152 Encyclopedia of the Solar System
FIGURE 2 Histogram of the elevation in 0.5 km bins for Earth
and Venus, normalized by area.
atmosphere. The atmosphere is 96.5% carbon dioxide, with
lesser amounts of nitrogen, sulfur dioxide, argon, carbon
monoxide, and water. The clouds are composed of 75% sul-
furic acid and 25% water. [SeeVenus: Atmosphere.]
2.4 Views of the Surface
Four Soviet landers have returned views of the surface of
Venus,Venera 9, 10 , 13 , and 14. These panoramas showed
relatively similar sites: rocky surfaces with varying amounts
of sediment (Fig. 3). Rocks at each site tend to be relatively
angular, suggesting minimal erosion and possible ejection
from an impact crater. All the sites are consistent with a vol-
canic origin, showing platy lava flows that have been covered
to varying extents by sediments. The sediments may be of
impact origin, produced by aeolian erosion or by chemical
weathering.
3. Impact Craters and Resurfacing History
There are approximately 940 identified impact craters on
the surface of Venus. They range in diameter from approx-
imately 1.5 km to 268.7 km. The dense atmosphere on
Venus causes impactors 1 km in diameter to break up be-
fore impacting the ground, reducing the number of craters
30 km in diameter. The shock waves that travel through
these small objects can cause them to explode in a man-
ner analogous to the Tunguska event on Earth. [SeePlan-
etary Impacts.] Atmospheric breakup and explosion, or
other dynamic effects in the atmosphere, are believed to
produce both radar-bright and radar-dark splotches on the
surface (Fig. 4). The brightness of a radar image is primarily
a function of how rough it is at the scale of the radar wave-
length (for theMagellanradar, 12.6 cm). The darker the im-
age, the smoother the surface. Very rough areas appear very
bright. Rough areas reflect the signal back to the spacecraft,
while smooth areas allow the radar waves to bounce off in
a direction away from the spacecraft. Approximately 400 of
these “splotch” regions have been identified. These regions
are believed to be either areas where fine-grained mate-
rial has been scoured away (radar-bright areas) or regions
where relatively fine-grain material has settled out of the
atmosphere (smooth, radar-dark areas). Additionally, most
impact craters have associated with them dark parabolas,
which are also part of fine-grained ejecta that are deposited
out of the atmosphere.
In the absence of samples returned from planetary bod-
ies, the only means of dating the surface is the analysis
of the impact crater population. A great deal of work has
been done on assessing the population of comets and as-
teroids available to impact the larger planetary bodies. [See
Comet Populations and Cometary Dynamics; Main
Belt Asteroids; andPlanetary Impacts.] Dating of
samples returned from the Moon has been used to tie the
record of lunar craters to an absolute age. The estimated
flux of impactors on the Moon must be extrapolated to other
bodies in the solar system, which have different dynamical
environments and thus different expected rates for impacts.
This introduces a major uncertainty into the estimated age
of a surface based on impact crater counts. Another ma-
jor factor is the history of the surface itself. Modification
of a surface by erosion, deposition, or tectonism can de-
crease the number of identifiable craters. Erosion can also
remove deposits that had covered a surface. Additionally,
secondary craters can form when large blocks of material are
ejected during an impact. Impactors can break up during
entry to the atmosphere, producing multiple smaller im-
pacts rather than a single large impact. Despite these issues
and the resulting uncertainties, estimated surface age is a
very important clue in deciphering the geologic history of a
planet.
The estimated age of resurfacing on Venus is∼750 mil-
lion years (Ma). Given all the possible uncertainties in this
age, estimates between 300 Ma and 1 Ga are permissible.
This age is in contrast to ages of 3–4 Ga on average for Mars
and the Moon. On Earth, new crust is continually forming
along spreading centers in the oceanic crust. Continental