Iron oxides. Iron oxides are common minerals formed at the surface of Earth and Mars that are likely to enrich for specific types of organic molecules.3 Organic acids may be enriched on the surface of these particles. Furthermore, organics adsorbed to iron oxides may be stable in low-pH environments.
Clays. Clays preferentially adsorb organic molecules at both their surface and within the interlayer. Absorption can occur in low- or high-pH systems.
The processes that lead to preservation of biosignatures on Earth are far from fully understood. Although researchers expect their martian counterparts to follow the same general sets of processes, the conditions on Mars that may have led to biosignature preservation are poorly constrained, and hence, the matrix of target environments is necessarily varied, including sites and samples with an uncertain potential for success.
A strategy to investigate martian biosignatures may focus on sites where the protection of biomolecules from degradation is expected. Three types of systems and specific sites are discussed below:
Hydrothermal deposits. Because hot springs are commonly characterized by mineral supersaturation and deposition, they would be good target sites for preservation of biosignatures. However, such hydrothermal systems have not been definitively identified on Mars via remote sensing or in situ imaging. Nonetheless, proposed sites of hydrothermal activity include areas near impact craters, slopes of volcanic structures such as Hecates Tholus and Ceraunius Tholus,4 and locations where ice melting may be occurring or have occurred.5,6 In addition, rift systems, such as the Cerberus Fossae, may have erupted both water and lava.7 These locations may also be sites favorable for the process of rapid mineralization and preservation of biomolecules.
Evaporites. Evaporation is known to occur on Mars, as evidenced by the sulfate-rich deposits analyzed by the rover Opportunity in Meridiani Planum8 and by detection from orbit of several thick deposits of sulfate minerals such as kieserite and gypsum, particularly in Valles Marineris.9 Evaporite deposits might be likely sites for entombed organic particles.
Concentrations of concretions. Concretions (termed “blueberries”) are well documented at Meridiani from investigations by the rover Opportunity.10 The dominant mineral of these concretions is hematite, an iron oxide that may have nucleated on organic centers and/or may have precipitated rapidly from concentrated solutions. These concretions have been concentrated as a lag deposit on the surface, where the more soluble evaporitic salts were removed by weathering processes. Sites with concretions should be considered as potential sites for biosignatures.
Sites of interest include carbon-bearing rocks and soils, layered sedimentary rocks, clays, weathered terrains, and iron oxide deposits.
Carbon-bearing rocks and soils. Deposits enriched in organic carbon are prime targets for detailed analysis, either with measurements in situ or on returned samples. Organic biomarker studies on Earth typically require carbon-rich samples, and there is no reason to believe that a strategy for Mars would be different. However, to date, no carbon-rich materials have been identified on Mars.
Layered sedimentary rocks. Ample evidence suggests that both surface water and groundwater existed on Mars, and that sediments accumulated from flowing surface waters and from lakes or oceans. Sedimentary rocks on Mars are found with various scales of stratification ranging from tens of meters (Figure 4.2) to very fine laminations visible in images from the rover Opportunity. Where deposits of sediments occurred in lakes or in a global ocean, biomolecule enrichment could have occured. Possible examples are deltas in the Holden crater.11