Mars: Surface and Interior 325
FIGURE 7 Outflow channels around the Chryse basin, to the north and east of the canyons. Dark areas are
the scoured channel floors. Where the channels enter the Chryse basin they scour broad tracts several
hundred kilometers across. (Mars Orbiter Laser Altimeter.)Major issues are concerned with how much water was
involved and where it all went. The size of the channels
suggests that the discharges were enormous, 1000 to 10,000
times the discharge of the Mississippi River. But we do not
know how long the floods lasted, so we do not know the total
volume of each flood. Nevertheless, large bodies of water, or
seas, must have been left in low-lying areas when the floods
were over. Efforts to find evidence for these seas has had
mixed results. Some researchers claim that Mars must have
had oceans as extensive as those on Earth; others claim that
seas larger than the Mediterranean were unlikely. Under
present conditions, such seas would have frozen, and the
ice would have slowly sublimed, thereby adding to the ice
at the poles. However, estimates of the amount of water
currently in the polar ice caps falls far short of even the
lowest estimates of the amounts of water involved in the
floods, so a mystery remains as to where the water went.
7.1.3 GULLIES
On many steep, poleward-facing slopes in the southern
hemisphere are gullies several meters across and hundreds
of meters long. They are forming during the present epoch.
They are most likely formed by melting snow during periods
of high obliquity. During these periods, water is driven off
the poles and accumulates as ice at lower latitudes. That
which accumulates on poleward-facing slopes may melt in
midsummer when the slopes are directly facing at the Sun
and almost permanently illuminated. Observations of gul-
lies emerging from under smooth-surfaced deposits (ice?)
on some crater walls support this suggestion.7.2 Ice
As indicated in Section 2.1, ice has been detected on the
surface at the poles and just below the surface at latitudes
higher than 40◦. Geomorphic evidence of ground ice is
pervasive at latitudes higher than about 30◦. At these lati-
tudes, when viewed at resolutions better than 50 m/pixel,
many surface features, such as ridges and crater rims ap-
pear rounded and subdued, as compared with the same
features at lower latitudes. The rounding or softening has
been attributed to slow, downhill movement of the near-
surface materials as a result of the presence of ground ice.
The rounding does not occur at lower latitudes because
ground ice is unstable, and likely absent in significant quan-
tities. Also at the higher latitudes, debris flows extend 20–
30 km away from almost all cliffs and steep mountains. The