Mars: Surface and Interior 319
elements, which would have been largely lost from the inte-
rior during the early global melting phase. It consists mainly
of iron-magnesium silicates. One difference between the
mantles on the two planets is that the Fe/Mg ratio is higher
in the martian mantle.
The crust is essentially a melt extract from the mantle. It
is probably mostly basaltic in composition. The thickness of
the crust varies considerably, ranging from 5 to 100 km, as
estimated from the relations between the global gravity field
and the global topography. The thickest crust is under the
high-standing cratered terrain in the southern hemisphere;
the thinnest is under the large impact basins of Isidis and
Hellas.
2.4 Global Topography and Physiography
The topography and physiography have a marked north-
south asymmetry, which is referred to as the global di-
chotomy. (See Fig. 1.) The dichotomy is expressed three
ways: as a change in elevation, a change in crustal thickness,
and a change in crater density. The southern uplands have
an average elevation 5.5 km higher than that in the north-
ern plains, the crust is roughly 25 km thicker in the uplands,
and most of the upland terrain is heavily cratered, dating
back to the period of heavy bombardment. (All the terres-
trial planets were heavily bombarded by meteoritic debris
early in their history. The period ended around 3.8 billion
years ago.) The plains are mostly younger surfaces, but there
must be an older surface at some depth beneath the younger
plains. The low-lying plains constitute roughly one third of
the planet and are mostly in the north. The cause of the
dichotomy is not known. Suggestions include a very large
impact, soon after the planet formed, or internal convection
sweeping most of the light, crustal material into one half of
the planet.
Superimposed on the global dichotomy is the Tharsis
bulge, which is more than 5000 km across and 10 km high
and is centered on the equator at 100◦W. Most of the
planet’s volcanic activity has been centered on the bulge,
which has the planet’s five largest volcanoes (Alba Patera,
Montes Olympus, Arsia, Ascreus, and Pavonis) on its north-
west flank. Tharsis is also at the center of a vast array of
radial faults and circumferential ridges that affect over half
the planet’s surface. To the east of the center of the bulge are
a series of vast canyons thousands of kilometers long and up
to 10 km deep. They are roughly radial to the bulge and ap-
pear to have formed largely by faulting, although they also
have been extensively modified by fluvial and mass-wasting
processes. At the east end of the canyons, extensive areas
of terrain have seemingly collapsed to formchaotic ter-
rainout of which emerge large dry riverbeds that extend
for thousands of kilometers downslope into the northern
plains. The bulge appears to be a massive accumulation of
volcanic rocks. Their accumulation started very early in theFIGURE 1 Topographic map of Mars between latitudes 65◦S and 65◦N. The highest elevations (whites and grays) are in Tharsis. The
lowest elevations (blues) are in Hellas and the northern plains. The dominant feature of the planet is the global dichotomy between
the low-lying northern plains and the cratered southern uplands. The main positive features are the volcanic provinces of Tharsis and
Elysium. The main negative features are the large impact basins Hellas, Argyre, Isidis, and the buried basin Utopia. The canyons
extending eastward from Tharsis, and large outflow channels such as Ares Vallis are visible even at this global scale. (Mars Orbiter
Laser Altimeter.)