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surface gravity and mass and assuming that
Venus is as well differentiated as Earth, only a
fraction of the basalt in Venus would have con-
verted to eclogite. This would make the uncom-
pressed density of Venus about 1.5% less than
Earth's without invoking any differences in com-
position or oxidation state. Thus, Venus may be
close to Earth in composition. It is possible that
the present tectonic style on Venus is similar to
that of Earth in the Archean, when temperatures
and temperature gradients were higher. If the
Moon, Mercury and Mars have molten iron core,
it is probable that Venus does as well.
The youthful age of the surface of Venus
has been attributed to a
global resurfacing
event
. The cratering record indicates that the
global resurfacing event, about 300 my ago, was
followed by a reduction of volcanism and tec-
tonism. Delamination of thick basaltic crust,
foundering of a cold thermal boundary layer and
a massive reorganization of mantle convection
are candidates for the resurfacing event. Mantle
convection itself is strongly controlled by surface
processes and changes in these processes.
Melt
Venus
1500
Earth
Partial melt
1000
Eclogite
500
Basalt
0
0
100
200
300
Depth (km)
Fig. 2.1
Schematic geotherms for the Earth with different
surface temperatures. Note that the eclogite stability field is
deeper for the higher geotherms and that a partial melt field
intervenes between the basaltic crust and the rest of the upper
mantle. Basaltic material in the eclogite field will sink through
much of the upper mantle and will be replaced by peridotite.
Shallow subduction of basaltic crust leads to remelting in
the case of Venus and the early Earth. Conversion to eclogite
leads to lower crustal delamination or deep subduction for the
present Earth. This figure explains why the low-density crust
can be no thicker than about 50 km on Earth. The depth scale
is for an Earth-size planet with the colder geotherm and
present crust and upper-mantle densities. For Venus, with
smaller gravity, higher temperatures and low-density crust
replacing part of the upper mantle, the depths are increased
by about 20%.
Mars
Mars is about one-tenth of the mass of Earth.
The uncompressed density is substantially lower
than that of Earth or Venus and is very similar
to the inferred density of a fully oxidized (minus
CandH
2
O) chondritic meteorite. The moment
of inertia, however, requires an increase in den-
sity with depth over and above that due to self-
compression and phase changes, indicating the
presence of a dense core. This in turn indicates
that Mars is a differentiated planet.
The tenuous atmosphere of Mars suggests that
it either is more depleted in volatiles or has
experienced less outgassing than Earth or Venus.
It could also have lost much of its early atmo-
sphere by large impacts. Geological evidence for
running water on the surface of Mars suggests
that a large amount of water is tied up in per-
mafrost and ground water and in the polar caps.
Whether Mars had standing water -- oceans and
lakes
of 100--170 km thickness on Venus composed of
basalt and partial melt. On the present Earth,
the eclogite stability field is entered at a depth
of 40--60 km. If the interior of Venus is dry it will
be stronger at a given temperature and will have
a higher solidus temperature.
A large amount of basalt has been produced
by the Earth's mantle, but only a thin veneer is at
the surface at any given time. There must there-
fore be a substantial amount of eclogite in the
mantle, the equivalent of about 200 km in thick-
ness. If this were still at the surface as basalt,
the Earth would be several percent less dense.
Correcting
--
for
long
periods
of
time
is
currently
40
Ar/
36
Ar ratio on Mars,
for
the
difference
in
temperature,
being debated. The high
