Geoscience Reference
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has rough textures due to sublimation of buried ice. The ice evaporates without
melting and leaves behind an empty space. Overlying material then collapses into
the void. Sometimes chunks of ice fall from the glacier and get buried in the land
surface. When they melt, a more or less round hole remains. Many of these “kettle
holes” have been identified on Mars.
Despite strong evidence for glacial flow on Mars, there is little convincing
evidence for landforms carved by glacial erosion, e.g., U-shaped valleys, crag and
tail hills, arêtes, and drumlins. Such features are abundant in glaciated regions on
Earth, so their absence on Mars has proven puzzling. The lack of these landforms is
thought to be related to the cold-based nature of the ice in most recent glaciers on
Mars. Because the solar insolation reaching the planet, the temperature and density
of the atmosphere, and the geothermal heat flux are all lower on Mars than they are
on Earth, modeling suggests that the temperature of the interface between a glacier
and its bed stays below freezing and the ice is literally frozen down to the ground.
This prevents it from sliding across the bed, which is thought to inhibit the ice's
ability to erode the surface.
8.3.6
Ice Ages
Mars has experienced large-scale changes in the amount and distribution of ice
on its surface in its relatively recent geological past, and as on Earth, these are
known as ice ages. Ice ages on Mars are very different from the ones that the Earth
experiences. During a Martian ice age, the poles get warmer, and water ice then
leaves the ice caps and is deposited in mid-latitudes. The moisture from the ice caps
travels to lower latitudes in the form of deposits of frost or snow mixed with dust.
The atmosphere of Mars contains a great deal of fine dust particles; the water vapor
condenses on these particles which then fall down to the ground due to the additional
weight of the water coating.
When ice at the top of the mantling layer returns to the atmosphere, it leaves
behind dust which serves to insulate the remaining ice. The total volume of water
removed is a few percent of the ice caps or enough to cover the entire surface of the
planet under 1 m of water. Much of this moisture from the ice caps results in a thick
smooth mantle with a mixture of ice and dust. This ice-rich mantle, a few meters
thick, smoothen the land at lower latitudes, but in places it displays a bumpy texture.
Multiple stages of glaciations probably occurred. Because there are few craters on
the current mantle, it is thought to be relatively young. It is thought that this mantle
was laid in place during a relatively recent ice age. Ice ages are driven by changes
in Mars' orbit and tilt, which can be compared to terrestrial Milankovich cycles.
Orbital calculations show that Mars wobbles on its axis far more than Earth does.
The Earth is stabilized by its proportionally large moon, so it only wobbles a few
degrees. Mars may change its tilt - also known as its obliquity - by many tens of
degrees. When this obliquity is high, its poles get much more direct sunlight and
heat; this causes the ice caps to warm and become smaller as ice sublimes. Adding
to the variability of the climate, the eccentricity of the orbit of Mars changes twice
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