Vision and Voyages for Planetary Science in the - Solar System ...

characteristics (water, gradients in heat, chemicals, and light, and oxidant supply) that make so many
environments habitable also cause them to be destructive to biosignature preservation. There are,
however, habitable environments with geochemical conditions that favor very early mineralization that
facilitate spectacular preservation. Authigenic silica, phosphate, clay, sulfate, and less commonly,
carbonate precipitation are all known to promote biosignature preservation. 10 The search for
environments that have been both habitable as well as favorable for preservation can be optimized by
pursuing an exploration strategy that focuses on the search for “windows of preservation”, remembering
that Mars may indeed have its own uniquely favorable conditions.
Assess the Past and Present Habitability of Mars
Understanding whether a past or present environment could sustain life will include establishing
the distribution of water, its geological history and the processes that control its distribution, identifing
and characterizing phases containing C, H, O, N, P and S, and determining the available energy sources.
Recent exploration has confirmed that the surface of Mars today is cold, dry, chemically
oxidizing, and exposed to intense solar ultraviolet radiation. These factors probably limit or even prohibit
any life near its surface, although liquid water might occur episodically near the surface as dense brines or
associated with melting ice. 11
Mars’s subsurface appears to be more hospitable. With mean annual surface temperatures close
to 215 K at the equator and 160 K at the poles, a thick cryosphere could extend to a depth of several
kilometers. Hydrothermal activity is likely in past or present volcanic areas, and even the background
geothermal heat flux could drive water to the surface. At depths below a few kilometers, warmer
temperatures would sustain liquid water in pore spaces, and a deep-subsurface biosphere is possible
provided that nutrients are accessible and water can circulate. 12
Biotic and abiotic pathways for the formation of complex organic molecules require an electron
donor closely coupled to carbon in a form suitable to serve as an electron acceptor. On Mars, igneous
minerals containing ferrous iron and/or partially reduced sulfur (e.g., olivine and pyrrhotite) are potential
electron acceptors for reduction of carbon. The report of methane in the martian atmosphere contends
that an active source is required to balance its destruction (its photochemical lifetime is