There have been numerous biosignature studies of terrestrial environments that have been considered analogs for Mars, particularly early in the planet’s history. Such studies have helped refine strategies for the astrobiological exploration of Mars, while defining new approaches for life detection.
Studies of hydrothermal springs over a broad range of pH and mineralogy (including siliceous,4,5,6 travertine,7,8 and iron oxide-precipitating systems,9,10) have shown that biosignatures of thermophiles are commonly captured and preserved in both surface and subsurface hydrothermal deposits.11 In addition, a variety of studies have revealed good organic preservation in deposits of low-temperature surface springs and streams, over a broad range of pH from acidic12,13,14 to alkaline.15,16 Studies of modern and ancient evaporite deposits have also shown that microorganisms and their remains are commonly entrapped in a variety of salts, particularly sulfates and halides, both as solid inclusions and within fluid inclusions (Figure 4.1; see also Figure 6.2).17,18,19,20,21,22 In addition, studies of sedimentary rocks that have experienced significant diagenesis, or alteration under low- to medium-grade metamorphism, also retain fossil organic materials (e.g., kerogen).23 Microbial biosignatures are not restricted to surface deposits but have also been described from mineralized subsurface fractures and other void spaces in subsurface volcanic rocks.24,25 Finally, glacial ice and permafrost have been shown to harbor a broad range of extant and dormant life forms, as well as their cryopreserved fossil remains within water and brine-filled voids.26,27
The recent discovery of sulfate-rich evaporite deposits by the Opportunity rover (see Figure 2.2) and both Mars Express and the Mars Reconnaissance Orbiter have elevated scientific interest in evaporite deposits as potential targets for a martian fossil record (Chapter 2). Similarly, recent orbital detections of phyllosilicates at many locations on Mars28 have stimulated interest in Mars-analog studies of clay-rich sedimentary systems, which on Earth often provide favorable conditions for the preservation of organic materials.29,30 Hydrated forms of silica have been shown to precipitate in a variety of aqueous settings, ranging from hydrothermal springs to low-temperature weathering environments. In terrestrial hot springs, silica has been shown to be a particularly favorable medium for preserving organic remains.31 Recent detections of amorphous hydrated silica (opal) by the Spirit rover32 and