Venus: Surface and Interior 153
FIGURE 3 These photographs of the surface of Venus were obtained by the SovietVenera 13
spacecraft.Venera 13was the first of theVeneralander missions to include a color camera. The
Venera 13lander touched down on March 3, 1982, near 305◦E, 5◦S, in the plains east of Phoebe
Regio. The arm on the surface in the top image is a soil mechanics experiment. A color bar for
calibration is visible in each image, as well as other spacecraft parts.
crust can be old as 4 Ga, but craters are erased by water
and wind erosion much more rapidly than on the other ter-
restrial planets.
There are two highly intriguing characteristics of the
Venus crater population. The first is that the distribution
of craters cannot be distinguished from a random pop-
ulation. The second is that very few of the craters are
modified by either volcanism or tectonism. Only∼17% of
the total population is either volcanically embayed and/or
tectonized. An example of a crater that is both embayed
and tectonized is Baranamtarra (Fig. 5). The low number
of modified craters on the surface of Venus means that
there is little record of the process or processes that re-
set the surface age to be less than 1 Ga. If volcanic flows
had covered the surface of Venus at a uniform rate, there
would be more partially buried craters. This observation
of the crater population initially led to the hypothesis of
global, catastrophic resurfacing. Subsequent detailed mod-
eling of resurfacing showed that the population is consistent
with a wide range of resurfacing models, allowing for dif-
ferent size areas to resurface at different rates. The small
number of modified craters is most consistent with resurfac-
ing occurring as small,∼400 km diameter, regions. How-
ever, yet another variable is whether or not some craters
with dark floors may in fact have been volcanically flooded.
However, even if a larger number of craters have been
modified by volcanism than initially estimated, the re-
gion covered by volcanism in these areas is small and still
most consistent with resurfacing proceeding in small, local
patches.
We can estimate the rate of volcanic resurfacing if we
assume that craters have been removed by burial un-
der volcanic flows. Crudely, if one takes the characteristic
resurfacing age to be 750 Ma and the average crater rim
height to be 0.5 km, then the rate of volcanic production is
∼0.3 km^3 /year. Alternatively, if we consider the hypothesis
that Venus resurfaced more quickly, in perhaps 50 Ma, the
production rate is∼4.6 km^3 /year. The relative volume of
lava extruded on the surface is believed to be small com-
pared to the volume intruded into the subsurface, perhaps
10% of the total. Thus, the total volume of melt produced
might be a factor of 10 larger. For comparison, the estimated
rate of volcanism for intrusive and extrusive volcanism on
Earth is 20 km^3 /year.
On planets with large numbers of craters, such as Mars,
the surface age of local regions can be estimated from the
crater populations. On Venus, some attempts have been
made to determine the relative ages of either populations
of specific types of geologic features or large areas on Venus.
However, statistical analysis of this approach indicates that
the very small number of craters on Venus makes attempts
at dating particular landforms or even large areas not reli-
able. Although traditional crater-counting methods are not
very useful, both the distribution of modified craters and the
distribution of dark crater parabolas suggest some variation
in surface age. In particular, the region with the highest