VENUS'S HIDDEN SURFACE:
Venus's surface is hidden by total cloud cover. Radar studies from the earth show that the planet's su rface possesses geological features suggestive of impact cratering, volcanism, and tectonic activity. Pioneer Venus radar studies confirm the earth-based observations, and a contour map produced from them. While 65 percent of the earth's surface is ocean basin lying on the average about 5 kilometers below sea level, fully 60 percent of Venus's surface lies within 0.5 kilometer of the mean radius of the planet, and only 5 percent is more than 2 kilometers above the mean radius. Despite this limited spread in elevation, the maximum distance between the highest and lowest points is about 13 kilometers, which is comparable to that of the earth.
The highland area, Ishtar Terra, in Figure 8.4 is unlike anything seen on the moon, Mercury, or Mars. It is larger than the continental United States and stands several kilometers above the mean planet radius. In elevation it is similar to the Tibetan Plateau on earth but about twice its size. Maxwell Montes on the eastern end of Ishtar Terra contains the highest point on the surface of Venus-about 11 kilometers above the mean radius, or 2 kilometers higher than Mount Everest is above sea level. The Venusian mountains containing Maxwell Montes rise out of the Ishtar Terra plateau in the same way that the Himalayas stand on the Asian plate. The other continentlike region, Aphrodite Terra, is also comparable in size to the continental United States.
Radar observations of the hidden surface disclose some almost circular structures between a few tens of and 1000 kilometers in diameter, which may be impact craters and basins. Other large features include a 1000 kilometer-long trough a few hundred kilometers wide and a few kilometers deep, which is similar in scale to the Martian Valles Marineris. Maxwell Montes is a large, low, circular dome some hundreds of kilometers across, with a central depression 100 kilometers or so in diameter, similar in many respects to a volcanic peak. If truly a volcano, it is about 25 percent larger than Olympus Mons on Mars. There are two other regions of suspected volcanic activity: Beta Regio and the eastern end of Aphrodite Terra.
Thus the possibility of impact craters and basins suggests a surface billions of years old, while the possibility of volcanoes, rift valleys, and plateaus point to youthful parts of the surface that may be only millions of years old. Venus apparently has not preserved as much of its early surface history as has the moon and Mercury, but it may be more like Mars than it is like the earth.
The two Soviet landers, Venera 9 and 10, sent back the first photographs of the Venusian surface in 1975, while Venera 13 and 14 provided four more, with at least one in color. Sunlight filtering through the
cloudy atmosphere supplies enough light to make the surface look like a dark, overcast day on the earth. The atmospheric color is decidedly orangish since the blue wavelengths of incoming sunlight are absorbed and scattered by the clouds. The Venera 13 view in Figure 8.6 shows a rock-strewn plain with a dark finegrained material interspersed between the rock outcroppings. Although having a number of noticeable differences, the view roughly resembles Martian terrain. Venera 14 landed on a terrain different from that for Venera 13. Its view was of a plain of broken rock layers that extend to the horizon. Samples were collected by both spacecraft and analyzed. Their composition suggests that the material is basaltic rock, an igneous rock extruded from the interior and generally silicon-poor and metal-rich. Such basaltic rock is common on the earth and moon.
