David Kring, Jeffrey Schweitzer, Charles Meyer, Jacob Trombka, Friedemann Freund, Thanasis Economou, Albert Yen, Soon Sam Kim, Allan Treiman, David Blake, Carey Lisse
Abstract. We identify the chemical elements and element ratios that should be analyzed to address many of the issues defined by COMPLEX. We determined that most of these issues require two chemically sensitive instruments to analyze the necessary complement of elements. In addition, it is useful in many cases to use one instrument to analyze the outermost planetary surface (e.g., to determine weathering effects), while a second is used to analyze a subsurface volume of material (e.g., to determine the composition of unaltered planetary surface material). This dual approach to chemical analyses will also facilitate the calibration of orbital and/or Earth-based spectral observations of the planetary body. We determined that in many cases the scientific issues defined by COMPLEX can only be fully addressed with combined packages of instruments which would supplement the chemical data with mineralogic or visual information.
1. Introduction
The Space Studies Board of the National Research Council has outlined four scientific goals for solar system exploration [COMPLEX, 1994]:
Because planetary surfaces are the most accessible portions of any solar system body, they are the logical targets of future spacecraft missions and planetary surface instruments. As the fourth goal suggests, however, the nature of the scientific issues or the evidence of the respective processes may not be equally represented on the surface of each planetary body. So, we will present separate discussions for: primitive asteroids; comets; differentiated asteroids; outer solar system satellites and Pluto; and differentiated terrestrial planets. We will then describe current flight instruments capable of making the necessary measurements and outline the analytical strategies that can be used. To fully address many critical scientific issues, the chemical analyses discussed here will often need to be supplemented with some other type of measurement (e.g., imaging or characterization of the mineralogy). We will identify these items and refer to the appropriate accompanying chapters.
We note that to justify planetary surface landers and in situ analyses, the critical scientific issues should be unresolvable from orbit (the cheaper and global strategy) or much resolved much cheaper in situ than by sample return missions (for they can rely on the full capabilities of terrestrial laboratories). One impetus might be complex geology (with more sample varieties than is possible to return; and too heterogeneous for orbital techniques). In this way, landers are ideal for in situ analyses on: large differentiated planets with complex surface processes (like Mars); complex rubble pile or differentiated asteroids; and comets, which may have heterogeneous ice and rock structures or have surface compositions that change with orbital position or with depth. Small body missions will be dominated initially by classification issues, to correlate asteroid spectra with meteorite types, and to assess the origin of near-Earth asteroid populations (from the Moon, Mars, or the main asteroid belt?). Large body missions must be more sophisticated, because they will involve planetary surfaces that are macroscopically complex. In all cases, the chemical composition of the surface will be used to infer conditions in the planetary interior.
2.1. Small, relatively primitive bodies
In situ analyses of asteroids and comets are needed to determine their elemental, molecular, isotopic, and mineralogic compositions (COMPLEX [1994]). Related questions include: identifying the sources of extraterrestrial materials that collide with Earth (or will in the future); determining if there are correlations between asteroids and comets; determining the surface geology of these objects; determining the types of carbonaceous materials in cometary nuclei; and determining the range of activity on comets. While asteroids and comets both represent relatively primitive material, we will discuss the analyses needed to address them separately, because these objects have different origins, volatile contents, and evolutionary histories.
Asteroids. Samples of primitive asteroids are available on Earth as chondrite meteorites, and have been analyzed with the best analytical instruments available. These samples, ~15,000 known, provide a good baseline of information for construction of advanced mission designs. Unfortunately, it has been impossible (so far) to correlate these meteorites with specific asteroids or groups of asteroids. Consequently, to fully leverage this database, the principal goal of any in situ analysis will be to determine whether or not the object has a composition similar to known meteoritic materials and, if so, which class of materials.
Primitive meteorites are classified as carbonaceous, ordinary, or enstatite, with many sub- categories. Classically, the discriminators among the chondrite meteorite group and sub- groups include: (1) the ratio of metallic Fe to total Fe in bulk analyses [Van Schmus and Wood, 1967]; (2) the weight percent ratio of total Fe to total SiO2 in bulk analyses [Urey and Craig, 1953]; (3) the weight percent ratio of total SiO2 to the total MgO in bulk analyses [Ahrens, 1964, 1965]; and (4) the ratio of FeO/(FeO+MgO) in olivine and pyroxene in the chondrites [Mason, 1963]. As described below (Section 3), many of the instruments used to measure the chemical composition of a planetary surface are unable to distinguish metallic from oxidized Fe, rendering discriminator 1 problematic for in situ analyses. Similarly, because many of the instruments determine bulk compositions rather than individual mineral compositions, discriminant 4 may not be useful. On the other hand, discriminants 2 and 3 are, in principal, useful with most categories of instruments, because most of the Si in any targeted asteroid should be in the form of SiO2 (there is relatively little metallic Si, even in enstatite chondrites), and all of the Mg should be in the form of MgO. Unfortunately, when one considers the errors inherent in the analyses of spacecraft hardware (e.g., an APX analysis, as described below), it may not be possible to distinguish among carbonaceous, ordinary, and enstatite chondrite asteroids based on only these two ratios. Other elemental ratios which are likely to be discriminating and measurable include Al/Si, Ca/Si, Fe/Mn, Fe/Sc, Fe/(Fe+Mg), and K/La (or proxy K/Sm and K/Gd). For our discussion (and the planning of future missions), we have compiled these ratios in Table 1. Because the absolute concentration of an element may also be a useful discriminator, the abundances of several important elements are listed in Table 2.
While analyses of an asteroid’s elemental composition are sufficient for classification, we note that several other types of measurements can also address this issue. For example, one could measure Fe0/FeT (method 1, above) using Electron Paramagnetic Resonance (EPR) techniques (Section 3.7 and Chapter IV) or Mössbauer techniques (Chapter VI). In addition, oxygen isotope compositions have proven to be a very useful classification tool among meteorites and could be used on an asteroid if sufficient precision is obtainable (see Chapter VIII).
In addition to determining the relationship between meteorites and a particular asteroid surface, it is also important to correlate the chemical composition of the asteroid with observed asteroid spectra. Earth-based observations of hundreds of asteroids have yielded a large library of reflectance spectra, but these spectra have not been correlated with meteorites or their compositions, frustrating attempts to address many of the issues outlined by COMPLEX. Consequently, a second analytical goal will be to coordinate the chemical analyses described above with spectral analyses of the surface, either from the lander (see Chapter VI), by the spacecraft during the approach phase of the mission, or from Earth-based telescopes. Analytical and flight strategies for asteroid analyses are discussed further in Section 4 of this chapter.
Comets. Comets, like asteroids, are important because their primitive materials may contain clues to physical and chemical conditions in the early solar system, like pressure, temperature, and mixing of chemical and isotopic precursors [COMPLEX, 1994]. Comets are unique, however, in containing both the rock-forming elements of asteroids and a large proportion of volatiles and organics. The last is particularly important for its implication about primordial life. Thus, it is important to measure the abundances of the rock-forming elements and any volatile and organic constituents.
Defining the analytical requirements for a landed comet mission is more difficult than for a landed asteroid mission, because there are no macroscopic (meteorite) samples of comets. The only cometary materials available for study thus far are interplanetary dust particles (IDPs). Based on analyses of these particles and current models of comet evolution, it is usually assumed that comets are composed of chondritic material (like CI chondrites) plus additional carbonaceous and icy material. Consequently, a lander on a comet should (like a lander on an asteroid) be able to discriminate among chondritic materials (e.g., Table 1 and Table2), and should be able to measure the elemental (and, ideally, isotopic) abundances of C, H, O, and N. Fortunately, some instruments designed to analyze rock-forming elements can also analyze for C, H, O, and N (e.g., the gamma-ray spectrometer (GRS), Section 3.2). Other useful instruments, designed to specifically analyze volatile constituents, are discussed in Chapters VI and VIII.
