Venus - Introduction to Planetary Geomorphology - page 108

Figure 6.1. (cont.)

From orbit, only the long wavelength of radar ( Fig. 2.14)
can penetrate the thick clouds of Venus to reveal the surface.
In 1978, the NASA Pioneer Venus spacecraft began orbit-
ing the planet and mapping the topography with a radar
altimeter, giving the first near-global perspective of Venus.
Figure 6.3 shows the general physiography and some of the
principal named features. Because Venus is in retrograde
motion, longitude increases toward the east, as set by con-
ventions of the International Astronomical Union. The
prime meridian for Venus, or the planet- xed reference for
longitude, passes through the central peak of crater Ariadne,
southwest of Alpha Regio. Pioneer Venus also obtained
information on gravity distributions and sub-meter surface
roughness related to rocks, lava textures, and fractures.
Pioneer Venus data showed important differences in the
distribution of elevations in comparison with Earth, sug-
gesting some fundamental differences in interior and sur-
face processes (Fig. 6.4) . Analysis of the topography
enabled major terrains to be discerned,

In addition to providing new data from orbit, Pioneer
Venus ejected four entry probes that plunged through the
dense atmosphere, measuring the temperature and pres-
sure as a function of altitude and providing some informa-
tion on atmospheric compositions and near-surface winds.
Of particular note was the measurement of a key isotopic
ratio, D/H (deuterium-to-hydrogen), indicating that Venus
might once have had extensive water on the surface
(Donahue et al., 1982 ), perhaps including vast oceans,
which has subsequently been lost.
The first clues to the geomorphology of individual sur-
face features came from the Soviet Ve n e r a 1 5 and 16 space-
craft in the early 1980s. These orbiters carried synthetic
aperture radar (SAR, Fig. 6.5) imaging systems operating at
wavelength 8 cm and giving images of resolution 1 - 2km
for about 25% of the surface. Ve n e r a 1 5 and 16 provided the
first clues as to the variety of volcanic landforms, impact
craters, and tectonic features. However, the data suggested
the absence of the signatures of plate tectonics. A few
years later, the Soviet Ve ga 1 and 2 spacecraft ew past

including rift

belts similar to those on Earth.