154 Encyclopedia of the Solar System
FIGURE 4 This radar image (approximately 125×140 km in
size) shows an impact splotch with a dark center and a bright
halo. The splotch is superimposed on a set of predominantly
northwest-trending wrinkle ridges. The spacing between major
ridges is roughly 10–20 km. These wrinkle ridges are part of the
set of ridges that wraps around Western Eistla Regio.
FIGURE 5 Crater Baranamtarra is both heavily embayed by
volcanic flows and fractured. It is 25.5 km in diameter and
centered at 17.94◦N, 267.80◦E.
density of volcanoes, coronae, and rifts appears to have a
lower density of haloes and more modified craters, suggest-
ing a younger age.
Overall, the crater population on Venus indicates it is a
comparatively active planet, completely resurfaced within
the last 1 Ga, possibly with resurfacing on-going today. Vol-
canic resurfacing rates are likely on the same order of mag-
nitude as those on Earth, but are a function of the poorly
constrained rate of resurfacing, which could be either con-
stant or variable. The distribution and modification of the
craters implies that there are limited differences in the ages
of large regions on Venus, unlike the dichotomy between
the age of oceanic and continental crust on the Earth. The
small number of modified impact craters leaves few clues
as to the process(es) that obliterated the earliest surface of
Venus. Below we discuss the implications of resurfacing for
the overall geologic evolution of Venus.4. Interior Processes
One of the greatest curiosities about Venus is that its global-
scale geologic processes are totally unlike that of Earth. On
Earth, the system that shapes the Earth’s large-scale phys-
iography and the majority of geologic features is plate tec-
tonics. The surface of the Earth is broken into dozens of
plates that move over the surface of the Earth at rates of up
to a few cm per year. The plates are tens to hundreds of kilo-
meters thick. Mountain belts form where plates meet, such
as where they collide, slide at an angle past each other, or
where one plate is pushed into themantlebeneath another
at subduction zones. Hot material wells up from the mantle
below along narrow ridges in the ocean crust, creating new
oceanic crust. These characteristics features are easily seen
in the topography for Earth, even at the relatively low res-
olution available for Venus (Fig. 1). Venus clearly does not
have plate tectonics. There is no evidence for this type of
geologic process in the topography or in the radar images.
[SeeEarth as a Planet: Surface and Interior.]
The energy that drives plate tectonics and other geologic
processes is predominantly generated by the decay of ra-
dioactive elements. For the terrestrial planets, the primary
contributors to radioactive decay are uranium (U), thorium
(Th), and potassium (K). Based on estimates of the abun-
dance of these elements on Earth and in chondrites [see
Meteorites], radioactive decay cannot account for the
total amount of energy. In addition, a significant amount,
perhaps 25%, of the heat lost from the interior results from
cooling of the planet over time, with some additional con-
tribution from the heat of initial planetary accretion. The
heat in the interior of the planet is predominantly transmit-
ted to the surface via convection in the interior. Convec-
tion in the mantle brings hot, low-density material from the
interior to the surface, or near the surface, allowing it to
cool.