Exobiology in the Solar System & The Search for Life on Mars - ESA

Table II.4.3/1. Environmental Conditions on Mars dur<strong>in</strong>g Different Climatic
Sett<strong>in</strong>gs.
Icehouse periods Greenhouse
Duration long short
Atmosphere th<strong>in</strong> thick
Water <strong>in</strong> subsurface at <strong>the</strong> surface
Biota bacterial activity <strong>in</strong> microbiota and algae
ground ice at <strong>the</strong> surface
dur<strong>in</strong>g several episodes throughout martian history; possibly it was present <strong>in</strong> epochs
as young as <strong>the</strong> Middle Amazonian. On <strong>the</strong> o<strong>the</strong>r hand, a large number of features
suggest <strong>the</strong> <strong>in</strong>termittent presence of water (both liquid and solid) on <strong>the</strong> surface.
Outflow channels have been active <strong>in</strong> Late Hesperian time (Maja and Ares Valles), <strong>in</strong>
Early Amazonian ( Kasei and Mangala Valles) and probably <strong>in</strong> Middle Amazonian
(channels of Elysim). Lacustr<strong>in</strong>e deposits show <strong>the</strong> same age ranges.
It is quite probable that water, released episodically from <strong>the</strong> subsurface, <strong>for</strong>med
large aqueous environments on <strong>the</strong> surface. <strong>The</strong> duration of <strong>the</strong>se events is hard to
def<strong>in</strong>e. Judg<strong>in</strong>g from <strong>the</strong> extensive feature described above, <strong>the</strong> surface liquid water
rema<strong>in</strong>ed active <strong>for</strong> a considerable geological time, allow<strong>in</strong>g <strong>the</strong> <strong>for</strong>mation of welldef<strong>in</strong>ed
morphologies. In order to susta<strong>in</strong> such a complex hydrological system, an
atmosphere entirely different from today’s must be envisaged.
If life developed on Mars, it is possible it happened be<strong>for</strong>e 3.5 Gyr, dur<strong>in</strong>g <strong>the</strong>
warmer and wetter phase (Carr, 1995). <strong>The</strong> atmospheric conditions at that time were
suitable <strong>for</strong> liquid water on Mars and life could have developed, as occurred <strong>the</strong>n on
Earth (Hansson, 1997). After 3.5 Gyr, <strong>the</strong> climatic conditions were unsuitable <strong>for</strong>
surface liquid water and consequently <strong>for</strong> <strong>the</strong> development and evolution of life.
Water has been reta<strong>in</strong>ed <strong>in</strong> <strong>the</strong> subsurface as permafrost. However, dur<strong>in</strong>g particular
events, which may be triggered by an <strong>in</strong>crease of <strong>the</strong> heat flux <strong>in</strong> <strong>the</strong> planet <strong>in</strong>terior,
permafrost melted and water was released at <strong>the</strong> surface. <strong>The</strong> evidence of water at <strong>the</strong>
surface <strong>for</strong> remarkable time spans suggests that <strong>the</strong> climatic conditions changed,
allow<strong>in</strong>g <strong>the</strong> <strong>for</strong>mation of large water bodies. <strong>The</strong>se phases can be described as
greenhouse periods with a thick atmosphere ma<strong>in</strong>ta<strong>in</strong><strong>in</strong>g <strong>the</strong> water liquid. <strong>The</strong>se wet
periods contrast with <strong>the</strong> dry conditions when <strong>the</strong> atmosphere was th<strong>in</strong> (icehouse
epochs). A sort of cyclicity may be envisaged (Baker et al., 1991), with long-last<strong>in</strong>g
periods of a dry Mars with a th<strong>in</strong> atmosphere and water <strong>in</strong> <strong>the</strong> subsurface, followed
by a short-term cycle of wet Mars, with a thick atmosphere and large stand<strong>in</strong>g bodies
of water. See Table II.4.3/1.
If life was present dur<strong>in</strong>g <strong>the</strong> planet’s early history (be<strong>for</strong>e 3.5 Gyr), <strong>the</strong> dramatic
change <strong>in</strong> climatic and atmospheric conditions that dried <strong>the</strong> planet must have affected
<strong>the</strong> life. That probably <strong>the</strong>n followed <strong>the</strong> fate of <strong>the</strong> water by enter<strong>in</strong>g <strong>the</strong> subsurface.
<strong>The</strong>re<strong>for</strong>e, dur<strong>in</strong>g <strong>the</strong> icehouse period <strong>the</strong> biota (if present) were restricted to <strong>the</strong> water
and ice layer underground. On Earth, <strong>the</strong> permafrost (<strong>the</strong> subsurface ice) is a place
where bacterial activity flourishes, and possibly this could be used as an analogue <strong>for</strong>
<strong>the</strong> martian permafrost. <strong>The</strong> release of water at <strong>the</strong> surface will br<strong>in</strong>g large quantities
of bacteria and associated life<strong>for</strong>ms back to <strong>the</strong> surface and, more importantly, it br<strong>in</strong>gs
large quantities of methane (produced by <strong>the</strong> bacteria) <strong>in</strong>to <strong>the</strong> atmosphere.
<strong>The</strong> energy driv<strong>in</strong>g <strong>the</strong> geophysical, geochemical and, eventually, <strong>the</strong> biological
processes of an <strong>in</strong>ner planet is predom<strong>in</strong>antly of solar orig<strong>in</strong>, except <strong>for</strong> a m<strong>in</strong>or
portion com<strong>in</strong>g from <strong>the</strong> planet’s <strong>in</strong>terior. Depend<strong>in</strong>g on its distance from <strong>the</strong> Sun, its
team I: exobiology and <strong>the</strong> mars surface environment/II.4
II.4.4 <strong>The</strong> Radiation
Environment
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