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

valley networks (Fig. II.2.5/2) are found all over <strong>the</strong> heavily cratered sou<strong>the</strong>rn terra<strong>in</strong>
but occasionally also on younger areas. <strong>The</strong>y are similar to terrestrial river valleys as
<strong>the</strong>y have tributaries and <strong>the</strong>ir widths <strong>in</strong>crease downstream. <strong>The</strong> networks extend
generally only over a few hundred kilometres, hence <strong>the</strong>y are much shorter than
terrestrial river systems. Water ice is unstable at <strong>the</strong> surface at low latitudes but may
be present at depths of a few to several hundred metres. <strong>The</strong> almost global presence
of lobate flows around impact craters larger than a few kilometres is taken as strong
evidence <strong>for</strong> <strong>the</strong> presence of ice at shallow depth at high latitudes and of ice or ground
water at greater depth at low latitudes. Under <strong>the</strong> present surface temperature
conditions, water can exist only as f<strong>in</strong>e water ice particles (Fig. II.2.7/1) or frost
(Fig. II.2.7/2).
<strong>The</strong> geochemical evidence suggests that <strong>the</strong> climate <strong>in</strong> <strong>the</strong> early days of Mars, be<strong>for</strong>e
about 3.5 Gyr ago, was wet and warm, allow<strong>in</strong>g <strong>the</strong> <strong>for</strong>mation of <strong>the</strong> observed
erosional features. It has been suggested that <strong>the</strong> temperature has been raised by a
strong greenhouse effect ow<strong>in</strong>g to a thick CO 2 atmosphere (Moroz & Mukh<strong>in</strong>, 1978;
Pollack et al., 1987). However, <strong>the</strong> required surface pressure of <strong>the</strong> order of 5-10 bar
would lead to <strong>the</strong> <strong>for</strong>mation of CO 2 ice clouds at high altitudes. <strong>The</strong>se would reflect
sunlight and also limit <strong>the</strong> amount of CO 2 resid<strong>in</strong>g <strong>in</strong> <strong>the</strong> atmosphere (Kast<strong>in</strong>g, 1991).
Hence, it seems necessary that <strong>for</strong> <strong>the</strong> time of a warm and wet climate <strong>the</strong>re were
greenhouse gases o<strong>the</strong>r than CO 2. Among o<strong>the</strong>rs, methane and ammonia have been
suggested.
In <strong>the</strong> light of <strong>the</strong> high sulphur content of <strong>the</strong> martian basalts (shergottites), SO 2 or
H 2S are also worth consider<strong>in</strong>g as possible additional greenhouse gases (Postawko &
Kuhn, 1986). <strong>The</strong> lifetime of <strong>the</strong> sulphur compounds <strong>in</strong> an atmosphere with traces of
water vapour is limited to a few months. Although it seems likely that SO 2 or H 2S
dom<strong>in</strong>ate <strong>the</strong> volcanic exhalations, that production rate appears to be too small <strong>for</strong> any
significant greenhouse effect. It has been argued, however, that SO 2 from volcanic
exhalation may have been stored <strong>in</strong> <strong>the</strong> <strong>for</strong>m of liquid or solid SO 2 tables at shallow
depth. A sudden release of <strong>the</strong> stored SO 2 by impact or volcanic eruption could <strong>the</strong>n
br<strong>in</strong>g enough <strong>in</strong>to <strong>the</strong> atmosphere <strong>for</strong> a substantial temperature rise that could,
supported by feedback effects, last hundreds of years (Wänke & Dreibus, 1994). <strong>The</strong>
large concentration of sulphur <strong>in</strong> <strong>the</strong> Vik<strong>in</strong>g soil, most likely as sulphates, may be <strong>the</strong>
<strong>the</strong> planet mars/II.2
Fig. II.2.5/2. An area about 200 km across of
valley networks on Mars. Although <strong>the</strong>y may
appear to resemble terrestrial dra<strong>in</strong>age systems,
<strong>the</strong>y lack small-scale feeder streams. It is
thought that <strong>the</strong>y might have been created by
groundwater flow ra<strong>the</strong>r than ra<strong>in</strong>water runoff.
Alternatively, because <strong>the</strong> valley networks are
conf<strong>in</strong>ed to relatively ancient regions, <strong>the</strong>y
might <strong>in</strong>dicate that Mars at one time had a
warmer and wetter climate. (NASA/Lunar &
Planetary Institute)
II.2.6 <strong>The</strong> Climate of Mars
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