Geoscience Reference
In-Depth Information
Terrestrial bodies were subjected to a high
flux of impacting objects in early planetary his-
tory. The high-flux period can be dated from
lunar studies at about 3.8 billion years ago. The
large basins on the surface of Mercury formed
duringthisperiodofhighbombardment.Later
cooling and contraction apparently were respon-
sible for global compression of the outer surface
and may have shut off volcanism. On the Earth,
volcanism is apparently restricted to the extend-
ing regions.
Earth's surface. The oceanic crust on the Earth
is renewed every 200 million years but the conti-
nents survive much longer.
If Venus had an identical bulk composition
and structure to the Earth, then its mean den-
sity would be about 5.34 g/cm
3
. By 'identical
structure' I mean that (1) most of the iron is
in the core, (2) the crust is about 0.4% of the
total mass and (3) the deep temperature gradi-
ent is adiabatic (an assumption). The high sur-
face temperature of Venus, about 740 K, would
have several effects; it would reduce the depth
at which the convectively controlled gradient is
attained, it would deepen temperature-sensitive
phase changes and it may prevent mantle cool-
ing by subduction.
The density of Venus is 1.2--1.9% less than
that of the Earth after correcting for the dif-
ference in pressure. This may be due to differ-
ences in iron content, sulfur content, oxidization
state and deepening of the basalt--eclogite phase
change. Most of the original basaltic crust of the
Earth subducted or delaminated when the upper-
mantle temperatures cooled into the eclogite sta-
bility field. The density difference between basalt
and eclogite is about 15%. Because of the high
surface temperature on Venus, the upper-mantle
temperatures are likely to be 200--400 K hotter in
the outer 300 km or so than at equivalent depths
on Earth, or melting is more extensive. This has
interesting implications for the phase relations
in the upper mantle and the evolution of the
planet. In particular, partial melting in the upper
mantle would be much more extensive than is
the case for the Earth except for the fact that
Venus is probably deficient in the volatile and
low-molecular-weight elements that also serve to
decrease the melting point and viscosity. Crust
can be much thicker because of the deepening
of the basalt--eclogite phase boundary.
Schematic geotherms are shown in Figure 2.1
for surface temperatures appropriate for Earth
and Venus. With the phase diagram shown, the
high-temperature geotherm crosses the solidus at
about 85 km. With other plausible phase rela-
tions the eclogite field is entered at a depth
of about 138 km. For Venus, the lower gravity
and outer-layer densities increase these depths
by about 20%; thus, we expect a surface layer
Ve nu s
Venus is 320 km smaller in radius than the Earth
and is about 4.9% less dense. Most of the dif-
ference in density is due to the lower pressure,
giving a smaller amount of self-compression and
deeper phase changes. Venus is a smoother planet
than the Earth but has a measurable triaxiality
of figure and a 0.34 km offset of the center of
the figure from the center of mass. This offset
is much smaller than those of the Moon (2 km),
Mars (2.5 km) and Earth (2.1 km).
In contrast to the bimodal distribution of
Earth's topography, representing continent--ocean
differences, Venus has a narrow unimodal height
distribution with 60% of the surface lying within
500 m of the mean elevation. This difference is
probably related to erosion and isostatic differ-
ences caused by the presence of an ocean on
Earth. For both Earth and Venus the topogra-
phy is dominated by long-wavelength features.
Most of the surface of Venus is gently rolling ter-
rain. The gravity and topography are positively
correlated at all wavelengths. On Earth most of
the long-wavelength geoid is uncorrelated with
surface topography and is due to deep-mantle
dynamics or density variations.
The other respects in which Venus differs
markedly from the Earth are its slow rotation
rate, the absence of a satellite, the virtual absence
of a magnetic field, the low abundance of water,
the abundance of primordial argon, the high sur-
face temperature and the lack of obvious signs
of subduction. From crater counts it appears
that the age of the surface of Venus is 300--500
million years old, much less than parts of the
