The Central Role of Water
Central to the search of recent environments for microbial lifeforms on Mars or any terrestrial body is the need for liquid water. Currently, aquifers on Mars are immobilized and frozen in the subsurface down to depths that can reach several kilometers.
One hypothesis is that life sought refuge down to these depths and is still thriving and active where the geothermal flux and a pressure allow water to stay liquid, developing comparable survival skills to that of the terrestrial subsurface microbial communities.
Another hypothesis is that living organisms are trapped in dormant state in the upper levels of the Martian regolith where the aquifer is permanently frozen. Terrestrial microbial communities in permafrost are capable of remaining viable for one million years in Pliocene permafrost of the Siberian Arctic and Antarctic Dry Valleys region. Cold-adapted bacteria from Siberian permafrost can incorporate acetate into lipids (vital for membrane integrity) in the laboratory at - 20°C, at which temperature they divide with a doubling time of 160 days. The continuous availability of liquid water for such prolonged periods is unlikely in the field. However, permafrost bacteria can remain dormant pending stochastic, seasonal, or cyclical favourable conditions. These permit opportunistic activity and reproduction before another phase of dormancy.
This is well illustrated by microbial communities up to 8 mm inside translucent sandstone in the Trans-Antarctic Mountains, whose carbon-turnover time (from C-fixation by photosynthesis to release as carbon dioxide by respiration) is about 10,000 years. This activity is entirely dependent on snowfall as a sporadic input of moisture. If analogous microbial communities had developed on Mars and managed to survive sheltered below the surface until today, plausible conditions for aquifer destabilization leading to liquid water circulation could provide a potential habitat for a biotic reservoir.
A source of thermal energy is important for maintaining liquid
water, even at sub-zero temperatures. Not only is the thermal receipt
from solar radiation into the surface important, but so is hydrothermal
activity for subsurface environments. Sources of hydrothermal energy
can originate both from recent volcanic activity and large impact craters.
Both these sources are discontinuous in time, but they may be of
sufficient duration for episodic metabolism and reproduction by
opportunistic microbial communities.
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