions, which can be meaningfully addressed using measurements from
instruments on the Pathfinder lander and rover. (a) Bedrock Lithology:
The camera filters on the Imager for Measure Pathfinder (IMP) can
discriminate major rock-forming minerals containing ferrous or ferric
iron. IMP is thus able to distinguish different spectral types of
blocks. Their elemental compositions can then be measured by the Alpha-
Proton-X-ray Spectrometer (APXS) on the rover, and their texures
observed by the rover camera. (b) Nature of Macrostructures: The stereo
capability and spatial resolution of IMP will show fractures and
bedforms in near-field blocks, and structurally influenced block and
knob shapes in the far-field. (c) Composition and Texture of Weathering
Products: Spectral measurements of "dust" by IMP will provide a basis
for comparison with telescopic and spacecraft spectral data and
determinations of elemental composition by APXS will allow comparison
with the Viking Lander sites. Both instruments and the rover camera, by
observing fresh and weathered surfaces of the same blocks, can together
determine the compositional and textural properties of weathered
coatings. Finally, measurements of any indurated "duricrust" may be able
to identify what phases are mobile and "enriched" in this material.

References: [1] Scott D. and Tanaka K. (1986) USGS Misc. Inv. Series,
Map I-1802-A. [2] Greeley R. and Guest J. (1987) USGS Misc. Inv. Series,
Map I-1802-B. [3] Gooding J. et al. (1992) in Mars (H. Kieffer et al.
eds.), Univ. of Arizona, 626-651. [4] Soderblom L. (1992) in Mars (H.
Kieffer et al. eds.) Univ. of Arizona, 557-593. [5] Christensen P. and
H. Moore (1992) in Mars (H. Kieffer et al. eds.) Univ. of Arizona, 686-
729. [6] Murchie S. et al. (1993) Icarus, 105, 454-468. [7] Tanaka K. et
al. (1992) USGS Misc. Inv. Series, Map I-2147.

Fig. 1, which appears here in the hard copy, shows digital image model
covering the Tartarus Colles region, showing the Pathfinder landing
ellipse. Coordinates are latitudes and longitudes of image corners.





                            Tuesday, April 19, 1994
    WHAT LONGITUDE ARE WE GOING TO? (i.e., WHERE DO WE TARGET THE ROCKET?)
                    WHAT WILL WE LEARN AFTER WE GET THERE?
                     220 DEGREES W-280 DEGREES W LONGITUDE
                                2:30 - 3:30 p.m.


Allen C. C.*
   Sampling Elysium Lavas (13 Degrees N, 203 Degrees W)

Elysium is the second largest volcanic province on Mars. A landing site
on this unit is proposed at 13 degrees N, 203 degrees W, in a dark
region north of Cerberus Rupes. The site was chosen to provide the
chemical composition and mineralogy of an Elysium lava flow.

Criteria for Landing Site Selection. The proposed landing site was
selected to utilize the Pathfinder APXS and multispectral cameras, in
order to characterize rock chemistry and mineralogy. Site selection was
based on three criteria:

Is the site important? Will the data from this site have planetary
significance?

Is the site accessible? Can the Pathfinder spacecraft land safely at
this site and perform the desired analyses?

Is the site representative? Will the samples analyzed by Pathfinder be
representative of the geology for which the site was chosen?

Is the Site Important? The chemical and mineralogical compositions of
major rock units are of extreme importance in deciphering a planet's
geologic history and interior structure. Our current knowledge of Mars
is deficient in this regard, lacking any analyses of unequivocal martian
rocks. Chemical and mineralogical analysis of at least one major rock
type should be a primary goal of any Mars lander.

The most widespread map unit in the Elysium province, and one of the
largest in the entire Pathfinder landing zone, is member 2 of Tanaka et
al. [1]. This unit, covering 1.06 x 106 km^2, is interpreted as lava
flows from the latest widespread volcanic activity in the area. Member 2
is mapped as lower Amazonian in age, with a cumulative crater density
(>2 km) of 329 +- 18 per 10^6 km^2. The lavas originated from Elysium
Mons and associated fissures. Flow fronts over 100 km in length have
been mapped. The landing site was chosen to sample this major volcanic
unit.

The friable material analyzed by the Viking landers is generally
interpreted as weathering products of mafic rocks [2]. The chemical
compositions of samples from both Viking sites were essentially
identical, and the samples spectral signatures matched those of
widespread martian bright areas. Thus, much of the planets surface is
thought to be mantled with windblown dust. Pathfinder analyses can show
whether or not the Elysium lavas are possible sources for this dust.

