Geoscience Reference
In-Depth Information
8.2.2.2
Surface Brines
Although liquid water does not appear at the surface of Mars, several modeling
studies suggest that potential locations on Mars could include regions where thin
films of salty liquid brine or perchlorate may form near the surface that may provide
a potential location for terrestrial salt- and cold-loving microorganisms (halophilic
psychrophile). Various salts present in the Martian soil may act as antifreeze and
could keep water liquid well below its normal freezing point, if water was present at
certain favorable locations. Astrobiologists are keen to find out more, as not much
is known about these brines at the moment. The briny water may or may not be
habitable to microbes from Earth or Mars. Another researcher argues that although
chemically important, thin films of transient liquid water are not likely to provide
suitable sites for life. In addition, an astrobiology team asserted that the activity of
water on salty films, the temperature, or both are less than the biological thresholds
across the entire Martian surface and shallow subsurface.
The damaging effect of ionizing radiation on cellular structure is one of the prime
limiting factors on the survival of life in potential astrobiological habitats. Even
at a depth of 2 m beneath the surface, any microbes would probably be dormant,
cryopreserved by the current freezing conditions, and so metabolically inactive
and unable to repair cellular degradation as it occurs. Also, solar ultraviolet (UV)
radiation proved particularly devastating for the survival of cold-resistant microbes
under simulated surface conditions on Mars, as UV radiation was readily and easily
able to penetrate the salt-organic matrix that the bacterial cells were embedded in. In
addition, NASA's Mars Exploration Program states that life on the surface of Mars
is unlikely, given the presence of superoxides that break down organic (carbon-
based) molecules on which life is based.
8.2.2.3
Cosmic Radiation
In 1965, the Mariner 4 probe discovered that Mars had no global magnetic field
that would protect the planet from potentially life-threatening cosmic radiation and
solar radiation; observations made in the late 1990s by the Mars Global Surveyor
confirmed this discovery. Scientists speculate that the lack of magnetic shielding
helped the solar wind blow away much of Mars' atmosphere over the course of
several billion years.
As a result, the planet has been vulnerable to radiation from space for about
4 billion years. Currently, ionizing radiation on Mars is typically two orders of
magnitude (or 100 times) higher than on Earth. Even the hardiest cells known could
not possibly survive the cosmic radiation near the surface of Mars for that long.
After mapping cosmic radiation levels at various depths on Mars, researchers have
concluded that any life within the first several meters of the planet's surface would
be killed by lethal doses of cosmic radiation. The team calculated that the cumulative
damage to DNA and RNA by cosmic radiation would limit retrieving viable dormant
cells on Mars to depths greater than 7.5 m below the planet's surface.
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