Because the surface of a comet is likely to be a heterogeneous mixture of rock and ice, chemical analyses should probably be done in conjunction with surface imaging so that any analyzed volume of material can be identified. Also, since comets experience different periods of activity, it is important for any lander to determine how surface compositions change with time and orbital position, and, very likely, with depth. Analyses of a comet’s elemental composition should also be supplemented with analyses of the oxygen and hydrogen isotope compositions of the solids and ices on the comet (Chapter VIII).
2.2. Small differentiated bodies, rocky or metallic
Many planetesimals in the solar system differentiated to produce metal and sulfide cores within shells of less dense silicate and oxide material. Because many (if not most) of these differentiated asteroids have been heavily cratered or disrupted, all of these differentiated components may be accessible to surface landers. As outlined by COMPLEX [1994], it is important to determine the thermal evolution and geochemical processes that produced the differentiated bodies by analyzing the compositions of the components in these heterogeneous bodies or the asteroid fragments of them. In one particular, it is hoped that we can identify the heat source(s) responsible for differentiation. Candidate sources include radionuclide decay, which is directly dependent on the chemical composition of the bodies (e.g., the abundances of Al, Fe, K, Th, and U), and induction heating, which is indirectly dependent on the chemical composition of the bodies (i.e., the abundance of electrical conductors like elemental Fe and C).
It is also important to correlate differentiated asteroids with meteorite samples. For silicate achondrites, this requires an instrument that can analyze many of the same rock- forming elements used to classify primitive asteroids (Table 3). Fortuitously, one of the best element ratios to use for the purposes of classifying these objects (K/U), also addresses the issue of radionuclide heating. Other useful ratios include Fe/Mn, Fe/Sc, and K/La (Fig. 1).
For metal-rich asteroids, it may be impossible to correlate them (by chemical composition) with individual meteorites or meteorite groups. Metal rich meteorites are commonly classified according to their abundances of the trace elements Ga, Ge, Ir and Ni. Of these, only Ni could reasonably be analyzed with available in situ instrumentation; the first three elements on Earth are analyzed by radiochemical neutron activation, a labor-intensive technique involving intense irradiation with neutrons, wet- chemical separations, and gamma-ray spectrometry. On the other hand, landers may be able to identify the specific class(es) of carbonaceous and hydrous material that sometimes affect the spectra of M-class asteroids.
As with relatively primitive asteroids, chemical compositions of materials from differentiated small bodies must be correlated with the objects’ reflectance spectra, although the spectra need not be collected by a landers. The utility of this type of information is clear from the results obtained after the correlation between Vesta and HED meteorites was discovered.
We also note that differentiated asteroids are sufficiently complex, that any chemical analyses need to be supplemented with an imager to identify the geologic context of the sample being analyzed. The lack of geologic context is one of the principal reasons why the igneous evolution of these types of planetesimals has not been resolved from studies of meteorites. Did our meteorites come from lava flows? Did they come from large magma chambers or narrow sills? Did they cool quickly because they were extruded or because they were quenched against the margin of a dike? It has also been difficult because we have had to extrapolate what we know of igneous processes on Earth to bodies with much less gravity and a much shorter thermal history. Consequently, while we may be able to begin to model the igneous evolution of small bodies, images of structures and lithologic contacts on these bodies will probably be needed to resolve these geologic processes. It would be immensely useful, for example, to find remnant lava flows and see to what extent chemical and mineralogical fractionation occurred within the flow (i.e., is crystal fractionation a greater function of shear than gravity on small bodies?) or to determine the extent that volatiles were important when magmas were emplaced (i.e., are there vesicles throughout the lava flow or concentrated only near the top?). Not only will coordinated imaging and sample returns help us resolve geologic issues on the specific body being sampled, by analogy, we can better interpret the evolution of all achondrites.
2.3. Resource Potentials of Small Bodies
Another significant goal for missions to asteroids and comets is to determine their potentials as resources for the human exploration and development of space. Asteroids and comets, particularly those with perihelia near the Earth ("near-Earth asteroids") could be sources of economic materials, including metals, water, and rocket propellant (e.g., carbon-rich compounds). As above, the meteoritic sampling of asteroids is not useful in evaluating the resource potential of a specific asteroid or comet, as we cannot now correlate specific samples with specific asteroid or comet types. Analyses of these objects for their resource potential must include both chemical analyses (to see if potential resources targets are available in commercial abundances) and mineralogic and textural analyses (see Chapters 7, 9, 10) to determine the proper beneficiation and refining methods.
2.4. Outer solar system satellites and Pluto
As outlined by COMPLEX [1994], it is important to characterize the surface chemistry of planetary satellites in the outer solar system and to determine their volatile inventories. In general, the principal measurements envisioned are bulk chemical analyses of ices, possibly hydrocarbons, and, in a few cases, rock-forming elements.
The surface of Europa, for example, appears to be almost pure ice, in which case instruments that can measure C, H, O, and N are needed. Some of the instruments described below (Section 3) can do so, but better packages of instruments designed specifically for ices should probably be considered (see Chapters VI and VIII). Many of the smaller saturnian satellites, also dominated by icy surfaces, fall into this same category.
In contrast, Io is believed to be covered with basaltic lavas and can, thus, be analyzed with the same types of instruments (and same capabilities) needed to analyze differentiated asteroids. In addition, since Io is partially covered with S-rich deposits, an instrument that can analyze S should also be available.
The surfaces of Ganymede and Callisto are dominated by (water) ice, but they also contain a dark phase which may be carbonaceous and/or silicate material. Consequently, instruments that can analyze rock-forming elements (Section 3, below) and/or organics (Chapter IX) are appropriate. Similarly, the surface of Rhea, one of the smaller saturnian satellites, and the surfaces of the uranian satellites should be analyzed with an instrument capable of analyzing rock-forming elements, because they appear to contain small amounts of carbonaceous (or some other dark) material mixed with ice.
The remaining saturnian satellite, Titan, is the first target of an attempted landing among the icy satellites. The landing will be attempted by the Huygens probe which will be launched from Cassini spacecraft. The probe is not designed to analyze the chemical composition of the surface (only the atmosphere during descent), but the imagery it provides (along with pressure, temperature and atmosphere composition data) should provide strong constraints on the chemical composition of Titan's surface materials. It is not yet clear how long the probe can survive on the surface (if at all), which is a problem that will need to be resolved if there are any future attempts to send landers.
Finally, COMPLEX [1994] has decided that it is important to know the composition and location of ices on Pluto and Neptune’s satellite Triton, and to determine the relationship of the ices to the tectonic and volcanic evolution of both bodies. Equally important are understandings of the evolution of organic matter on these bodies, and of the long-term motion of volatiles in and above them (e.g., volatile exchange between the surface and atmosphere) Triton, for example, has a complex seasonal cycle, so it will be important to monitor the compositions over an extended baseline to determine how they may change over time. Also, because the planetary surfaces have been affected by tectonic, impact cratering, and/or volcanic processes, any chemical analyses should be accompanied by imaging so that the geologic context is clear. The principal target of the analyses are C, H, O, N and their molecular and isotopic forms (see Chapter VIII).
2.5. Differentiated terrestrial planets
The large terrestrial planets have complex surfaces that reflect the extended actions of endogenic processes. While the Moon, Mercury, Venus, and Mars are all possible targets of future missions, we focus on Mars because it is the most likely target of missions in the near future [SSES, 1994]. Mars also presents a good example of the range of issues that can be addressed by chemical analyses of surface materials. Spacecraft instruments have already provided in situ chemical analyses of all these bodies except Mercury; these in situ data have been augmented significantly by analyses of meteorites from the moon and particularly the 'SNC' meteorites from Mars.
