326 Encyclopedia of the Solar System
FIGURE 8 Teardrop-shaped islands and scour in Ares Vallis.
The islands formed where flow was diverted around preexisting
craters. Flow is from lower right to upper left. The image is
19 km across. (Mars Orbiter Camera.)
simplest explanation is that debris shed from high ground
contains ice that enables it to flow. At low latitudes, be-
cause of the absence of ice, debris flows do not form. Ma-
terial eroded from the cliffs remains adjacent to the cliff
and protects it from further erosion. The process is particu-
larly evident in what has been termed fretted terrain. These
are high-latitude sections of the plains/upland boundary in
which wide, flat-floored valleys reach deep into the upland.
Material has flowed away from the walls to widen the valleys
and form the flat floors. Again, the simplest explanation is
that the upland materials at these high latitudes contain ice
that facilitates flow of eroded debris. (See Fig. 9.)
FIGURE 9 Ice-rich flow in the fretted terrain. Ice-rich material
shed from the cliff at the top of the image has flowed away from
the cliff and converged on a gap in hills to the south. At the
latitude of this image (40◦N), similar flows occur at the base of
almost all cliffs and hills, which suggests that there is abundant
ice in the ground. (THEMIS.)A wide range of other observations, particularly in the
low-lying northern plains, have been interpreted to be the
result of ground ice or glaciers. These include polygonally
fractured ground (analogous to arctic-patterned ground?),
closely spaced, curvilinear, parallel ridges (moraines?), lo-
cal hollows (left by removal of ice?), branching ridges (sites
of former subglacial streams?), and striated ground (glacial
scour?). In addition, several features adjacent to volcanoes
in Tharsis have been interpreted as glacial in origin. They
are thought to have formed during periods of high obliq-
uity when ice, driven off the poles, accumulated on the
volcanoes.7.3 Wind
We know that the wind redistributes material across the
martian surface. We have observed dust storms from or-
bit and the changing patterns of surface markings that they
cause. The 2004 rovers have made movies of dust devils,