prebiotic chemistries. Of these potential sites to preserve biosignatures, the most likely sites are sediments, evaporites, and hydrothermal systems based upon observations of terrestrial rock systems.
The committee approaches a consideration of martian sites from a process-oriented perspective and discusses first the processes most likely to generate and preserve biosignatures. As pointed out in Chapter 3, biosignatures can be molecular, isotopic, morphological, mineralogical, or elemental in nature. This section finishes with a discussion of the characteristics of specific sites where biosignatures of past life might be found.
Entombment involves processes characterized by rapid mineralization which preserve microorganisms and organic molecules against degradation. Such processes are especially important for reduced compounds preserved in highly oxidizing environments. Examples of processes leading to entombment include the following:
Evaporation. Evaporation leads to mineralization driven by increases in solute concentrations as water is removed. Minerals often crystallize on nuclei provided by microbial cells or organic particles. The relatively rapid precipitation that can occur in evaporative settings often captures particles and compounds within the mineral matrix. In some cases, such entrapped particles can provide evidence of biogenicity.
Freezing. Concentration of brine solutions by freezing is also a likely process of evaporite salt formation and entombment of particles. Its effectiveness as a preservation mechanism may be enhanced by the low temperatures experienced on Mars.
Temperature and pressure changes. Supersaturation due to cooling often leads to precipitation in and around hot springs. Supersaturation can also occur because of changes in pressure. The rate at which precipitation occurs can be rapid enough to entomb living cells or to protect biomolecules. For example, some micro- and macrostructures in and around hot spring deposits show morphological relationships that are uniquely biogenic.1 The precipitation that occurs owing to supersaturation can also occur in the subsurface where hydrothermal flow is driven by a buried heat source. The emplacement of igneous intrusives such as dikes infiltrating sedimentary rocks can generate subsurface hydrothermal flow and enhance mineralization reactions. For example, chert, a common authigenic mineral formed by hot springs in igneous terrains, can often preserve biosignatures.
Diffusion-driven reactions. Concretions are precipitates that occur within a stratigraphic horizon in response to variations in the concentrations of solutes. While the precipitation of the concretion is controlled by the degree of supersaturation and the diffusional gradients, the kinetics of concretion formation are not well known. Precipitation processes related to concretions can entomb potential biosignatures.
Enrichment processes include all phenomena that lead to enhanced concentrations of compounds or particles. The search for biosignatures on Mars may be limited to sites with local enrichment of biosignatures, given that analytical equipment may be hampered by detection limits. Sedimentary rocks in general often enrich biomarkers on Earth, while iron oxides and clays, wherever they are found, can incorporate organic molecules.
Sedimentary rocks. Sedimentation may enhance biosignature preservation in a number of ways. Sediments can provide an environment in which biomass accumulates and is buried. When sedimentation occurs as a pelagic process, settling particles may concentrate microbes that are scavenged from the water column.2 Sediments may also preserve morphological biosignatures: Trace fossils, wrinkle marks, stromatolites, and microbialites are identifiable sedimentary forms thought to involve the interaction between sediments and microbiota.