Geoscience Reference
In-Depth Information
The lithosphere on Mars is capable of support-
ing large surface loads. The evidence includes
the roughness of the gravity field, the heights
of the shield volcanoes, the lack of appreciable
seismicity and thermal history modeling. There
is some evidence that the lithosphere has thick-
ened with time.
Olympus Mons
is a volcanic con-
struct with a diameter of 700 km and at least
20 km of relief, making it the largest known vol-
cano in the solar system. It is nearly completely
encircled by a prominent scarp several kilometers
in height and it coincides with the largest grav-
ity anomaly on Mars. The origin of the
Olympus
Mons scarp
is controversial; it may be, in part,
due to spreading of the volcano, and, in part, to
erosion.
The surface of Mars is more complex than
that of the Moon and Mercury. There is abundant
evidence for volcanic modification of large areas
after the period of heavy bombardment, subse-
quent to 3.8 Ga. Mars has a number of gigan-
tic shield volcanoes and major fault structures.
In contrast to Mercury there are no large thrust
or reverse faults indicative of global contraction;
all of the large tectonic features are extensional.
The absence of terrestrial-style plate tectonics is
probably the result of a thick cold lithosphere. In
any event, the absence of plate tectonics on other
planets provides clues to the dynamics of a planet
that are unavailable from the Earth. Apparently,
a planet can be too small or too dry or too old
or too young, for it to have plate tectonics. Liquid
water, magnetic fields, plate tectonics and life are
all unique to Earth and there may be a reason for
this.
tain about 30% iron, which is consistent with
the composition of stony meteorites and the non-
volatile components of the Sun. They therefore
fit into any scheme that has them evolve from
solar material. Mercury is overendowed with
iron, which has led to the suggestion that part of
the mantle was blasted away by impacts. Because
iron is the major dense element occurring in the
Sun, and in the preplanetary solar nebula, the
Moon is clearly depleted in iron, and in a num-
ber of other elements as well. A common char-
acteristic of many of these elements and their
compounds is volatility. Calcium, aluminum
and titanium are the major elements involved
in high-temperature condensation processes;
minor refractory elements include barium, stron-
tium, uranium, thorium and the rare-earth ele-
ments. The Moon is enriched in all these ele-
ments, and we are now sure that more than
iron--silicate separation must be involved in lunar
origin.
The surface samples of the Moon are remark-
ably depleted in such volatile elements as
sodium, potassium, rubidium and other substan-
ces that, from terrestrial and laboratory experi-
ence, we would expect to find concentrated in
the crust, such as water and sulfur. The refrac-
tory trace elements -- such as barium, uranium
and the rare-earth elements -- are concentrated
in lunar surface material to an extent several
orders of magnitude over that expected on the
basis of cosmic or terrestrial abundances. Some
of these elements, such as uranium, thorium,
strontium and barium, are large-ion elements,
and one would expect them to be concentrated
in melts that would be intruded or extruded
near the surface. However, other volatile large-
ion elements such as sodium and rubidium are
clearly deficient, in most cases, by at least several
orders of magnitude from that expected from
cosmic abundances. The enrichment of refractory
elements in the surface rocks is so pronounced
that several geochemists proposed that refractory
compounds were brought to the Moon's surface
in great quantity in the later stages of accre-
tion. The reason behind these suggestions was
the belief that the Moon, overall, must resem-
ble terrestrial, meteoritic or solar material and
that it was unlikely that the whole Moon could
Moon
Strange all this difference should be
'Twixt tweedle-dum and tweedle-dee.
John Byrom
The Moon is deficient in iron compared with the
Earth and the other terrestrial planets. It is also
apparently deficient in all elements and com-
pounds more volatile than iron. The density of
the Moon is considerably less than that of the
other terrestrial planets, even when allowance is
made for pressure. Venus, Earth and Mars con-