Studies of the terrestrial planets are aimed at understanding the internal structure and dynamics of at least one convecting terrestrial planet other than the Earth; studying the crust-mantle structure of this body; determining the geochemistry of surface units, morphological and stratigraphic relationships, and absolute ages for all solid planets; and determining how chemical and physical processes (impact cratering, surface weathering, and so on) affect planetary surfaces (COMPLEX, 1994). These goals and some of the key questions they represent can be directly or indirectly addressed with chemical analyses of surface units. Some of these key questions (cast in terms of Mars) and the analyses they require are:
What was the thermal state of Mars during differentiation and how has it evolved?
Does Mars have a different bulk composition than Earth? What is the density of the
mantle? What is the size of the core? Did Mars have a magma ocean?
What is Mars' internal chemical structure?
Are the heavily-cratered (old) uplands of Mars a remnant of an early primary crust
or a reworked crustal component?
What type of volcanism modified the surface of the planet? Based on the
compositions of the extrusions, what can we infer about parent (mantle) compositions,
magmatic temperatures, and volatile content? Did the magmas change with time or are
they correlated with specific types of terranes, and, if so, what can we then infer about
the mineralogical, chemical, and physical properties of the interior of the planet and how
they have changed with time?
How much SiO2 is in the magmas?
Where are the volatiles on Mars and how have these reservoirs evolved with
time? Did a reservoir of prebiotic organic compounds ever exist and is there any
evidence that might indicate that organic matter underwent prebiotic chemical
evolution?
Did life emerge on Mars? What was the form of this life? Does life exist in any form
on Mars today? The types of chemical analyses one expects of the initial robotic
surveyors will not answer these questions directly. However, in preparation of future
missions, one would want to determine whether or not carbonates, phosphates, cherts,
and/or evaporites were deposited;
What are/were the chemical interactions between Mars' surface and its
atmosphere?
Are SNC meteorites really from Mars and can we really use them to infer the origin
and evolution of that planet?
3.Analytical capabilities of spacecraft instruments
There are several categories of elements that can be analyzed with available instruments.
Many major and minor rock-forming elements, for example, can be measured with an
alpha-proton-X-ray spectrometer (APX), a gamma-ray spectrometer (GRS), an X-ray
diffraction and X-ray fluorescence spectrometer system (XRD and XRF), and an X-ray
stimulated photon spectrometer (XPS). Analyses of volatiles are also possible with an
APX (C, N, O, and S), a GRS (H, C, N, O, and S), an XRD/XRF (C and S), and an XPS
(C, O, N, and S). These instruments and their analytical capabilities are described below.
Some related instruments are described in those chapters discussing isotopic, mineralogic,
or organic compositions (Chapters 4, 7, and 8 respectively).
3.1. Alpha-proton-X-ray instrument (APX)
The alpha-proton-X-ray instrument (APX) has evolved from the simpler alpha-particle
instrument that was used to conduct the first chemical analyses of the lunar surface during
the Surveyor program. In its original form, the target was irradiated with alpha particles
from a source like 242Cm. The energies of backscattered alpha particles were then used
to analyze light elements (except H) and the energies of protons produced by (alpha,p)
nuclear reactions were used to analyze slightly heavier elements (Z = 9 to 14), including
the rock-forming elements Na, Mg, Al, and Si, in the outermost few microns of the
sample. These types of analyses can now be augmented with an additional mode which
utilizes the X-rays produced in the sample by the same alpha particle source (e.g.,
Economou and Turkevich, 1976; Turkevich and Economou, 1993). This mode is
comparable to X-ray fluorescence (see Section 3.3) and can approach ppm sensitivity for
heavier elements. Examples of the accuracies and sensitivities expected for some major,
minor, and trace elements are listed in
Table 4 and
Table 5.
Because the APX system has such
an extensive heritage, the errors associated with the technique are understood relatively
well. Consequently, we will use it as an example of how to determine the capabilities of
an instrument relative to the goals of a particular planetary surface mission.
Consider, for example, a mission to a primitive asteroid. As outlined above, the classic
criteria for identifying and classifying chondritic materials are the ratios Fe0/FeT,
FeT/SiO2 (or Fe/Si), and SiO2/MgO (or Mg/Si) in bulk samples, and FeO/(FeO + MgO)
in olivine and pyroxene. The first ratio will not be useful, because an APX cannot
distinguish metallic Fe from oxidized forms; neither will the fourth ratio be useful,
because an APX determines bulk compositions rather than individual mineral
compositions. On the other hand, the second and third ratios can, in principal, be utilized.
However when one considers the error inherent in an APX analysis
(Table 6), it is clear
that one may not be able to distinguish between carbonaceous, ordinary, and possibly
enstatite chondrite materials. To illustrate this point, let us assume that the APX was
dropped on an H-chondrite body and we were trying to identify it as such. In this case, an
APX analysis might indicate an Mg/Si ratio of 0.82 ±0.08, which could be interpreted to
represent H-chondrite material, but it could also represent L-, LL-, or CM-chondrite
material. The other classic ratio, Fe/Si, may be similarly ambiguous; if the same APX
analysis indicated an Fe/Si ratio of 1.60 ±0.10, the body could still consist of either H or
CM material. Fortunately, ratios of other elements help clarify the issue. In this case,
Ca/Si is particularly useful, because an analyzed ratio of 0.073 ±0.010 would clearly
correspond to H-chondrite material rather than CM-chondrite material. Also, the absolute
concentrations of individual elements may help. In the case of an H-chondrite body, the
atomic percent Fe is substantially greater than that in a CM-body (27.45 vs.
21.64; Table 2), even though both have the same Fe/Si value. The difference between
these values is large enough that it should not be blurred by the errors associated with an
APX analysis (±0.2 atom % for Fe).
This exercise indicates that unambiguous identification of the target may not be as
straightforward using an APX analysis (or other planetary surface instruments) as it
would be analyzing a meteorite using methods typically available in terrestrial
laboratories. Nonetheless, it appears that combinations of element concentrations and
element/element ratios can be used in many cases to successfully determine the nature of
the target asteroid surface.
Similarly, an APX can be used to analyze a series of lithologies on the surfaces of
differentiated asteroids or planets, as illustrated in Table 7,
which shows a series of
analyses for igneous rocks, a carbonate, and a tektite under simulated martian conditions.
As these sample analyses illustrate, an APX can provide absolute abundances of elements
rather than just relative abundances.
3.2. Gamma-ray spectrometer (GRS)
Gamma-ray spectroscopy (GRS) is a well-established technique [e.g., Evans
et al., 1993; Boynton et al., 1993] for determining the elemental
compositions of planetary bodies. Such measurements can be performed from orbit or on
the surface. Previous missions have all used the ambient cosmic-ray flux to produce
neutron-induced reactions on elements in the planetary surface which, in turn, produces
the characteristic gamma-rays that are used to determine the elemental concentrations.
The last spacecraft GRS was built for Mars Observer [Boynton et al., 1992] and
was designed to operate from orbit and provide information on the global surface
elemental concentrations and their variations over large spatial regions. Extending these
measurements to a surface lander is important, because they can provide a direct analysis
without having to compensate for atmospheric effects or contributions. Thus, surface
measurements can verify and extend the interpretation of orbital measurements and
provide a better estimate of the variance that can be assigned to orbital measurements. A
surface GRS can also identify specific lithologies and, thus, enable one to evaluate local
heterogeneities and perform detailed mapping, perhaps from a rover-based system. In
addition, a surface GRS can provide information about diurnal and seasonal variations of
constituents, like those that might be produced in a region with permafrost. Typical GRS
systems are capable of detecting essentially all major rock-forming elements, as well as
volatile components like H, C, O, and S (See
Table 8 for a comparison of APX and
GRS analyses of a model comet.). It should also be noted that a GRS measurement can be
integrated with a penetrator, where such an approach is desired for determining the true
intrinsic planetary body composition by analyzing material beneath any disturbed surface,
whether it be distillation product on a comet [Evans et al., 1986], a weathering
patina on an asteroid, or an evaporitic crust on a terrestrial planet.