The SNC meteorites, generally believed to be derived from Mars, are all
basalts [3]. Chemical and isotopic differences among the meteorites show
derivations from several lavas, separated either vertically or
horizontally on the martian surface. The SNCs strongly indicate that
basalts occur somewhere on Mars. Compositional data from Elysium lavas
can show whether or not they are a reasonable source for the SNCs.

A lava analysis from a known site would provide valuable ground truth
for photogeologic interpretation. Chemical and mineralogical composition
can be used to determine lava viscosity. With this calibration point,
measurements of flow dimensions can be used to derive eruption
parameters basic to the understanding of the volcanic province.

Geophysical models require the compositions and densities of martian
crust and mantle rocks. The current uncertainty as to rock type allows
for a wide range in geophysical parameters. Knowledge of the chemical
composition of an extensive unit like the Elysium lavas could strongly
constrain these models.

Knowledge of martian lava composition is of considerable importance to
the study of comparative planetology. To zero order the surface of the
Earth is dominated by eruptive mafic and intrusive sialic rocks.
Knowledge that the lunar rocks show the same basic dichotomy is
fundamental to our understanding of that body. The Venera and Vega
analyses strongly suggest that the same two rock types dominate the
surface of Venus [4]. Pathfinder should provide the composition of one
of these major rock types on Mars.

Is the Site Accessible? The proposed site, 13 degrees N, 203 degrees W,
is within the Pathfinder landing zone. The site lies between the 0 and -
1 km contours, on a regional slope of approximately 1:500 [5]. Viking
orbiter imagery shows no scarps or large craters at the landing site.

The landing ellipse is entirely within an ENE-WSW trending dark area,
which measures 1500 km by 300 km. A small percentage of the area is
covered by light-toned NE-SW trending streaks, interpreted as dust
deposits in the wind shadows of topographic obstructions. The dark
material is interpreted as lava flows denuded of dust by the wind [6].
Thus, Pathfinder should have a high probability of landing on a
relatively dust-free lava flow.

The Viking landers touched down in bright areas dominated by windblown
dust deposits. Both sites, however, contained numerous large rocks,
which could have been analyzed by a mobile system such as that on the
Pathfinder rover. Thus, even if Pathfinder were to touch down on a dust
deposit, it should be able to find lava outcrops or boulders to analyze.

Is the Site Representative? The Columbia River Basalt Group (CRBG) in
the US Pacific northwest is one of the largest (200,000 km^2) and
youngest (17-6 Ma) flood basalt provinces on Earth [7]. Fissure
eruptions produced flows tens to hundreds of meters thick, with some
flows traceable for over 300 km. Over 5000 samples, representing all of
the flows in the CRBG, have recently been analyzed by XRF [8]. These
analyses are a unique dataset by which to judge Pathfinder analyses from
Elysium.

The CRBG can be divided into six chemically distinct formations, with
the Grande Ronde Formation comprising 85% of the volume of the entire
Group [9]. Individual lava flows within the Grande Ronde display
striking chemical uniformity. Flows can be reliably distinguished, based
on major and minor element compositions, even hundreds of km from their
sources [9].

If Pathfinder landed anywhere in the Columbia River basalts, APXS
analysis of a random dark rock would be indistinguishable from any other
analysis of the same lava flow, which could be hundreds of km in length.
To first order, in fact, such a random analysis would be highly
representative of the entire CRBG. By analogy, the composition of any
lava rock from 13 degrees N, 203 degrees W on Mars should be
representative of the fresh lavas across much of the Elysium province.

Conclusions. The proposed Pathfinder landing site presents the
opportunity to determine chemical and mineralogical compositions of an
Elysium lava flow. The flow is part of a geologic unit of planetary
signif1cance. The proposed site appears suitable for landing, and lava
surfaces should be accessible to the Pathfinder instruments. By analogy
to terrestrial flood basalts, any lava analyzed by Pathfinder is likely
to be representative of the entire Elysium province.

References. [1] Tanaka K. L. et al. (1992) Map I-2147, USGS. [2] Banin
A. et al. (1992) in Mars, Univ. Arizona, 594-625. [3] Wood C. A. and
Ashwal L. D. (1981) Proc. LPSC 12, 1359-1375. [4] Surkov et al. (1986)
Proc. LPSC 17th, E215-E218. [5] Mars Topography (1991) Map I-2160, USGS.
[6] Scott D. H. and Allingham J. W. (1976) Map I-935, USGS. [7] BSVP
(1981) Basaltic Volcanism, 1286, Pergamon. [8] Hooper P. R. and
Hawkesworth C. J. (1993) J. Petrol., 34, 1203-1246. [9] Reidel S. P.and
Tolan T. L.(1992) GSA Bull., 104, 1650-1671.


