OPTIMIZATION OF AN '05 SAMPLE RETURN FOR MARS EXOPALEONTOLOGY

Jack D. Farmer
(NASA Ames Rsearch Center)


INTRODUCTION

Since Viking, Mars Exobiology has shifted focus to include the search for evidence of an ancient biosphere. The failure of the Viking biology experiments to detect life at the two sites sampled has been broadly accepted as evidence that life is probably absent in surface environments, a view consistent with the lack of liquid water, the high UV flux, and oxidizing conditions observed in the surface soils sampled. The detailed hydrological history of Mars is unknown. But in broad outline, it appears that clement conditions for life probably existed at the surface of Mars early in its history, particularly during the time that widespread valley networks were formed within the ancient cratered highland terranes of the southern hemisphere (Carr 1996). Crater ages suggest that this period of surface water occurred toward the end of late bombardment, perhaps 3.5-4.0 Ga, during the time that life was emerging on the Earth (McKay and Stoker 1989).

Despite the fact that present surface conditions are inhospitable to life, it is quite plausible that life may yet persist in subsurface environments on Mars where liquid water could be present owing to higher temperatures and pressures (Boston et al., 1992). However, such environments are unlikely to be explored prior to manned missions, perhaps decades hence. Thus, if life ever arose on Mars, we will likely discover evidence of its former presence in the rock record at the surface long before we are able to drill for liquid water perhaps hundreds to thousands of meters beneath the surface. Clearly, the exploration for a fossil record of life on Mars requires a much different strategy than the search for extant life (see Kerridge et al. 1995), and this strategy is presently embodied in Mars Exopaleontology, a new subdiscipline of geology that borrows its scientific heritage from Precambrian paleontology, microbial ecology, biosedimentology, biogeochemistry and Mars surface science (Farmer 1995).

The strategy for Mars Exopaleontology is founded on a few basic principles gleaned from studies of the Precambrian fossil record, as well as studies of fossilization processes in modern environments on Earth that are regarded to be good analogs for the early Earth and Mars. Such studies are revealing not only the ways in which biological information is captured and preserved in sediments, but also suggest optimal methods for extracting biological information from ancient rocks on Earth or returned from Mars.

It is noteworthy that even if life never developed on Mars, the prebiotic organic chemical record preserved there is an equally important scientific objective for Exopaleontology. The absence of a plate tectonic cycle on Mars suggests that old geologic terranes may be much better represented there. The destructuve processes of burial metamorphism are likely to be much less a problem on Mars, although impact metamorphism and brecciation of the surface have undoubtedly overprinted the early record to some extent. But, the prebiotic chemical record found on Mars, of vital importance in understanding the origin of life on Earth, is likely to be much better preserved, and may provide access to a record of prebiotic proceeses long ago destroyed on our own planet.

On Earth, >98% of all the organic carbon fixed by organisms is destroyed and recycled. The small amount of organic carbon that escapes recycling persists in the crust because it is rapidly buried in fine-grained low permeability sediments, and isolated from destructive biocehemical processes. This organic carbon reservoir, makes up the chemical portion of the fossil record, preserving biological information as a variety of organic biomarker compounds (e.g., hopanes, the degradation products of cell wall lipids) and isotopic signatures (e.g., characteristic carbon isotope ratios reflecting biological fractionation processes).

The preservation of organic carbon occurs under a very restricted set of geologic environments and conditions which are fairly well understood on Earth. But even where organic compounds are destroyed by oxidation, biosignatures may yet persist in sedimentary rocks as fabrics produced by microorganisms (e.g., mesoscopic features like stromatolites or related biolaminated sediments, and characteristicv microfabrics contained therein), or "biominerals" (e.g., carbonates or phosphates formed as the byproduct of various physiological processes).

A basic tenet that has emerged from paleontological studies is that the long-term preservation of biological information as fossils is favored in environments where aqueous minerals precipitate rapidly from aqueous solutions, or where fine-grained, clay-rich detrital sediments accumulate very rapidly, entombing living organisms or their byproducts, before they can be degraded (Farmer and Des Marais 1995). The most favorable aqueous minerals are those that form fine-grained, impermeable host rocks that form a closed chemical system, isolating organic materials from oxidation. Favorable host minerals are also those that are chemically and physically stable and resistant to major fabric reorganization during diagenesis.

