Science Priorities for Mars Sample Return

V.SAMPLES REQUIRED TO ACHIEVE THE SCIENTIFIC OBJECTIVES

V.A.Sedimentary materials rock suite.

Chemical precipitates formed under aqueous conditions could be used to constrain the role of water in Martian surface environment (e.g., Clark et al., 2005; Tosca et al., 2005). Precipitates could form within the water column and settle to the sediment surface or they could crystallize directly on the sediment surface as a crust. Any investigation that involves habitability, evidence of past or present life, climate processes, or evolution of the Martian atmosphere would be enabled by the acquisition of these rocks(Farmer and Des Marais, 1999). Some, but not all, chemical precipitates have interlocking crystalline textures with low permeability, potentially allowing preservation of trapped labile constituents such as organic compounds and sulfides (e.g., Hardie et al., 1985). Thus, intact samples of chemical precipitates would be critical for unravelling the history of aqueous processes, including those that have influenced the cycling of carbon and sulfur.

Siliciclastic sedimentary materials are moved as solid particles and are deposited when a transporting agent loses energy. Variation in grain size and textural structures at scales from millimeters to meters are important indicators of depositional processes and changing levels of energy in the environment (Grotzinger et al., 2005). Secondary mineralization of sedimentary materials is likely to be minimal if pores spaces are filled with dry atmospheric gases but is likely to be substantial if pore spaces are filled with fresh water or brine (McLennan et al., 2005). Sub-mm textures at grain boundaries are indicative of processes that have modified the sedimentary deposit.. Thus, individual samples of siliciclastic sedimentary materials would provide insights into transporting agents, chemical reactions, availability of water in surface environments, and the presence of currents or waves. A series of samples through a sedimentary sequence would provide critical insight into rates and magnitudes of sedimentary processes. Certain deposits such as chemically precipitated sediments, varved sediments, ice, etc. could provide insights into climatic cycles. Siliciclastic sedimentary materials are central to investigations involving past and present habitability and the evolution of the Martian surface. Fine-grained siliciclastic materials rich in phyllosilicates are likely to have low permeability, thus increasing the potential for preservation of co-deposited organic matter and sulfide minerals (Potter et al., 2005). Like chemical precipitates, samples of phyllosilicates that were deposited in aqueous environments would be critical for unravelling the carbon and sulfur cycle on Mars.

V.B.Hydrothermal rock suite

Another important aspect of the habitability of hydrothermal systems is the ready availability of nutrients. High temperature aqueous reactions leach volcanic rocks and release silica, Al, Ca, Fe, Cu, Mn, Zn and many other trace elements that are essential for microorganisms. Because hydrothermal fluids are rich in dissolved minerals, they create conditions favorable for the preservation of biosignatures, i.e., traces of the life forms that inhabit them. Although the organic components of mineralized microfossils can be oxidized at higher temperatures (>100°C), more recalcitrant organic materials (e.g., cell envelopes and sheaths) can be trapped and preserved in mineral matrices at lower temperatures (<35^oC; Cady and Farmer, 1996; Farmer, 1999), thus allowing chemical and isotopic analysis of organic biosignatures. Minerals implicated in the fossilization of hydrothermal microorganisms include silica, calcium carbonate and iron oxide.

Some of the earliest life forms on Earth might have inhabited hydrothermal environments (Farmer, 2000). Hyperthermophiles occupy the lowest branches of the tree of life (Woese et al., 1990). Indeed, hydrothermal vent environments, with their organic molecule-forming reactions, chemical disequilibria and high nutrient concentrations are considered as a possible location for the origin of life (Russell and Hall, 1996). However some would argue that the position of hyperthermophiles at the base of the tree of life is an artifact caused by the fact that such environments would have represented protected habitats during the late heavy bombardment period when a large part of the world ocean was probably volatilized (Sleep et al., 1989). But the fact that hydrothermal environments could serve as protected habitats in hostile conditions is relevant to the early history of Mars.

Recently, it has been suggested that the suites of minerals found at the surface of Mars (including silica and sulfates) could be related to hydrothermal/fumarolic activity (e. g. Bishop et al., 2002; Squyres et al., 2007; Yen et al., 2007; Squyres et al., Science, submitted). Hydrothermal activity is to be expected because volcanic activity has occurred at the surface within the last couple of million years, demonstrating that active heat sources still exist (Neukum et al., 2004). Hypothesizing that life arose on Mars and flourished at the surface during the first 500 My of its history, the gradual deterioration in surface conditions would have confined life forms beneath the surface, perhaps to be preserved in the cryosphere and elsewhere. Conceivably, life might have adapted to subsurface environments during the first 500 My and has persisted there since. The subsurface environment might have sustained only very low rates of productivity, but it is also the most stable environment and a potential haven for life during large impacts. Volcanic activity in the vicinity of the cryosphere would lead to active hydrothermal systems that, in some cases, might extend to the surface (Clifford, 1987).

The detection of hydrothermal activity on Mars is extremely significant since these environments could represent ideal habitats for microorganisms that obtain their carbon and energy from inorganic sources. They might host extant life as well as the fossilized traces of its ancestors. Returning intact samples of this lithology might be difficult for geologically recent material, which tends to be friable. It would therefore be very important to document the geologic context of such samples in case they do not survive the return trip whole.

Criteria for sample size, selection, and acquisition protocol would be the same as for the sedimentary suite. Examples of possible lithologies for the hydrothermal suite include samples from subsurface veins, fumarole deposits, surface spring deposits from vent areas to distal apron environments, as well as altered host rocks.