Craddock R. A.*
   Rationale for Isidis Planitia (15 Degrees N, 275 Degrees W) as a Back-up
   Landing Site for the Mars Pathfinder Mission

As discussed previously [1], the present engineering constraints imposed
on the Mars Pathfinder mission leave only three broad regions available
for site selection: Amazonis, Chryse, and Isidis Planitiae. Because of
the knowledge gained by the Viking 1 mission, Chryse Planitia would make
an ideal primary landing site. The principal objectives of this mission
should be to determine the composition and distribution of surface
materials. Analysis of rocks in Chryse Planitia would build upon results
obtained by the Viking 1 lander and answers questions concerning
composition and origin of these materials. Of particular interest would
be determining whether proposed bedrock materials are indeed in situ
materials and perhaps the degree of weathering these materials have
undergone. Because these materials may represent Hesperian age ridged
plains, they could potentially be the key to understanding the absolute
ages of the martian epochs. Results of the Mars Pathfinder could
determine whether the bedrock materials are indeed Hesperian ridged
plains materials, which could influence the priorities of future sample
return missions.

Isidis Planitia also contains material that is Hesperian in age [2].
These materials, however, are from the Late Hesperian epoch and mark the
end of this period. Nonetheless, identifying exposed bedrock materials
from this unit would also be important as radiometric age dates obtained
by future missions could determine which model for the absolute ages of
the martian periods is correct (Fig. 1). Analysis of the temperature
contrast measured by the Viking Infrared Thermal Mapper [3] suggests
that the spatial distribution of rocks in Isidis Planitia may be as high
as 20%, similar to that observed at both Viking landing sites. Central
and northeastern Isidis Planitia appear to be much smoother, containing
<10% rocks. These data suggest that Mars Pathfinder could expect to find
similar or perhaps more favorable conditions than observed at the Viking
1 landing site. In addition, extensive Viking orbiter data with
resolution <50 m/pixel exists for most of Isidis Planitia. Without an
additional orbiter imaging system, these data will be critical for
determining the location of the lander once on the surface. Similar data
were useful in determining the location of the Viking 1 lander to within
~50 m [4].

The material contained in the interior of Isidis Planitia is frequently
interpreted to be an aeolian deposit [2,5,6] based on the morphology and
relative ages of these units. Examination of high resolution Viking
orbiter images suggest that these materials also exhibit a systematic
pattern of terrains from the interior outward to the basin rim [6].
These investigators suggested that Isidis Planitia had been the location
of a thick, volatile-rich debris layer, which was subsequently removed.
Crater statistics and the buried morphology of craters contained within
the annulus of material surrounding the basin also suggest some type of
resurfacing event. However, it has also been proposed that water may
have stood in Isidis Planitia [7], perhaps as part of an ocean covering
the northern hemisphere of Mars [8]. Such a hypothesis may also explain
the stratigraphy. Examination of the grain-size and distribution of
surface materials may yield clues as to the viability of either of these
hypotheses. Particularly useful may be the analysis of material
excavated by the crater resulting from the jettisoned Mars Pathfinder
aeroshell, if it could be located. A long rover traverse (i.e., several
kilometers and out of the view of the lander), however, may ultimately
be required to examine such unit differences on the surface.

A variety of mechanisms has been proposed to explain the enigmatic
mounds and arcuate ridges in the interior of Isidis Planitia, including
volcanic cinder cones [9-11], pingoes [12], and glacial features
[6,12,13]. Because of their close spacing (tens of meters), it is very
likely that Mars Pathfinder would land in the vicinity of one of these
features. Surface images and compositional analyses of surface material
would provide valuable clues as to their origin. Such information is
important for understanding the geologic history of Mars and the
climatic transition that planet may have experienced from the late
Hesperian into the Amazonian.

As in Chryse Planitia, the Isidis basin contains both lunar-like and
rampart craters. "Rampart" [14] or "fluidized ejecta" craters [15] have
been suggested as forming from the incorporation of volatile material
from depth [14] or from the interaction of the ejecta curtain with a
thin atmosphere during emplacement [16]. The derived volatile content
and/or sediment distribution from a rampart crater near the Mars
Pathfinder landing site could provide clues as to which formation
mechanism is the most viable.

Fig. 1, which appears here in the hard copy, shows that geologic time
for the Earth can be broken into the Archean (Ar), Proterozoic (Pr), and
Phanerozoic (Ph) periods. The dashed line represents the amount of time
life is known to have existed on this planet. Geologic time for Mars is
divided into the Noachian (N), Hesperian (H), and Amazonian (A) periods.
The absolute ages are estimated from two different models of cratering
rates. A sample of Hesperian material could determine whether Model 1
[17] or Model 2 [18] is correct. Because geologic evidence suggests that
liquid water existed on the martian surface during both the Noachian and
Hesperian, absolute ages based on Model 2 imply that microbial life may
have also evolved on Mars. (Reproduced from [19].)

