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as much as Earth's eccentricity. As the poles sublime, the ice is deposited closer to
the equator, which receives somewhat less solar insolation at these high obliquities.
Computer simulations have shown that a 45 ı tilt of the Martian axis would result in
ice accumulation in areas that display glacial landforms. A 2008 study provided
evidence for multiple glacial phases during Late Amazonian glaciation at the
dichotomy boundary on Mars.
8.3.7
Evidence for Recent Flows
8.3.7.1
Branched Gullies
Liquid water cannot exist in a stable form on the surface of Mars with its present
low atmospheric pressure and low temperature, except at the lowest elevations for a
few hours. So, a geological mystery commenced when observations from NASA's
Mars Reconnaissance Orbiter revealed gully deposits that were not there 10 years
ago, possibly caused by flowing salty water (brine) during the warmest months on
Mars. The images (below) were of two craters called Terra Sirenum and Centauri
Montes which appear to show the presence of liquid water flows on Mars at some
point between 1999 and 2001.
There is disagreement in the scientific community as to whether or not gullies
are formed by liquid water. It is also possible that the flows that carve gullies are dry
or perhaps lubricated by carbon dioxide. Even if gullies are carved by flowing water
at the surface, the exact source of the water and the mechanisms behind its motion
are not well understood (Fig. 8.10 ).
In August 2011, NASA announced the discovery by Nepalese student Lujendra
Ojha of current seasonal changes on steep slopes below rocky outcrops near crater
rims in the southern hemisphere. Dark streaks were seen to grow downslope during
the warmest part of the Martian summer, then to gradually fade through the rest of
the year, recurring cyclically between years. The researchers suggested these marks
were consistent with salty water (brines) flowing downslope and then evaporating,
possibly leaving some sort of residue. Because these flows have been the flows form
and fade in sync with heat flux into the surface, many scientists feel these recurrent
slope lineae are probably the best candidates for features formed by flowing water
on Mars today. The rate of growth of these features has been shown to be consistent
with shallow groundwater flow downslope through a sandy substrate (Figs. 8.11
and 8.12 ).
8.3.8
Habitability Assessment
Life is understood to require liquid water, but it is not the only essential require-
ment for life. These requirements include water, an energy source, and materials
necessary for cellular growth, while all under appropriate environmental conditions.
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