The use of neutron-induced gamma-ray production for evaluating elemental content has
also been applied to subsurface measurements on the Earth [Schweitzer, 1993; Herron
et al., 1993]. While some measurements make use of natural gamma-ray
production from K, U, and Th, the most significant multi-element analyses are performed
with a pulsed neutron generator (PNG). The use of such a generator for surface planetary
measurements is practical, as it is a reasonably compact, rugged device with a power
requirement that is well within typical power budgets. Current systems use about 20 W
during operation (which would typically be no more than 50% of the time during
continuous spectroscopic measurements). Systems have been envisioned whose power
requirements during operation would be reduced to 1 to 2 W, though with lower neutron
output. The main advantage of such a device is that it produces an ambient neutron flux
which is approximately 5 orders of magnitude more intense than that produced by the
ambient cosmic-ray flux, a factor that only increases if the planetary body has an
atmosphere. This means that if a cosmic-ray flux based measurement would take a month
to achieve the desired statistical level, the same GRS detector with a PNG could perform
the measurements to the same statistical level in about half a minute. This makes it
practical to sample many locations or to monitor temporal variations, such as daily or
seasonal variations in volatile components in, for example, a martian permafrost layer. A
further advantage of a PNG is that timing of spectral acquisition relative to the neutron
production permits a separation of gamma-rays produced by different types of reactions,
all of which are combined when the ambient cosmic-ray flux is used as a source. This
timing capability significantly increases the signal-to-noise content of the detected
spectra, improving the sensitivity for detecting elemental concentrations for the same
neutron flux and gamma-ray detector system.
A further development that has improved the potential utility of GRS measurements is the
growth in viable materials, both scintillators and semiconductors, that can be used for
gamma-ray detectors. Unique properties of these new materials can improve the signal-to-
noise content of a spectrum, reduce the weight of a system without sacrificing spectral
information, reduce the sensitivity to varying ambient conditions, or make possible a
measurement that would be impractical with traditional detector materials. An example is
the design of a spectrometer using a PNG with a gamma-ray detector using Ce-doped Gd-
oxyorthosilicate [Bradley et al., 1995] that has been proposed as a GRS for measurements
on the surface of Venus.
Surface GRS instruments are intrinsically portable and are, thus, ideally suited for rover
applications. A GRS system on a planetary surface, or within a planetary body, has a
typical measurement volume of about 1000 cm3 and, thus, can determine a reasonable
site average composition without being affected by small scale heterogeneities.
Important performance parameters of a GRS include energy resolution, detector
efficiency, insensitivity to radiation damage, and ability to extract gamma-ray induced
detector signal into an electronic pulse that can be reliably processed. For orbital
measurements, the use of anti-Compton shielding, of the same or another detector
material, appreciably improves the quality of the spectra by rejecting gamma-rays that do
not come from the planetary surface and from cosmic-ray interactions in the detector
material or the spacecraft. When a neutron generator is used on the planetary surface, the
significant weight of this shielding can generally be eliminated, as the direct counting rate
far exceeds the counting rate from background events.
As an illustration of the sensitivities that can be achieved with a GRS on a lander, we
consider a model of the martian surface [Boynton et al., 1993; see, also, Boynton
et al., 1992] that was developed to test the analytical capabilities of orbital
measurements designed to determine elemental concentrations to a relative precision of
10%. The calculated sensitivities obtained in the study of an orbiter (using only the
cosmic-ray flux) are here divided by a factor of 2 to allow for the improvement for
placing the GRS on the surface. These improved sensitivities are then compared with
what the same GRS would achieve on the surface when coupled with a neutron generator.
In Table 9, the final column illustrates the improvement
in measurement time to be expected with a neutron generator producing about 108 neutrons/sec. The values for K, Th, and U are not included in the table since their detection sensitivity does not depend on the neutron source intensity. It is clear from the results in the last column that all the listed elements can be determined to 10% precision with a neutron generator in under an hour, with the exception of Ni. This is quite sensible with regard to the expected mode of operation of a rover. Where it is necessary to achieve higher levels of precision, an approximately 10 hour measurement would attain a precision for these concentrations of 1%.
There are a number of developments that would enhance the current capabilities of GRS
systems. Neutron generator development needs to be taken the final step to proving space
worthiness. Current systems are rugged and stably operate over a wide temperature range.
However, final layout of the high voltage supply and controlling electronics for satellite
configuration needs to be completed. In addition, a smaller, lighter, lower-power version
has been envisioned that would be appropriate where only a few watts of power are
available. This version is anticipated to produce about 2 or 3 orders of magnitude more
flux than is provided by cosmic-rays. New semiconductor and scintillator materials can
significantly improve GRS performance. However, many of these materials need to be
more carefully evaluated for radiation damage effects and to establish the packaging
requirements for space worthiness. In addition, for scintillators, recent developments in
compact photosensing devices need to be pursued to provide the optimum spectral
response characteristics and to provide low background, non-absorbing material in their
design.
3.3 X-ray Fluorescence (XRF)
X-ray fluorescence (XRF) is a powerful and well-established method of chemical analysis
for geological materials; XRF instruments have a venerable spaceflight heritage, having
operated on the surfaces of Mars (Viking; Clark et al., 1977) and Venus (Vega
and Venera; Zurkov et al., 1986; Barsukov, 1992). In XRF, the target sample is
irradiated with relatively hard (high-energy) X-rays, which (among other processes)
ionize atoms in the target by removing inner shell electrons. The resulting inner shell
vacancies are filled by electrons from outer shells of the same sample atom, and the
difference in energy between the two electron orbitals appears as an X-ray photon (a
secondary X-ray). The energies of secondary X-rays are characteristic of the elements
from which they are emitted and the electronic transitions involved, and the number of X-
rays and their energies can be translated into major, minor and trace element abundances.
Secondary X-rays can be excited by any high-energy incident radiation: alpha particles (as
in APX, see above), protons (as in PIXE analysis), electrons (as in electron microprobe),
and primary X-rays, as in XRF analysis. In laboratory XRF, primary X-rays are produced
be electron tubes, in which high energy electrons impinge on metal targets, usually Cu,
Mo, or Fe. Tube sources tend to be massive and require considerable power at high
voltage, but new designs are reducing both of these drawbacks. Primary X-rays can also
come from radioactive decay of selected radio-isotopes. The Viking XRF instrument was
of this sort, and used 55Fe and 109Cd to produce primary X-rays (Clark et al., 1977); the
Venera probes used 55Fe and 238U (Surkov et al., 1986). Isotopic sources tend to
produce X-rays of narrow energy ranges, but with limited intensities.
Secondary X-rays of different energies (different elements) can be discriminated by a
diffractometer (wavelength dispersion) or by semiconductor sensor (energy dispersion).
The former is favored for laboratory use because of its excellent resolution, and is usually
implemented with a moving scintillator/photomultiplier to detect X-rays. A diffraction
geometry could also be implemented without a moving X-ray detector by using CCD
arrays in the instrument's focal circle. Semiconductor X-ray sensors are common in SEM
and TEM instruments on Earth, and have been used in spacecraft instruments because of
their small mass and mechanical simplicity.
X-ray fluorescence can be sensitive to all elements except H and He, but is rarely used
for elements lighter than F or N. Detection limits are in the ppm range for heavier
elements. XRF is a bulk analytical method, as secondary X-rays readily penetrate
hundreds of microns of silicate material. Thus, the analyzed sample volume is relatively
large; for the Viking XRF experiment sample volumes were ~25 cm3 (Clark et al., 1977).