The most favorable host minerals for the long-term preservation of organic materials are those with long crustal residence times. These tend to be minerals that are most resistant to chemical weathering. High priority minerals in this category include silica, phosphates, clays, Fe-oxides and carbonates. Not coincidentally, such compounds are also the most common host minerals for the microbial fossil record on Earth, and the classic microbiotas of the Precambrian are almost exclusively preserved in such lithologies. Other aqueous minerals, including a wide variety of evaporite minerals (salts), and even ice, also provide excellent media for preserving microorganisms. However, an important caveat with these classes of minerals is that residence times in the Earth's crust tend to be quite short (100's of Ma for evaporites owing to dissolution and 100's of thousands of years for ice due to long-term climatic warming). However, it is quite likely that residence times for aqueous mineral deposits on Mars will be different. The hydrological cycle on Mars appears to have died very early and evaporites may yet persist there as surficial deposits in paleolake basins. But the chaotic obliquity of Mars suggests that the present Martian chryosphere is likely to be very young owing to periodic global warming and therefore unlikely to hold evidence of an early biosphere . Nevertheless, the present Martian ice caps could be an important source of information about extrinsic inputs of organics (e.g., IDP's or commetary impacts) during the recent history of the planet.

The basic criteria outlined above suggest that the long-term preservation of a fossil record on Mars is likely to have occurred in a comparatively small number of geologic environments. The oldest terranes on Mars, those formed during the early wet period, offer the greatest interest for Exopaleontology. However, the discovery of favorable paleoenvironments on Mars will require a more detailed knowledge of the surface geology and mineralogy of the Martian surface. Unfortunately, we have yet to determine the mineralogy of Martian surface or even identify one aqueous mineral deposit there with any certainty. Thus, a first step in implimenting a strategy for Mar Exopaleontology is the identification of aqueous mineralogies on the surface.


HIGH RESOLUTION ORBITAL IMAGING IN '01

As noted above, perhaps the most basic requirement for implimenting a strategy to explore for an ancient biosphere on Mars is the identification of key geologic environments and aqueously-deposited mineralogies from orbit (Kerridge et al., 1995). In order for the proposed '05 sample return to legitimately address the concerns of Exopaleontology, rock samples of appropriate mineralogy should be returned from a yet to be identified high priority site in the southern highlands of Mars. Given the coarse spatial resolution of the Thermal Emission Spectrometer (TES) which will be flown in 1996 (3 km/pixel; Christensen et al., 1992), it will likely prove difficult to resolve the precise spatial location of target mineral deposits. In addition, at the scale mapped, each pixel of TES data is likely to involve a complex mixture of mineralogies, and deconvolution of discrete mineral spectra may likewise prove difficult to impossible. Obviously, the solution to such problems is higher spatial resolution data from orbit or high altitude. Therefore, in order to optimize site selection for samples of exopaleontological interest during the '05 sample return, high resolution (100 m/pixel) compositional mapping is deemed essential.

It is unlikely that a mid-IR orbital instrument that can achieve an acceptable signal to noise ratio within existing cost/weight guidelines, and therefore it is recommended that high spatial resolution data be obtained using a near-IR (1-5 um range) hyperspectral (10 nm bandwidths) imaging system to create maps of high priority target areas for future landed missions. The technologies needed to accomplish this task are relatively mature, because near-IR mineral mapping is a standard exploration tool in the minerals industry. High resolution data obtained from orbit will not only provide a basis for detailed site studies for future landed missions, but will also yield valuable information that will assist the interpretation of data obtained by the Thermal Emission Spectrometer during its global mapping exercise (e.g. deconvolution of mineral spectra and precise spatial location of deposits of interest)

In order that high resolution mineralogical mapping be able to assist in site selection for an '05 sample return, it should be obtained during the '01 opportunity. However, we do not recommend the substitution of an orbital for landed mission in '01, because a lander in '01 will be needed to prepare for landed science in both '03 and '05.


BALLOON MISSIONS

A high priority mission proposal that presently lies outside of the MGS Program is a mid-latitude aerobot/balloon mission to Mars that would carry a high spatial resolution mid-IR spectral imager. This would facilitate the mapping of surface mineralogy at the desired100 m/pixel spatial resolution, while providing highly resolved spectral data to assist in interpreting the global TES data set. Optimally, the spectral range of this instrument should be in the 5-12 um range where many fundamental vibrations of high priority aqueous minerals (e.g., carbonates, silica, evaporites) can be detected. It would be preferable from the standpoint of Exopaleontology to deploy such a mission over southern highland terranes at a latitude that would transect several high priority targets.


