Because exploration strategies for past and present life are ... detailed study in Earth-based labs will probably be required for definitive answers (1). It is also.
Concepts and Approaches for Mars Exploration
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Strategies for the Astrobiological Exploration of Mars Jack Farmer (Arizona State University)
Introduction The search for evidence of past and present life and/or prebiotic chemistry has been identified as the primary focus of the current Mars Surveyor (MS) Program. In this context, recent exploration strategies have emphasized the need to explore three basic geological environments (1): A) sites of ancient surface water, B) sites of ancient subsurface water and C) sites of present subsurface water. In previous implementation strategies it has been generally assumed that if subsurface water exists on Mars today it will be located at a depth of several km (2). Access will require deep drilling that is beyond the capabilities of current robotic platforms (3). Logically, the exploration for deposits of ancient hydrological systems may be much easier and has, therefore, given priority. However, recent discoveries from the Mars Global Surveyor (MGS) mission have demonstrated that we still have a lot to learn about past and present Martian environments and the potentia l for life. Advances in our understanding of Martian surface topography, geomorphology (4) and composition (5), as well as in our knowledge of life in extreme environments on Earth (6,7), indicate the value of considering a broadly-based, flexible strategy that will balance elements of both Exopaleontology (the search for a fossil record) and Exobiology (the search for extant life). Because exploration strategies for past and present life are fundamentally different (8), it is appropriate to consider each separately before seeking to define a program architecture that will effectively combine both aspects during future robotic exploration. Exploring for a Fossil Record Important clues to guide the search for a evidence of an ancient Martian biosphere can be gleaned from studies of the fossil record on Earth, as well as modern geomicrobiological systems that are analogs for the early biosphere. The most basic requirement for life is widely regarded to be liquid water, which is a universal necessity for life as we know it. One underlying assumption is that if life ever started on Mars, it's origin and evolution was controlled by the distribution of liquid water. Hence, the current dictum of Mars exploration, "follow the water" (Carl Pilcher, personal comm. 2000). Tracing the past distribution of aqueous habitats is a necessary first step in implementing exploration for ancient life. This means identifying the distribution of aqueouslyformed sediments that are the most likely repositories for fossil information. But the search for ancient water is only a first step. Studies of the Precambrian fossil record and of microbial fossilization processes in modern Earth environments, indicate that preservation is common in only a few types of geologic settings (9). These environments typically share certain features in common, including: A) the rapid deposition of sediments under conditions favorable for life and B) the incorporation of organisms or their by-products into impermeable host sediments of stable mineralogy (e.g. silica, phosphate, or carbonate). The Archean fossil record on Earth is basically preserved in two types of sedimentary deposits: A) fine-grained, clay-rich detrital sediments and/or water-lain pyroclastics and B) finely crystalline chemical sediments (e.g. evaporite deposits of terminal paleolake basins, spring deposits, (inclusive of hydrothermal) and mineralized zones (hard-pans) within ancient soils). The most reliable basis for identifying the paleoenvironments cited above is mineralogy. Thus, mapping the distribution of aqueous mineral deposits is of paramount importance for implementing missions to explore for a record of past life (10). As noted, the systematic exploration for a Martian fossil record will depend critically upon locating accessible surface outcrops of aqueously-formed sediments which possess the preservational properties identified above. Logically, exploration should begin by mapping the distribution of high priority sedimentary deposits from orbit to provide the precursor information needed to target landed missions to the most favored sites for in situ exploration and sample return. This effort could be optimized by acquiring high spatial resolution remote sensing data over a broad range of wavelengths be able to identify discrete mineral signatures from complex
Concepts and Approaches for Mars Exploration
mixtures. To further reduce risk in site selection, orbital observations of high priority sites need to be confirmed by in situ surface exploration. Because the modern Martian regolith and surface soils do not provide favorable environments for the preservation of fossil biosignatures, landed missions need to be able to analyze rocks (10). The most definitive fossil biosignatures are typically microscale geochemical and/or microfabric features. Thus, sample return followed by detailed study in Earth-based labs will probably be required for definitive answers (1). It is also likely that discover fossil biosignatures, we will need to sample returns from several different types of sites (1). Given the science requirements, proper sample selection for return to Earth will be of paramount importance. This can be best accomplished by the use of properly equipped mobile science laboratories that can survey and sample a broad range of lithotypes at a site . The Athena payload has been optimized for this task. Crucial instrument capabilities needed for sample selection include in situ mineralogical analysis (e.g. by surface spectral methods), the ability to access unweathered interior rock surfaces (e.g. by coring or chipping) and capabilities for imaging rock surfaces at hand lens magnifications (x10-20). Exploring for Extant Life The exploration for extant Martian life must take a fundamentally different path than that just outlined for Exopaleontology (8). The crucial element of this strategy is to locate potentially habitable zones of liquid water in the Martian subsurface. Prior to deep (multiple km) drilling by humans it may be possible to access near-surface liquid water "oases" that could sustain life or prebiotic chemistry using surface robots. The potential for subsurface hydrothermal systems sustained by magmatic sources within the shallow crust (e.g. 11, 12) could provide for the convective upflow of subsurface water into the shallow cryosphere. Where recent upflows of subsurface water have replenished near surface aquifers, subsurface miroroorganisms or prebiotic chemistry may have been incorporated into shallow ground ice. Depending on the geologic context, such sites would provide direct robotic access to cryopreserved organic materials through shallow drilling. Locating the most favorable sites for such investigations could be undertaken initially from orbit by searching for spatially confined thermal anomalies and/or localized concentrations of vapor emissions (e.g. water and/or reduced gases like methane). Recent observations favor the presence of zones of basal melting within polar ice caps (13). This suggests an alternative site type, namely periglacial (marginal polar) environments where sub-glacial outfloods of the Icelandic type (14) may have occurred. These are logical targets for robotic exploration. Outflow features possibly formed this way have recently been identified adjacent to both polar caps on Mars, providing exciting new opportunities for future exploration.
References: (1) NASA, 1996. Mars Expeditions Strategy Group (D. McCleese, Chair) Unpublished report. (http://www.hq.nasa.gov/office/oss/mccleese.html); (2) Clifford, S.M. J. Geophys. Res. 98, 10973-11016, 1993; (3) NASA, Mars Deep Drilling Workshop (Briggs, G., Organizer), unpublished report, 1996. (4) Head III, James W. et al., Science 286, 2134-2137, 1999; (5) Christensen, P.R., et al., J. Geophys. Res., 105, 9623-9642, 2000; (6) Stevens, T.O., and J.P McKinley, Science 270, 450-454, 1996; (7) Siegert, Martin J., American Scientist 87:6, 1999; Karl, D.M., et al., Science 286, 2144-2147, 1999; (8) Farmer, J. D., Palaios 10 (3), 197-198, 1995; (9) Farmer J. D. and D. J. Des Marais, Journ. Geophys. Res. 104 (E11), 26,977-26,995, 1999; (10) Farmer, J.D., p. 58-65, In Julian Hiscox (Editor) The Search for Life on Mars, Special Issue, Brit. Interplanet. Soc., London, 1999; (11) Farmer, J.D., pp. 273-299, In G. Bock, and J. Goode (eds) Evolution of Hydrothermal Ecosystems on Earth (and Mars), John Wiley & Sons Ltd., Chichester, 1996; (12) Gulick, V., Journ. Geophys. Res., 103 (E8), 19,365-19,387, 1998; (13) Clifford, S.M., J. Geophys. Res. 92 (B9), 9135-9152, 1987; (14) Brandsdóttir, B., http://www.hi.is/~mmh/gos/photos4.html, 1996.
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