Mars Astrobiology: Recent Status and Progress - Planetary Exploration and Science: Recent Results and Advances - page 163

8.3.5.2 Equatorial Frozen Sea

Surface features consistent with existing pack ice have been discovered in the
southern Elysium Planitia. What appear to be plates of broken ice, ranging in size
from 30 to 30 km, are found in channels leading to a flooded area of approximately
the same depth and width as the North Sea. The plates show signs of breakup and
rotation that clearly distinguish them from lava plates elsewhere on the surface
of Mars. The source for the flood is thought to be the nearby geological fault
Cerberus Fossae which spewed water as well as lava aged some 2-10 Ma. It was
suggested that the water exited the Cerberus Fossae then pooled and froze in the
low, level plains and that such lakes may still exist. Not all scientists agree with
these conclusions.

8.3.5.3 Polar Ice Caps

Both the northern polar cap (Planum Boreum) and the southern polar cap (Planum
Australe) are thought to grow in thickness during winter and partially sublime
during summer. In 2004, the MARSIS radar sounder on the Mars Express satellite
targeted the southern polar cap and was able to confirm that ice there extends to a
depth of 3.7 km (2.3 mi) below the surface.
In the same year, the OMEGA instrument on the same orbiter revealed that
the cap is divided into three distinct parts, with varying contents of frozen water
depending on latitude (Bibring et al. 2004 ).
The first part is the bright part of the polar cap seen in images, centered on the
pole, which is a mixture of 85 % CO 2 ice and 15 % water ice. The second part
comprises steep slopes known as scarps, made almost entirely of water ice, that
ring and fall away from the polar cap to the surrounding plains. The third part
encompasses the vast permafrost fields that stretch for tens of kilometers away
from the scarps and is not obviously part of the cap until the surface composition
is analyzed. NASA scientists calculate that the volume of water ice in the south
polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a
depth of 11 m (36 ft). Observations over both poles and more widely over the planet
suggest melting all the surface ice would produce a water equivalent global layer
35 m deep (Fig. 8.8 ).
On July 2008, NASA announced that the Phoenix lander had confirmed the
presence of water ice at its landing site near the northern polar ice cap (at 68.2 ı
latitude). This was the first ever direct observation of ice from the surface. Two
years later, the shallow radar on board the Mars Reconnaissance Orbiter took
measurements of the north polar ice cap and determined that the total volume of
water ice in the cap is 821,000 cubic kilometers (197,000 cubic miles). That is
equal to 30 % of the Earth's Greenland ice sheet or enough to cover the surface of
Mars to a depth of 5.6 m. Both polar caps reveal abundant fine internal layers when
examined in HiRISE (High Resolution Imaging Science Experiment is a camera
on board the Mars Reconnaissance Orbiter) and Mars Global Surveyor imagery.