Venus - Introduction to Planetary Geomorphology - page 113

Earth and that the zone, or asthenosphere, of the

upper mantle is probably absent. This would reduce the
possibility for plate tectonics and perhaps result in a stagnant
“ lid ” for capping heat loss from the interior, which could lead
to the formation of individual plumes, or hot spots.

“ slippery ” quality of the data is not as good as can be obtained with

current technologies. Nonetheless, the generally basaltic
compositions for most of the landing sites are consistent
with the recent VIRTIS data from the Venus Express
orbiter. Until new missions are flown to the surface of
Venus, the Venera and Vega landers provide the only data
from direct surface measurements.
Radar data reveal upland regions, such as Maxwell
Montes, to have surprisingly low emissivities. These
regions are thought to represent some unusual surface
compositions. One possibility is that the rocks are coated
with materials that form only where the temperatures and
pressures are lower at the high elevations on Venus.
Although intuitively strange, the term metallic “ frosts ”
has been used to suggest that volatile metals such as
tellurium and bismuth might be deposited on surface
materials, which would then yield low radar emissivities.
Others have suggested that concentrations of minerals
such as pyrite could explain the radar signatures and that
wind-winnowing and removal of lower-density grains
could leave a lag surface of higher-density minerals; how-
ever, remarkably large amounts of pyrite would be
required in order to explain the radar signatures.

6.4 Surface compositions

Information on venusian surface compositions comes
mostly from the Soviet missions and from the Ve nu s
Express VIRTIS mapper. Because of operational con-
straints, the Ve n e r a and Ve ga landing sites are restricted
mostly to the equatorial regions (Fig. 6.3) . The Venera sites
are on the eastern flanks of Beta Regio, a major upland
region, while the Ve ga sites are on Rusalka Planitia near
Aphrodite, another major upland. Measurements frommost
of the lander sites show that the rocks are similar to basalts
foundonEarth ' ssea oor (Fig. 6.7) . It is important to note
that the 300 km radius uncertainty in the exact location of
the Ve ne r a and Ve ga landing sites makes the geologic
context poorly constrained.
The Venera 8 site is particularly important because it
has a very high thorium content, which is indicative of
signi cant chemical differentiation. In addition, its high
potassium content is comparable to that of felsic rocks on
Earth, suggesting to some investigators the presence of
granite-like materials and the idea that the upland plateaus
might be roughly comparable to Earth-like continents.
However, the Soviet instruments used to measure surface
compositions were developed decades ago, and the

6.5. Geomorphology

6.5.1 General physiography

The first hints of the major terrains on Venus came from
Earth-based radar data (Fig. 6.8) in which certain distinctive

Figure 6.7. Venus compositions from Venera
and Vega measurements compared with
Earth mid-ocean-ridge basalts (MORB),
basalts in Gusev crater on Mars, martian
meteorites, and lunar basalts (after Treiman,
2007 ).