It is important to note that XRF instrumentation need not stand alone. XRF is readily
implemented with other techniques that involve X-ray sources, like X-ray diffraction and
Mossbauer spectroscopy (see Chapter 7). Two XRD/XRF instruments intended for
remote planetary applications are currently under development, and are described in the
Mineralogy chapter of this report. One prototype instrument is designed for analysis of
rock surfaces (at NASA Ames). Another prototype is matchbox-sized and designed for
analysis of particles approximately 100 micrometers diameter (at NASA Ames). The
APX instrument described above relies on similar principles and utilizes similar
detectors.
3.4 Scanning Electron Microscope and Particle Analyzer (SEMPA)
This instrument is designed to image important textures and analyze microscopic
components in a target. The basic design of the instrument has been described by Albee
and Bradley (1987). A preliminary version weighs 11.9 kg, requires 22 W of power, can
analyze all elements of Z > 11 (Na) with concentrations > 0.2% by weight, and
carries imaging and X-ray standards onboard. The instrument was designed to collect dust
particles in the tail of a comet (CRAF), and should probably considered as breadboard
stage of development. If the SEMPA were to be flown on missions that land on planetary
surfaces, then sampling mechanics would have to be re-designed to accommodate surface
samples. The instrument would also have to be tested to determine if it could withstand a
hard landing.
3.5. X-ray Stimulated Photon Spectroscopy (XPS) and Auger Electron
Spectroscopy (AES)
The XPS technique, also known as electron spectroscopy for chemical analysis (ESCA),
uses a monochromatic X-ray source in conjunction with an electron energy analyzer to
determine the chemical composition and chemical state (or oxidation state; e.g.,
S2-, S0, S4+O3, or S6+O4) of the topmost (50 to 100 micrometer thick) surface layer of
solid samples [Bubeck and Holtkamp, 1991; Barr, 1991; Ebel and Ebel, 1990; Perry,
1986]. In principle, any monochromatic X-ray source can be used, though most laboratory
studies have utilized Mg or Al K (alpha) radiation [Perry, 1986; Henrich, 1987]. Such Mg
and Al K (alpha) sources have a typical power requirement of several watts and need
water cooling. For space applications, alternative X-ray sources may have to be
considered. XPS analyzes all elements except H, and its underlying physical principle is
as follows: monoenergetic X-rays impinge on the sample surface and cause electrons
from core levels of the target atoms to be ejected. To first approximation, the energy of
these photoelectrons is determined by the energy of the impinging X-rays minus the
binding energy of the electrons to the atomic nuclei (plus a correction term for the
workfunction of the instrument). To second approximation, the energy of the
photoelectrons is also influenced by the electron density in the outer (valence) shell and
therefore reflects changes in the oxidation and ligands of the target atoms. XPS data
correlate with theoretically calculated chemical shifts [Maksic and Supek, 1989]. Besides
the oxidation states of S, which are often quoted as "textbook examples," XPS is widely
used to determine the bonding and oxidation state of C in C-bearing compounds [Bubeck
and Holtkamp, 1991]. With respect to other geological problems, XPS can potentially be
used to determine Fe3+ and Fe2+, the oxidation states of other transition metal cations,
and some limited information about the proportions of O2-, O-, OH-, and H2O.
A technique that is related to XPS is auger electron spectroscopy (AES) [Chambers et
al., 1994]. AES is based on the measured energy of electrons emitted from the target
by an internal photoeffect. This photoeffect is produced by the same primary process that
gives rise to XPS, or by irradiation with high energy, typically 10 to 30 kV electrons, such
as in an electron microscope. This produces an electron hole in a core level which is then
filled with an electron from a higher level. The energy produced by this internal process is
transferred to another electron within the same atom, which is then emitted as an auger
electron, carrying information about the element from which it emerges. AES and XPS
have similar surface sensitivities [Bubeck and Holtkamp, 1991], but AES does not
contain information about the oxidation state or ligands of the target atom.
Typical XPS and AES laboratory instruments require ultrahigh vacuum, both to minimize
surface contamination that may otherwise mask the chemistry of the underlying sample,
and to avoid electron-gas collisions in the long path through the electron energy
analyzers. By making the electron energy analyzer small and reducing the path length for
the electrons to a few millimeters, the vacuum requirements for electron-gas scattering
can be somewhat relaxed; i.e., on Mars where the ambient atmospheric pressure
is low. On airless bodies like an asteroid or inactive comet, the issue disappears and both
XPS and AES would be suitable.
XPS laboratory instruments have the capacity to focus X-rays into a 100 µm spot. Larger
spot sizes may be used on a lander instrument, perhaps several square millimeters. AES
laboratory instruments typically do not use X-rays for excitation, but rather electron
beams that have much better spatial resolution.
AES has a higher quantum yield than XPS and therefore provides stronger signals. AES
also has a significantly higher quantum yield than XRF for relatively light elements (Z<
Na). AES would well compete with XRF as a chemical analysis tool, in particular for
low-Z elements, were it not limited by its extreme surface sensitivity which makes any
AES analysis strongly dependent on surface contamination. In cases of even moderate
levels of surface contamination, of the order of a monolayer, it is impossible to obtain
reliable information about the composition of the underlying bulk sample.
While XPS suffers from similar surface sensitivity, this apparent disadvantage may be
instead used as an advantage. XPS is unique among spectroscopic techniques because it
provides information about the presence and nature of C-bearing compounds spread over
the surfaces of mineral grains, even at a monolayer level. XPS is therefore a technique
that might be able to address issues related to the Exobiology Program (also see Chapter
IX).
Dust particles and soil grains are obvious candidate samples for XPS (and AES). If the
samples have to be introduced into a high vacuum system for analysis, then robotic
sample selection and handling requirements are severe. One possibility that lends itself to
fine-grained soil samples is the use of sticky tapes or grids. Even though such collection
devices will probably use organic "glue" that could interfere with the search for in
situ organics, they can be "overloaded" with sample material so as to mask any
chemical signature from the underlying tape or grid. Larger solid rock or ice samples with
relatively smooth surfaces could be studied directly, if the appropriate robotic handling
capacity is available to position them inside the XPS (or AES) instrument.
3.6. Charge Distribution Analysis (CDA)
CDA is a technique that is still very new to planetary sciences, but it has unique
capabilities that cannot be provided by any other analytical method [F. Freund et
al., 1993, 1994a; M.M. Freund et al., 1989]. Currently under development at
the Ames Research Center and in industry, CDA determines the dielectric polarization of
solids at the 0 Hz limit. It does so by measuring the force in an electric field gradient of
reversible polarity. The measurements are typically carried out as a function of
temperature (ambient to 800K) or of UV flux. CDA provides two parameters that are of
interest to minerals and planetary materials: (I) bulk polarization and (ii) sign and
magnitude of a surface charge.
The scientific rationale for CDA is based on the recognition that "water" dissolved as
OH- in nominally anhydrous magmatic (olivine, pyroxene, feldspar, etc.) or
metamorphic (garnet, quartz, feldspar, etc.) minerals [Bell and Rossman, 1992;
Aines and Rossman, 1984] undergo, at least in part, a particular internal redox reaction by
which OH- pairs convert into H2 molecules (reduced) plus peroxy entities (oxidized)
such as peroxy anions, O22-, or peroxy links, X/OO\Y with X, Y = Si, Al, etc.
[F. Freund et al., 1989; F. Freund and Oberheuser, 1986; King and F. Freund,
1984]. The significance of this is that minerals that have crystallized or recrystallized in
an H2O-laden environment, especially at high pressures, will always contain some
"impurity" OH-. If these dissolved OH- undergo redox conversion, the infrared
spectroscopic signature for dissolved "water" may disappear completely or nearly
completely. Even in terrestrial laboratory studies such minerals would then appear free of
OH- and would likely be (wrongly) classified as having formed under anhydrous
conditions. As a result of the redox conversion of OH-, the minerals contain peroxy
entities which represent electronic defects in the O2-sublattice. As long as the O- are
spin-paired and diamagnetic, they are dormant and undetectable. Upon heating or UV
irradiation, however, the O--O- bound dissociates into paramagnetic O-, equivalent to
defect electrons or "positive holes" [F. Freund et al., 1994a].
