Extant life at or near Mars’s surface would have to be able to endure present-day conditions of low temperature, low water activity, and intense radiation, as well as a lack of organic matter (unless methane is a factor). Such organisms may manifest only briefly, seasonally and in restricted zones where liquid water exists. Subsurface environments, such as aquifers, groundwater, and the sediments discussed above, would still be characterized by low temperature.

Cold, High, and Dry Environments

Several studies have been conducted in Earth environments with conditions closest to modern-day conditions on Mars, including high-altitude regions in South America (Altiplano) and Australia (Paralana Springs) that receive intense ultraviolet radiation, the Arctic (Svalbard/Spitsbergen-Arctic Mars), Antarctica’s Dry Valleys, the Atacama Desert, basaltic rocks in the cold, dry deserts of Idaho and Oregon, and geological terrains that contain Earth’s oldest rocks and earliest traces of life. The types of microorganisms associated with these environments span a range of phylogenies but are largely characterized by their ability to survive desiccation and, for those exposed at the surface, ultraviolet radiation. Organisms related to Deinococcus radiodurans are one example of the type of organism commonly found in many of these types of habitats.28 The ability to withstand the damaging affects of drying and radiation, often leading to similar types of cellular damage, is a common trait. In addition to yielding information on the occurrence and survival of extremophilic microorganisms, these environments have also been used to test various analytical and sample-handling instruments proposed for in situ applications on Mars. Continued studies in Mars surface analog environments are critical for new discovery and for testing biosignature techniques and biosignature preservation.

Subsurface Refugia

As noted above, the extreme oxidizing conditions and high radiation flux at the surface of Mars provide conditions that are generally not conducive to extant life or the preservation of biomarkers. Under this premise, subsurface habitats may be the most likely refuge for life on Mars and may also provide conditions favorable for the preservation of biomarkers.

Crystalline Rocks

Some of the more intriguing analogs are deep crystalline rocks where the presence of autotrophic microbial populations is supported solely by H2 generated via abiotic reactions such as weathering of Fe(II)-bearing silicates. The concept of a subsurface lithoautotrophic microbial ecosystem, or “SLiME,” was first described for the deep basalt aquifers within the Columbia River Basalt (CRB) of south-central Washington state. The CRB microbial communities were described by Stevens and McKinley as the first discovered that are completely independent of solar-derived energy.29 The importance of the result was not that this particular microbial ecosystem was independent of the Sun. Rather, it was the fact that communities can, in principle, exist independently of photosynthesis.

Indeed, the Stevens and McKinney results were subsequently challenged by Anderson and colleagues.30 However, the original results are now supported by more recent results reported by Chapelle and co-workers following studies at another site.31 The microbial communities associated with the CRB are numerically dominated by autotrophic microorganisms, bacteria capable of growth by oxidizing H2 and fixing CO2, including high populations of acetogens.32 Subsequent cultivation-independent molecular analysis revealed that Archaea also accounted for 1 to 2 percent of the population of the CRB.33 Due to the common occurrence of similar rocks on Mars—identified by their spectroscopic and morphological characteristics—and the likelihood of liquid water in the subsurface, SLiME is an attractive analog in the search for microbial life on Mars. This same basic concept has been extended from the CRB to hydrothermal waters circulating through igneous rocks in southern Idaho and to the deep groundwater of the Fennoscandian Shield. Radiolysis of water has been implicated in the millimolar concentrations of H2 observed in the groundwater of several Precambrian Shields. These concentrations are well



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