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to the conditions found on the surface of Mars. Some teams determined that some
bacteria may be capable of cellular replication down to 25 mbar, but that is still
above the atmospheric pressures found on Mars (range 1-14 mbar). In another study,
26 strains of bacteria were chosen based on their recovery from spacecraft assembly
facilities, and only
Serratia liquefaciens
strain ATCC 27592 exhibited growth at 7 m
bar, 0
ı
C, and CO
2
-enriched anoxic atmospheres.
8.3
Water on Mars
Water on Mars exists today almost exclusively as ice, with a small amount present
in the atmosphere as vapor. The only place where water ice is visible at the surface
is at the north polar ice cap (Carr
1996
). Abundant water ice is also present beneath
the permanent carbon dioxide ice cap at the Martian south pole and in the shallow
subsurface at more temperate latitudes (Feldman et al.
2004
). More than five million
cubic kilometers of ice have been identified at or near the surface of modern Mars,
enough to cover the whole planet to a depth of 35 m (Christensen
2006
). Even more
ice is likely to be locked away in the deep subsurface.
Some liquid water may occur transiently on the Martian surface today but only
under certain conditions (Hecht
2002
). No large standing bodies of liquid water exist
because the atmospheric pressure at the surface averages just 600 Pa (0.087 psi) -
about 0.6 % of Earth's mean sea level pressure - and because the global average
temperature is far too low (210 K (
63
ı
C)), leading to either rapid evaporation
or freezing. Before about 3.8 billion years ago, Mars may have had a denser
atmosphere and higher surface temperatures (Pollack
1987
), allowing vast amounts
of liquid water on the surface, possibly including a large ocean that may have
covered one-third of the planet. Water has also apparently flowed across the surface
for short periods at various intervals more recently in Mars' history. On December
9, 2013, NASA reported that, based on evidence from the Curiosity rover studying
Aeolis Palus, Gale Crater contained an ancient freshwater lake which could have
been a hospitable environment for microbial life.
Many lines of evidence indicate that water is abundant on Mars and has played
a significant role in the planet's geologic history. The present-day inventory of
water on Mars can be estimated from spacecraft imagery, remote sensing techniques
(spectroscopic measurements, radar, etc.), and surface investigations from landers
and rovers. Geological evidence of past water includes enormous outflow channels
carved by floods; ancient river valley networks, deltas, and lake beds; and the detec-
tion of rocks and minerals on the surface that could only have formed in liquid water.
Numerous geomorphic features suggest the presence of ground ice (permafrost) and
the movement of ice in glaciers, both in the recent past and present. Gullies and slope
lineae along cliffs and crater walls suggest that flowing water continues to shape the
surface of Mars, although to a far lesser degree than in the ancient past.
Although the surface of Mars was periodically wet and could have been
hospitable to microbial life billions of years ago, the current environment at the
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