The O- are of dual interest. (I) They are electronic charge carriers that propagate through
the O2-sublattice with little interference from the cation sublattice, even if the latter
contains transition metal cations in low oxidation states [F. Freund et al., 1993].
(ii) They are highly oxidizing radicals [Freund et al., 1990]. While propagating
through the mineral lattice, the O- cause an increase in the electric conductivity which is
very hard to measure [F. Freund et al., 1993], but also a diagnostic increase in the
dielectric polarization which can easily be determined by CDA. When trapped at a
surface, the O- cause this surface to acquire a positive charge which can be detected by
CDA. Concomitantly, trapped surface O- represent a powerful oxidant which can oxidize
H2 to H2O or subtract an H atom from CH4 to produce CH3 radicals [Yamamoto et
al., 1993; Lunsford et al., 1988]. The latter issue is relevant to the
exploration of Mars and the characterization of the still enigmatic martian soil oxidant.
The currently prevailing opinion is that the soil oxidant consists of a physisorbed layer of
H2O2 molecules formed photochemically from traces of water vapor in the martian
atmosphere and frosted into the soil. Thus, CDA can determine (I) whether or not a
mineral formed in a H2O-laden environment and (ii) whether the martian soil oxidant
consisted of an H2O2 frost formed from traces of water vapor in the martian atmosphere
or of a layer of trapped surface O- radicals photodissociated in the bulk of peroxy-bearing
mineral grains and trapped on the mineral surfaces.
Given that CDA is a new technique, an instrument suitable for planetary exploration is
only in the design stage [F. Freund et al., 1994b]. The core device is a
miniaturized atomic-force-microscope-type force sensor (license AT& T Bell
Laboratories [Griffith and Griggs, 1995]) with a tip carrying a special electrode to which
the positive and negative bias voltages are applied. The tip has to be brought into
proximity (0.1 to 1 mm) of the sample to be studied. The sample will typically consist of
a small grain (1 to 3 mm) and it has to be heated to temperatures up to 800K. Robotic
operation requires a manipulator to select, grab, and accurately (± 0.1 mm) position a
sample grain.
3.7. Electron Paramagnetic Resonance (EPR)
This technique, which is also referred to as electron spin resonance (ESR), uses a
microwave (~9 Ghz) source and magnetic field for characterization and quantification of
paramagnetic transition metal ions, radicals, and defect centers (created by high energy
radiation) in minerals. The EPR spectra usually show species specific signatures such as
splitting factors (g values), hyperfine splittings, and spectral line shapes that can be used
for characterization. The technique is for molecular characterization as well as for
determining the oxidation states of transition metal ions. It cannot detect, however, Fe
metal. The technique is also limited by the total amount of Fe in sample; it will not work,
for example, if there is more than 10% FeO in olivine. Consequently, while it may be a
very good technique when analyzing anorthosites and gabbros, it may not be useful with
some primitive materials.
This is a well-established technique in terrestrial laboratories and has a typical sensitivity
at the ppb level. For flight instruments, it is estimated that sensitivities at ppm level can
be obtained. An EPR spectrometer for a prototype flight instrument is being developed at
JPL. It has a mass of ~300 g and a power requirement of < 5 W. Mössbauer is a
competing technique (see Chapter VI).
3.8. Nuclear Magnetic Resonance (NMR)
This technique uses a radio frequency (~13 MHZ) source and magnetic field (3 Kgauss)
for detection and quantitative measurement of various forms of water: adsorbed and
chemically bound H2O, -OH, H, etc. Other nuclei with nuclear spins are
detectable with appropriate RF ranges.
This, too, is a well established technique in terrestrial laboratories. A NMR flight
instrument prototype in a penetrator configuration is being built at JPL. The instrument
mass is ~150 g and requires < 5 W power.
4. Analytical and flight strategies
In many mission scenarios, the scientific issues require two chemically sensitive
instruments to analyze the necessary complement of elements (e.g., rock-forming
elements plus volatile elements and their isotopes). Two chemically sensitive instruments
are also needed in many cases so that one can analyze the outermost planetary surface
(e.g., to determine weathering effects) while a second can analyze a subsurface
volume of material (e.g., to determine the composition of unalterd planetary
material). It is also necessary sometimes to coordinate the chemical analyses with
measurements designed to determine other properties (like the mineralogy of the surface).
Examples of these requirements are outlined below in the context of the issues that
pertain to specific types of planetary bodies.
4.1. Small, relatively primitive bodies
Asteroids are airless bodies and, thus, do not have the protective shield of an atmosphere.
Consequently, micrometeoritic and solar particle damage could have significantly altered
the near surface environment. To ensure that an analysis of unaltered material is obtained,
a technique that analyzes the subsurface (> 1 cm deep?) is preferred. This could
involve devices that dig trenches, drill cores, or bury instrument packages in penetrators.
Alternatively, an instrument that analyzes a large volume of material, like a GRS, could
be employed. To quantify the chemical effects of any surface modifications, one could
use a GRS in conjunction with a surface-sensitive instrument, like an XPS or APX.
Because asteroids are likely to be rubble piles of material with different chemical or
petrologic properties, any chemical analyses should probably be coordinated with an
imaging system. In some cases, bulk chemical analyses will need to be supplemented with
individual mineral analyses (see Chapter VI). Similarly, because impact processes are
constantly modifying the surfaces of asteroids, and there are hints that these processes
juxtapose material with different spectral properties (e.g., Galileo’s observations
of Ida), the chemical analyses should be supplemented with reflectance spectra.
Depending on the capabilities of spectral systems, this task could be conducted during
approach, from orbit, or from the lander. This task is particularly important if one is ever
going to be able to link the meteoritic database with the library of asteroid spectra.
Comets have a lot more activity occurring on their surfaces than asteroids and, thus, it
will be important to design systems that can measure compositional variations over an
extended period of time (as the orbit evolves) and to determine if the surface activity has
produced a layered structure in the surface materials. To obtain vertical compositional
profiles, devices that dig trenches, drill cores, or bury an instrument package in
penetrators should be considered. In the case of comets, instruments must be selected that
can analyze both the rock-forming elements and volatile constituents. A GRS is a good
candidate, because it analyzes many of the rock-forming elements plus H, C, O, N, and S.
However, instruments that are designed specifically to analyze volatiles and organics
should also be included (see Chapters VI, VIII, and IX). For comparison of IDP’s with
particles on the comet surface or in the surrounding coma, instruments like the XPS or
SEMPA should be considered. In all cases, the heterogeneity of the target, plus the
expected activity, suggest that any chemical analyses could best be interpreted if they
were integrated with an imaging technique.
4.2. Small, rocky or metallic, differentiated bodies
Geologic context is the watchword here. These bodies will probably have very
complicated surfaces, produced first by the volcanic, tectonic, and impact cratering
processes that affect geologically active planetary surfaces, and then modified by an
extended period (> 4 billion years) of collisional evolution that has either cratered or
disrupted the bodies. For that reason, it is imperative that good imaging systems be
utilized in conjunction with any chemical analyses. Because these surfaces are likely to
be heterogeneous, mobile systems are also required. Possible candidates include rovers
or hoppers, both of which are compatible with most of the instruments described above.
The chemical analyses should be governed by the same criteria used to examine primitive
asteroids, and, to again link the meteoritic database with the library of asteroid spectra,
any chemical analyses should be conducted in regions where the reflectance spectra is
also being determined.
