Geoscience Reference
In-Depth Information
12.7 Mars
Mars has a radius of 3386 km, a density of 3940 kg m
−
3
and its gravity is 38% of that of
the Earth. Mars is a heavy planet for its size, which is usually taken as a sign of a high
iron content. Its modern magnetic field is essentially nil but the very ancient highlands are
magnetized along broad stripes, which attests to the presence of a dynamo very early in
Mars' history. The planet retains, admittedly somewhat imperfectly, an atmosphere, with
a pressure equivalent to 0.7% of the Earth's atmospheric pressure and which is capable
of generating violent winds. The composition of the atmosphere, reported by the Viking
landers, is dominated by CO
2
(95.3%) with traces of N
2
(2.6%) and Ar (1.6%). The orbital
satellite MOLA told us that the thickness of the Martian crust varies from 40 to 80 km.
There is a marked contrast between the two hemispheres: the southern hemisphere is com-
posed of elevated volcanic plateaus, which are heavily cratered and therefore very old.
The surface of the southern hemisphere is eroded by what are unquestionably fluviatile
systems. The northern hemisphere is a vast flat depression covered with eolian sands. Large
old craters show in the basement through the sedimentary blanket of the northern plains.
Orbiter spectroscopic observations of the sands show the ubiquitous presence of olivine,
pyroxene, and ferric oxide, with occasional clay minerals. The rovers Spirit and Oppor-
tunity provided evidence of abundant iron sulfate in these sediments, whereas carbonates
are missing. Whatever water masses have been present at the surface of the planet, they
must have been too acid (pH
6) for carbonate to precipitate. Extrapolating the lunar
cratering chronology indicates that most of the southern hemisphere and the basement of
the northern hemisphere are covered with very large craters older than
<
3.9 Ga, i.e. nearly
contemporaneous with the Late Heavy Bombardment of the Moon. Cratering chronology
for the sediments of the northern hemisphere supports an age in excess of 3.2 Ga. Some
strato-volcanoes, such as those of the Tharsis plateau, have a rather fresh morphology:
although their deeper layers are very old (
≈
3.4 Ga), lava flows younger than 200 Ma are
undoubtedly present. A polar ice cap composed mainly of carbonic ice forms and melts
annually at high latitudes.
It is believed that Mars never went through a plate tectonic regime, probably because
the lithosphere quickly became too rigid (too dry, too thick) ever to bend. The mantle is
certainly convective, but beneath the thick lithosphere: this is the so-called stagnant lid
regime, rather inefficient at extracting heat, and which probably also smothered the core
dynamo very early on in the planet's history.
Martian stratigraphy is divided into three stratigraphic systems with boundaries ten-
tatively set at 3.7 and 3.0 Ga by cratering chronology. The ancient cratered and rugged
terranes of the southern hemisphere belong to the oldest Noachian, and terminated with the
heavy bombardment period. This period was a time of generalized volcanic activity. The
overlying Hesperian corresponds to the activity of the major strato-volcanoes, of the out-
flow channels (Valles Marineris), and to the infilling of the northern plains. The Amazonian
is the youngest period, which saw very little volcanic and sedimentary activity.
Although there have not yet been any missions to recover samples from Mars, the rovers
analyzed a variety of sedimentary and volcanic samples in place. It was concluded that
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