Geology Reference
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from reactions of atmospheric carbon dioxide with lms
of liquid water on soil particles.
circular features beneath the lowland plains surface that
are thought to be large (100 to ~450 km) buried impact
craters that formed in Mars ' early history. Although there
is debate as to whether the dichotomy formed as a result of
endogenic processes (such as mantle convection) or rep-
resents one or more mega-impacts (the basins which
would form the lowlands), it would appear that the dichot-
omy was present very soon after Mars ' crust developed
and that the lowlands were resurfaced.
Superposed on the border of the northern lowland
plains and the southern cratered uplands is the volcanic
Tharsis rise, which includes four enormous shield volca-
noes and numerous other structures. The shields, dome
volcanoes, and associated lava flows in this area constitute
the Tharsis Volcanic Province. Other volcanic regions
include Elysium, Syrtis Major, and the Circum-Hellas
Volcanic Province.
The Hellas basin is found in the southern cratered
uplands and, at more than 1,800 km across, is one of the
largest impact structures in the Solar System. A similar but
smaller impact basin, Argyre, is also found in the southern
hemisphere. The floors of both structures have been partly
filled with a variety of materials, while their ejecta depos-
its have been mantled or so heavily modi ed that they are
not clearly recognized.
Valles Marineris is the
7.5 Geomorphology
7.5.1 Physiography
Super cially, Mars can be divided into two primary ter-
rains somewhat similar to the Moon, the northern lowland
plains and the southern cratered uplands, constituting a
so-called global crustal dichotomy (Fig. 7.4) . The north-
ern lowland plains are generally below the 0 km reference
elevation. Lacking a sea level, elevations on Mars were
rst de ned following the Mariner 9 mission as the ele-
vation at which atmospheric carbon dioxide pressure is
6.1 mbar, the triple point of water on Mars. Subsequently,
MOLA data were used to rede ne elevations referenced to
the center of mass for Mars. Both reference systems are in
the literature, and caution must be exercised in using the
data uniformly, since there are signi cant differences
between the two systems on the planet. Note also that
both east and west longitudes are used in papers and on
Mars maps as geographic coordinates; it is best to check
for the system used in any given case.
The southern cratered uplands represent the final stages
of early heavy bombardment and generally stand at ele-
vations higher than 1 km above datum. As discussed
below, this terrain has been heavily modi ed by surface
processes, some of which are probably active today. The
relative lack of impact craters exposed on the surface of
the lowland plains suggests its relative youth. However,
radar sounder data reveal
of Mars, which
stretches more than 3,800 km eastward from the Tharsis
rise across the northern equatorial region. This tectonic
feature has multiple canyon systems, with some oors
being more than 3 km below the surrounding terrain.
Mars has distinctive north and south polar regions that
are easily seen by noting the presence of white deposits of
frozen carbon dioxide (Fig. 7.1) . These polar frosts
Grand Canyon
the presence of dozens of
Figure 7.4. An image of Mars
generated from Mars Orbiter
Laser Altimeter data showing
the
between the sparsely cratered
northern lowlands and the
heavily cratered southern
uplands. The Tharsis volcanic
province (1) is superposed on the
boundary between the two
prominent terrains; also shown
are the Elysium (2), Syrtis Major
(3), and Syria Planum (4) volcanic
provinces and the Hellas impact
basin (5).
global dichotomy
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