4.3. Outer solar system satellites and Pluto
Because many of the bodies have surfaces dominated by ices, the best package of
instruments and analytical strategy are described in Chapters VI and VIII. In those cases
where silicate or carbonaceous material is present (like Ganymede, Callisto, and Rhea)
one or more of the instruments described in Section 3 should also be on board. In the case
of Io, instruments that analyze the rock-forming elements should take priority. Because Io
still has active volcanism, analyses should be coordinated with a high-quality imaging
survey to identify lava flows and other morphological features. If possible, the imaging
systems (See Chapter ??) should also be able to constrain the mineralogy of the
lithologies being analyzed. Because Io has a S-rich surface, this element is a particularly
important analytical target. It may also be necessary to have an analytical system that can
measure material below a surficial blanket of S.
4.4. Differentiated terrestrial planets
The surfaces of these types of planetary bodies are complex and may require more than
one analytical instrument for chemical analyses.
Sometimes, for example, it is useful to compare and contrast the composition of the
outermost surface layer and the underlying volume of rock (e.g., to determine the
interaction of the surface with the atmosphere via weathering ). This approach
was illustrated on the USSR’s Vega 2 mission to Venus, which carried both a GRS and
an XRF [Barsukov, 1992]. The GRS, which analyzed the deepest and largest volume of
material, measured 0.4 ± 0.2 wt.% K. In contrast, the XRF, which measured the
outermost surface material, measured 0.08 ± 0.07 wt.% K. One interpretation of this
differerence in analyzed K contents is that the uppermost surface of Venus is depleted in
K relative to deeper material. If so, the true crustal K abundance requires a method like
GRS technique is critical. On the other hand, if one wants to processes that affect the
surface of Venus, techniques like GRS and XRF are both critical. The utility of a coupled
GRS-XRF package has also been discussed in the context of a mission to Mars [Yin
et al., 1988]. In a feasibility study for the proposed Lunar Geoscience Observer
(LGO) mission, a GRS combined with an X-ray spectrometer (XGRS) was found to be
capable of properly distinguishing at least 14 different lithologies on the Moon [LGO-
SWM, 1986]. While this instrument couplet was designed for a orbiter, it illustrates the
utility of this approach, which can also be implemented in a configuration suitable for a
lander.
This two-instrument concept is attractive on Mars where weathering processes seem
likely (e.g., a crust was observed in Viking images). Instead of a GRS-XRF
system, one could utilize a GRS-APX system
(Figure 2). This system can be used
passively, without any modification of the planetary surface. Alternatively, one could
send a single instrument if trenching or some other mechanical method is used to expose
successively deeper layers of the planetary surface.
References
Ahrens L.H. (1964) Si-Mg fractionation in chondrites. Geochim. Cosmochim.
Acta 28, 411-423.
Ahrens L.H. (1965) Observations on the Fe-Si-Mg relationship in chondrites.
Geochim. Cosmochim. Acta 29, 801-806.
Aines R.D. and Rossman G.R. (1984) Water in minerals - a peak in the infrared? J.
Geophys. Res. 89, 4059-4071.
Albee A.L. and Bradley J.G. (1987) SEMPA - Scanning electron microscope and particle
analyzer for the CRAF mission. Lunar Planet. Sci. XVIII, 13-14.
Barr T.L. (1991) Recent advances in X-ray photoelectron spectroscopy studies of oxides.
J. Vac. Sci. Technol. 9A.3 (Part 2), 1793-1805.
Barsukov V.L. (1992) Venusian igneous rocks. In Venus Geology, Geochemistry, and
Geophysics: Research Results from the USSR (eds. V.L. Barsukov, A.T.
Basilevsky, V.P. Volkov, and V.N. Zharkov), pp. 165-176, University of Arizona
Press, Tucson.
Bell D.R. and Rossman G.R. (1992) Water in Earth’s mantle: The role of nominally
anhydrous minerals. Science 255, 1391-1397.
Boynton W.V. and 19 other authors (1992) Science applications of the Mars Observer
Gamma Ray Spectrometer. J. Geophys. Res. 97, 7681-7698.
Boynton W.V., Evans L.G., Reedy R.C., and Trombka J.I. (1993) The composition of
Mars and comets by remote and in situ gamma ray spectrometry. In Remote
Geochemical Analysis: Elemental and Mineralogical Composition (eds. C.M.
Pieters and P.A.J. Englert), pp. 395-411. Cambridge University Press,
Cambridge.
Bradley J.G., Schweitzer J.S., Truax J.A., Rice A., and Tombrello T.A. (1995) A neutron
activation gamma ray spectrometer for planetary surface analysis. Acta
Astronautica 35, Suppl., 109-118.
Bubeck C. and Holtkamp D. (1991) Optical and surface-analytical methods for the
characterization of ultrathin organic films. Adv. Mater. 3.1, 32-
38.
Chambers S.A., Tran T.T., and Hileman T.A. (1994) Auger electron spectroscopy as a
real-time compositional probe in molecular beam epitaxy. J. Vac. Sci.
Technol. 13 A.1, 83-91.
Clark B.C. III and 9 other authors (1977) The Viking X ray fluorescence experiment:
Analytical methods and early results. J. Geophys. Res. 82, 4577-
4594.
COMPLEX (1994) An Integrated Strategy for the Planetary Sciences: 1995-
2010. Space Studies Board of the National Research Council, Washington,
D.C., 199 p.
Ebel M.F. and Ebel H. (1990) Quantitative surface analysis by X-ray photoelectron
spectroscopy and X-ray excited Auger electron spectroscopy. Mikrochim.
Acta 2.1-6, 49-54.
Economou T.E., Turkevich A.L., Patterson J.H. (1973) An alpha particle experiment for
chemical analysis of the martian surface and atmosphere. J. Geophys. Res.
78, 781-791.
Economou T.E. and Turkevich A.L. (1976) An alpha particle instrument with alpha,
proton, and X-ray modes for planetary chemical analyses. Nuclear Instruments
and Methods 134, 391-400.
Evans L.G., Trombka J.I., and Boynton W.V. (1986) Elemental analysis of a comet
nucleus by passive gamma ray spectrometry from a penetrator. Proc. Lunar
Planet. Sci. Conf., 16th (Part 2), J. Geophys. Res.
91, D525-D532.
Evans L.G., Reedy R.C., and Trombka J.I. (1993) Introduction to planetary remote
sensing gamma ray spectroscopy. In Remote Geochemical Analysis: Elemental
and Mineralogical Composition (eds. C.M. Pieters and P.A.J. Englert), pp.
167-198. Cambridge University Press, Cambridge.
Freund F. and Oberheuser G. (1986) Water dissolved in olivine: a single crystal infrared
study. J. Geophys. Res. 91, 745-761.
Freund F., Batllo F., and Freund M.M. (1989) Dissociation and recombination of positive
holes in minerals. In Spectroscopic Characterization of Mineral and Their
Surfaces (eds. L.C. Coyne, S.W.S. McKeever, and D.F. Blake), pp. 310-329.
Am. Chem. Soc., Washington, D.C.
Freund F., Maiti G.C., Batllo F., and Baerns, M. (1990) O- identified at high temperatures
in CaO-based catalysts for oxidative methane dimerization. J. Chim. Phys.
87, 1467-1477.
Freund F., Freund M.M., and Batllo F. (1993) Critical review of electrical conductivity
measurements and charge distribution analysis of magnesium oxide. J.
Geophys. Res. 98, 22209-22229.
Freund F. et al. (1994a) Positive Hole-Type Charge Carriers in Oxide Materials. In
Grain Boundaries and Interfacial Phenomena in Electronic Ceramics (ed.
L.M. Levinson), pp. 263-278. Am. Ceram. Soc., Washington, D.C.
Freund F., Freund M.M., Plombon J.J., and Butow S.J. (1994b) Studying Martian soil and
surface rocks by Charge Distribution Analysis. In Mars Surveyor Science
Objectives and Measurements Requirements Workshop, JPL Tech. Rep.
D12017 (eds. D.J. McCleese et al.), pp. 58-59. Jet Propulsion Laboratory,
Pasadena.