LANDED MISSIONS

Obviously, exobiological site recommendations for future landed opportunities will necessarily reflect a balance of programmatic goals. But to achieve maximum science return for Exopaleontology, certain milestones (listed below) should be met iduring precursor landed missions in '01 and '03.

Mobility: The rover in '03 should be capable of multiple km traverses during nominal mission times to provide access to a broad sampling of geologic targets at a site of exopaleontological interest.

Sample Selection: Rovers for '01 to '05 should be able to survey rock fields and pre-select individual target rocks for in situ analysis and (in '05) sample return based upon mineralogy. This capability will require high resolution visible range cameras and a rover-mounted (preferably mid-) IR spectrometer.

Microscopic Imaging: Once targets have been identified, rovers should be able to image weathered and fresh rock surfaces at 'hand lens' magnifications (0.1 mm resolution) in order to visualize microtextures of rocks. Optimally, ilumination systems for rover hand-lenses should deliver visible, infrared and UV wavelengths to assist in textural and compositional evaluation. UV could be particularly valuable because many minerals and organic materials autoflouresce and exhibit unique spectral signatures.

Access to Rock Interiors: Rovers should have ability to access rock interiors by exposing fresh surfaces either through breakage or abrasion. This capability is regarded as a key requirement for all analytical tools that seek to evaluate composition.

In Situ Mineralogical Analysis: Although elemental analysis is regarded as important for exopaleontology in providing information about the biogenic elements, in order to address the important issues of Exopaleontology instrument payloads must also provide information about molecular structures that will lead to an understanding of mineralogy. Key technologies foe mineralogical analysis include more qualitative tools like IR spectroscopy, and Laser Raman which operate in a reflected energy mode, and more definitive methods, such as Xray Diffraction which, using the optimal transmission mode geometry, require a powdered sample.

Redox Analysis: In order to optimize the recognition of samples likely to preserve organic matter, instrumentation should provide capability for determining oxidation state of iron in potential rock samples. The various mineralogical instruments described above provide important tools for evaluating redox, and Mossbauer is especially effective for iron-bearing minerals.


SITE SELECTION

Because only selected areas will be mapped at high resolution during upcoming orbital missions, targets for high resolution imaging should include high priority sites for Exopaleontology. A catalog of such sites is being assembled to assist mission planners (e.g., Farmer et al., 1994). Although there are a number of geological sites of potential exopaleontological interest (Farmer and Des Marais 1994), the most easily identified targets from orbit are most likely to be those within ancient paleolake basins in the southern highland terranes of Mars (e.g., Farmer et al. 1995). Such sites may provide access to a variety of aqueous mineralogies, including fine-grained, aqueously-deposited detrital sediments (e.g. claystones and shales) and mineral precipitates such as evaporite deposits, spring-deposited carbonates, and more broadly-distributed sedimentary cements.

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REFERENCES:

Boston, P.J., M.V. Ivanov, C.P. McKay. 1992. On the possibility of chemosynthetic ecosystems in subsurface habitats on Mars. Icarus 95: 300-308.

Carr, M.H. 1996. Water on Mars. Oxford Univ. Press 229 p.

Christensen, P. R., D. L. Anderson, S. C. Chase, R. N. Clark, H. H. Kieffer, M. C. Malin, J. C. Pearl, J. Carpenter, N. Bandiera, F. G. Brown and S. Silverman 1992. Thermal emission spectrometer experiment: Mars Observer Mission. Journal of Geophysical Research 97, 7719-7734.

Farmer, Jack D. 1995. Mars Exopaleontology. Palaios 10 (3): 197-198.

Farmer, J., Des Marais, D., Greeley, R., Landheim, R. and Klein, H. 1995. Site selection for Mars Exobiology. Advances in Space Research 15(3): (3)157-(3)162.

Farmer, J., and D. Des Marais. 1994. Exopaleontology and the search for a fossil record on Mars. Lunar Planetary Science 25: 367-368.

Farmer, J., Des Marais, D., Klein. 1994. Mars Site selection for Exobiology: Criteria and Methodology. pp. 11-16. In Greeley, R. and P. Thomas (ed.). Mars Landing Site Catalog. (Second Edition) NASA Reference Publication 1238, 392 p.

Kerridge, J. (ed.) 1995. An Exobiological Strategy for Mars Exploration. NASA Special Publication 530, 55 pp.

McKay, C.P. and C.R. Stoker. 1989. The early environment and its early evolution on Mars: Implications for life. Rev. Geophys. 27, 189-214.

JD Farmer, Mars Sample Return Workshop, March 1996