References. [1] Craddock R. A. (1994) Rationale for a Mars Pathfinder
mission to Chryse Planitia and the Viking 1 lander, this volume. [2]
Greeley R. and Guest J. E. (1987) USGS Misc. Invest. Series Map I-1802B,
Denver. [3] Christensen P. R. (1986) Icarus, 68, 217-238. [4] Morris E.
C. and Jones K. L. (1980) Icarus, 44, 217-222. [5] Meyer J. D. and
Grolier M. J. (1977) USGS Misc. Invest. Series Map I-995 (MC-13), scale
1:5,000,000, Denver. [6] Grizzaffi P. and Schultz P. H. (1989) Icarus,
77, 358-381. [7] Scott D. H. et al. (1992) Proc. LPS 22, 53-62. [8]
Parker T. J. et al. (1993) JGR, 98, 11061-11078. [9]Moore H. J. and
Hodges C. A. (1980) NASA TM-82385, 266-268. [10] Plescia J. B. (1980)
NASA TM-82385, 263-265. [11] Frey H. and Jarosewich M. (1982) JGR, 87,
9867-9879. [12] Rossbacher L. A. and Judson S. (1981) Icarus, 45, 39-59.
[13] Lucchitta B. K. (1981) Icarus, 45, 264-303. [14] Carr M. H. et al.
(1977) JGR, 82, 4055-4066. [15] Mouginis-Mark P. J. (1979) JGR, 84,
8011-8022. [16] Schultz P. H. and Gault D. E. (1981) Third International
Colloquium on Mars, 226-228. [17] Neukum G. and Wise D. U. (1976)
Science, 194, 1381-1387. [18] Hartmann W. K. et al. (1981) In Basaltic
Volcanism on the Terrestrial Planets, Pergamon. [19] Craddock R. A.
(1992) Proc. Third Int. Conf. of Eng. Construc. and Oper. in Space,
1488-1499.


Parker T. J.*   Rice J. W.
   Scientific Rationale for Selecting Northwest Isidis Planitia (14-17 Degrees
   N, 278-281 Degrees W) as a Potential Mars Pathfinder Landing Site

The northwest Isidis Basin offers a unique opportunity to land near a
fretted terrain lowland/upland boundary that meets both the latitudinal
and elevation requirements imposed on the spacecraft. The landing site
lies east of erosional scarps and among remnant massif inselbergs of the
Syrtis Major volcanic plains. The plains surface throughout Isidis
exhibits abundant, low-relief mounds that are the local expression of
the "thumbprint terrain" that is common within a few hundred kilometers
of the lowland/upland boundary. These typically occur in arcuate chains,
often with a summit pit or trough. They have been variously interpreted
as volcanic cinder cones or psuedocraters [1], pingos [2], or eolian
deposits that formed at the edge of a sublimating ice sheet [3]. We have
used photoclinometry to measure similar mounds in Cydonia, using
photoclinometry, at no more than a few tens of meters high, implying
very gentle slopes.

Cinder cones should exhibit slopes determined by the ballistic
emplacement of unconsolidated material and so they commonly approach the
angle of repose. Pingos are typically conical shaped ice cored hills up
to 100 meters high and 600 meters in diameter [4]. Many pingos often
exhibit dilation cracks radiating from the apex of the hill. These
fractures are created as a result of the growth of the ice core. This
process exposes the ice core and allows it to thaw out, thereby
producing a collapsed summit area. Pingos are usually located in lowland
areas, especially lake beds and deltas. Lander and rover observations
should be able to confirm whether these landforms are pingos or cinder
cones based on the presence of dilation cracks or slopes approaching the
angle of repose. The discovery of pingos would be of high importance to
future missions to Mars, both robotic and "folked". The pingo ice core
could contain relatively pure water ice within several meters of the
surface. The massif inselbergs are not as numerous nor as massive as
those in fretted terrains to the northwest, so local slopes are not
expected to be steep. Neither feature should pose a serious threat to
the lander. Landing on or adjacent to one of these features would
enhance the science return and help to pinpoint the landing site in
Viking and subsequent orbiter images by offering views of landmarks
beyond the local horizon.

References: [1] Lucchitta B. K.(1981) Icarus, 45, 264-303. [2] Frey H.
and Jarosewich M. (1982) JGR, 87, 9867-9879. [3] Grizzaffi P. and
Schultz P. H. (1989) Icarus, 77. [4] Washburn A. L. (1980) Geocryology,
Wiley, 406.