Geoscience Reference
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Above 60 degrees latitude, ice is highly abundant. Pole wards on 70 degrees of
latitude; ice concentrations exceed 25 % almost everywhere and approach 100 % at
the poles. More recently, the SHARAD and MARSIS radar sounding instruments
have begun to be able to confirm whether individual surface features are ice rich.
Due to the known instability of ice at current Martian surface conditions, it is
thought that almost all of this ice must be covered by a veneer of rocky or dusty
material.
The Mars Odyssey's neutron spectrometer observations indicate that if all the
ice in the top meter of the Martian surface were spread evenly, it would give a water
equivalent global layer (WEG) of at least
14 cm - in other words, the globally
averaged Martian surface is approximately 14 % water. The water ice currently
locked in both Martian poles corresponds to a WEG of 30 m, and geomorphic
evidence favors significantly larger quantities of surface water over geologic history,
with WEG as deep as 500 m. It is believed that part of this past water has been lost
to the deep subsurface and part to space, although the detailed mass balance of these
processes remains poorly understood. The current atmospheric reservoir of water is
important as a conduit allowing gradual migration of ice from one part of the surface
to another on both seasonal and longer timescales. It is insignificant in volume, with
a WEG of no more than 10 m(Fig.
8.7
).
8.3.5.1
Ice Patches
On July 28, 2005, the European Space Agency announced the existence of a crater
partially filled with frozen water; some then interpreted the discovery as an “ice
lake.” Images of the crater, taken by the high-resolution stereo camera on board the
European Space Agency's Mars Express Orbiter, clearly show a broad sheet of ice
in the bottom of an unnamed crater located on Vastitas Borealis, a broad plain that
covers much of Mars' far northern latitudes, at approximately 70.5
ı
North and 103
ı
East. The crater is 35 km wide and about 2 km deep. The height difference between
the crater floor and the surface of the water ice is about 200 m. ESA scientists
have attributed most of this height difference to sand dunes beneath the water ice,
which are partially visible. While scientists do not refer to the patch as a “lake,” the
water ice patch is remarkable for its size and for being present throughout the year.
Deposits of water ice and layers of frost have been found in many different locations
on the planet.
As more and more of the surface of Mars has been imaged by the modern
generation of orbiters, it has become gradually more apparent that there are probably
many more patches of ice scattered across the Martian surface. Many of these
putative patches of ice are concentrated in the Martian mid-latitudes (
30 - 60
ı
N/S of the equator). For example, many scientists believe that the widespread
features in those latitude bands variously described as “latitude-dependent mantle”
or “pasted-on terrain” consist of dust- or debris-covered ice patches, which are
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