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gas produced can be an important energy source for chemosynthetic organisms or it
can react with CO
2
to produce methane gas, a process that has been considered as
a non-biological source for the trace amounts of methane reported in the Martian
atmosphere. Serpentine minerals can also store a lot of water (as hydroxyl) in
their crystal structure. A recent study has argued that hypothetical serpentinites in
the ancient highland crust of Mars could hold as much as a 500-m-thick global
equivalent layer (GEL) of water. Some serpentine minerals have been detected on
Mars, and widespread outcroppings are evident from remote sensing data from
NASA's Mars Reconnaissance Orbiter spacecraft. McLaughlin Crater, one of the
deepest craters on Mars, contains evidence for Mg-Fe-bearing clays and carbonates
that probably formed in an alkaline, groundwater-fed lacustrine setting (Michalski
et al.
2013
). This fact could hypothetically indicate the presence of large amounts
of serpentinite hidden at depth in the Martian crust. Deposits formed as a result of
groundwater upwelling on Mars, such as those in McLaughlin Crater, could preserve
critical evidence of a deep biosphere on Mars (Michalski et al.
2013
)andmaybea
good place to look for signs of life that pooled there from underground (Fig.
8.3
).
8.3.1.3
Weathering Rates
The rates at which primary minerals convert to secondary aqueous minerals vary.
Primary silicate minerals crystallize from magma under pressures and temperatures
vastly higher than conditions at the surface of a planet. When exposed to a surface
environment, these minerals are out of equilibrium and will tend to interact with
available chemical components to form more stable mineral phases. In general,
the silicate minerals that crystallize at the highest temperatures (solidify first in a
cooling magma) weather the most rapidly. On Earth and Mars, the most common
mineral to meet this criterion is olivine, which readily weathers to clay minerals in
the presence of water. Olivine is widespread on Mars, suggesting that Mars' surface
has not been pervasively altered by water; abundant geological evidence suggests
otherwise.
8.3.2
Evidence of Water in Martian Meteorites
Over 60 meteorites have been found that came from Mars. Some of them contain
evidence that they were exposed to water when on Mars. Some Martian meteorites
called basaltic shergottites appear (from the presence of hydrated carbonates and
sulfates) to have been exposed to liquid water prior to ejection into space. It has
been shown that another class of meteorites, the nakhlites, were suffused with liquid
water around 620 Ma ago and that they were ejected from Mars around 10.75 Ma
ago by an asteroid impact. They fell to Earth within the last 10,000 years.
In 1996, a group of scientists reported the possible presence of microfossils in
the Allan Hills 84001, a meteorite from Mars. Many studies disputed the validity
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