Freund M.M., Freund F., and Batllo F. (1989) Highly mobile oxygen holes in magnesium
oxide. Phys. Rev. Lett. 63, 2096-2099.
Griffith J.E. and Griggs D.A. (1993) Dimenional metrology with scanning microscopes.
J. Appl. Phys., in press.
Henrich V.E. (1987) Electron spectroscopic studies of perfect and defect metal oxide
surfaces. Phys. Chem. Miner. 14.5, 396-400.
Herron S.L., Herron M.M., Grau J.A., and Ellis D.V. (1993) Interpretation of chemical
concentration logs and applications in the petroleum industry. In Remote
Geochemical Analysis: Elemental and Mineralogical Composition (eds. C.M.
Pieters and P.A.J. Englert), pp. 507-537. Cambridge University Press,
Cambridge.
Jarosewich E. (1990) Chemical analyses of meteorites: A compilation of stony and iron
meteorite analyes. Meteoritics 25, 323-338.
Kallemeyn G.W. and Wasson J.T. (1981) The compositional classification of chondrites -
I. The carbonaceous chondrite groups. Geochim. Cosmochim. Acta
45, 1217-1230.
Kallemeyn G.W., Rubin A.E., Wang D., and Wasson J.T. (1989) Ordinary chondrites:
Bulk compositions, classification, lithophile-element fractionations, and
composition-petrographic type relationships. Geochim. Cosmochim. Acta
53, 2747-2767.
King B.V. and Freund F. (1984) Surface charges and subsurface space charge distribution
in magnesium oxide containing dissolved traces of water. Phys. Rev.
B29, 5814-5824.
LGO-SWM (1986) Contributions of a Lunar Geoscience Observer (LGO) Mission to
Fundamental Questions in Lunar Science. Prepared by M.J. Drake (Univ.
Arizona) for the LGO Science Workshop Members (R. Phillips, chair).
Lunsford J.H., Lin C.H., Wang J.X., Campell K.D. (1988) The role of M+O-centers in
the activation of methane on metal oxides. Mater. Res. Soc. Symp. Proc.
111, 305-314.
Maksic Z.B. and Supek S. (1989) Critical appraisal of some current semiempirical
methods in calculating ESCA chemical shifts. J. Mol. Struct. 198,
427-434.
Mason B. (1963) Olivine compositions in chondrites. Geochim. Cosmochim.
Acta 27, 1011-1023.
Palme H., Schultz L., Spettel B., Weber H.W., Wanke H., Michel-Levy M.C., and Lorin
J.C. (1981) The Acapulco meteorite: chemistry, mineralogy and irradiation effects,
Geochim. Cosmochim. Acta 45, 727-752.
Perry D.L. (1986) Applications of surface techniques to chemical bonding studies of
minerals. ACS Symp. Series, Geochem. Processes on Mineral Surfaces
323, 389-402.
Schweitzer J.S. (1993) Subsurface nuclear measurements for geochemical analysis. In
Remote Geochemical Analysis: Elemental and Mineralogical Composition
(eds. C.M. Pieters and P.A.J. Englert), pp. 485-505. Cambridge University Press,
Cambridge.
Schmitt R.A., Goles G.G., Smith R.H., and Osborn T.W. (1972) Elemental abundances in
stone meteorites. Meteoritics 7, 131-213.
SSES (Solar System Exploration Subcommittee) (1994) Solar System Exploration
1995-2000. Solar System Exploration Division, Office of Space Science,
NASA, Washington, D.C., 35 p.
Surkov Yu.A., Moskaloyova L.P., Kharyukova V.P., Dudin A.D., Smirnov G.G., and
Zaitsev S.Ye. (1986) Venus rock composition at the Vega 2 landing site. Proc.
17th Lunar Planet. Sci. Conf., J. Geophys. Res. 91 Suppl., E215-E218.
Turkevish A. and Economou T. (1993) Elemental analysis of extraterrestrial surfaces by
alpha-particle and radiation sources. In Remote Geochemical Analysis:
Elemental and Mineralogical Composition (eds. C.M. Pieters and P.A.J.
Englert), pp. 471-483. Cambridge University Press, Cambridge.
Urey H.C. and Craig H. (1953) The composition of the stone meteorites and the origin of
the meteorites. Geochim. Cosmochim. Acta 4, 36-82.
Van Schmus W.R. and Wood J.A. (1967) A chemical-petrologic classification for the
chondritic meteorites. Geochim. Cosmochim. Acta 31, 747-
765.
Warren P.H. and Kallemeyn G.W. (1989) Elephant Moraine 87521: The first lunar
meteorite composed of predominantly mare material. Geochim. Cosmochim.
Acta 53, 3323-3300.
Yamamoto Hiroshi, Chu H.Y., Xu M.T., Shi C., Lunsford J.H. (1993) Oxidative coupling
of methane over a lithium (+1)/magnesia catalyst using nitrous oxide as an oxidant.
J. Catal. 142.1, 325-336.
Yanai K. (1994) Angrite Asuka-881371: Preliminary examination of a unique meteorite
in the Japanese collection of Antarctic meteorites, Proc. NIPR Symp. Antarctic
Met. 7, 30-41.
Yin L.I., Trombka J.I., Evans L.G., and Squyres S.W. (1988) In-situ XRF and gamma ray
spectrometer for Mars sample return mission. In Workshop on Mars Sample
Return Science, LPI Tech. Rep. 88-07, 182-183.
Zurkov Y.A., Moskalyova L.P., Kharyukova V.P., Dudin A.D., Smirnov G.G., and
Zaitseva S.Y. (1986) Venus rock composition at the Vega 2 landing site. Proc.
Lunar Planet. Sci. Conf., 17th (Part 1), J. Geophys. Res.
91, E215-E218.
Are some elements (like Ca, Al, Th, and U) concentrated in the crust? What is the
ratio of incompatible and refractory elements?
What are the Mg/(Fe + Mg) ratios in magmas?
What are the absolute and relative abundances of the rare earth elements (REE) in
magmas or any other crustal reservoirs?
What is the oxidation state of the surface?
Is the Mg/(Mg + Fe) ratio in lavas in the uplands higher, lower, or the same as it is
elsewhere?
How do the relative and absolute abundances of the REE in lavas in the uplands
compare with those in lavas from elsewhere?
Similarly, how do these elemental components compare in impact melts (which
represent bulk crustal melts) from both types of regions?
What are the abundances of the rock-forming elements?
What are the Na/Ca and Mg/Fe ratios in the basalts or in lavas that have different
ages (where the ages are determined stratigraphically, by crater counts, or
radiometrically)?
Within a single volcanic province, how has the (Na2O + K2O)/SiO2 ratio changed
with time?
What is the distribution of H2O?
What are the Fe2+/Fe3+, and/or Fe2+/Fe0 ratios in fresh lavas, fresh impact melts,
dust, and obviously altered surface components?
How is C distributed in the crust?
What are the abundances of Si, Mg, Fe, Ca, S, P and C (or CO2) in surface
lithologies?
Are there any C-rich organic deposits?
What are the Mg/(Mg + Fe), Fe2+/Fe3+, and/or Fe2+/Fe0 ratios in lavas and
weathered products?
What are the abundances of the rock-forming elements in the dust?
What is the distribution of H2O? Are there any aqueously altered lithologies?
Do bulk element analyses of the rock-forming elements indicate there are chemically
fractionated units that may correspond to fluvial sedimentation?
What are the distributions of S, Cl, Fe, P, C (or CO2)? Are there evaporite deposits,
salts, banded iron formations, phosphates, and/or carbonates?
What is the Fe/Mn, K/U, and K/La ratios in mafic and ultramafic igneous rocks on
the planetary surface?
What are the oxygen isotope compositions of those same rocks?
What is the Ca/Na and Mg/(Mg + Fe) ratios in basalts and how do they compare with
those in shergottites?
