Geoscience Reference
In-Depth Information
the extinct radioactive nuclides
146
Sm and
182
Hf and their variation from one meteorite to
another further indicate that the Martian lithosphere is nearly as old as the planet itself.
Plate tectonics is therefore quite unlikely to have modeled the planet's surface. Recent Pb-
Pb work suggests that shergottites are actually older than 4.1 Ga and that the younger ages
inferred from mineral isochrons reflect later impact events. It is therefore a very long time
since Mars experienced any intense internal activity, with the possible exception of the few
volcanoes visible at its surface, and that even its metal core is frozen as it does not generate
a magnetic field. Although some young lava flows are undoubtedly present, the dynamics
of Mars' interior, just as that of the Moon's interior, was not strong enough to maintain a
surface volcanic activity for more than about a billion years after planet formation.
However, it is generally accepted that Mars once had a denser atmosphere than today
and possibly oceans. The Viking probes measured the composition of the Martian atmo-
sphere: high D/H ratios with
4000 and
15
N/
14
N ratios 60 percent larger than that
of the Earth indicate that hydrogen and nitrogen were quickly lost by the planet. The lack
of a Martian magnetic field did not let a stable magnetosphere form that would protect
it from the solar wind. Water photolysis by solar UV radiation and hydrogen loss may
have been accompanied by oxygen production which was either lost to space or used by
oxidation of ferrous iron. The presence of sulfates and of low-temperature carbonates in
SNC meteorites are evidence of water circulation, corroborating satellite observations of
hydrographic networks, probably of very ancient age. The current status of groundwater
is still under discussion. The existence of biological activity - a hypothesis given much
media coverage by NASA thanks to photographs of minute elongated calcitic objects in
the Martian meteorite ALH84001 discovered in Antarctica - encounters a certain degree of
disbelief among the scientific community. The wider implications of the claim for society
are such that the possibility must, of course, be further investigated. If traces of biological
activity are to be found, it will probably be deep in the sedimentary layers of the northern
hemisphere, far from the lethal solar radiation.
δ
D
=
12.8 Venus
Venus has a radius (6052 km), a surface gravity (8.87 m s
−
2
), and a density (5243 kg m
−
3
)
very similar to the terrestrial values. There is not much detailed geochemistry we know for
sure about Venus. The atmospheric pressure at the surface is 90 times the terrestrial pres-
sure. The 460
◦
C surface temperature makes the planet an inferno of CO
2
(96.4%) and N
2
(3.4%), with clouds of sulfuric acid! Venus has essentially no magnetic field. The Venera
Soviet probes survived hardly long enough to analyze the atmosphere and take a few pic-
tures of a soil covered with volcanic fragments. The Magellan radar experiments showed
a pronounced topography with highlands, volcanoes, and circular volcanic structures of
unknown origin. Although the range of elevations (13 km) is comparable to that on Earth,
the planet does not show the distinctive dichotomy of continental and oceanic reliefs that,
on Earth, results from plate tectonics. The surface lacks the large and old craters that would
attest to the presence of very ancient terrane. Adapting the cratering rates of the Moon to
