7. Venus: The veiled planet
Unveiling Venus with radar
Magellan and its predecessors
No human eye has ever gazed on the surface of Venus; it can only be sensed by radio transmissions. Radar, an acronym for radio detection and ranging, uses its own source of radio radiation, and does not need sunlight to probe the planet, gathering data day or night. Only radar is capable of piercing the thick clouds of sulfuric acid that blanket Venus.
Magellanís radar images revealed a rich and varied landscape with stunning and unprecedented clarity, describing a surface whose nature and history turned out to be quite different from those of the Earth. The surface of Venus is covered by massive, global outpourings of lava, punctuated by unique volcanic constructs never seen before, scarred by sparse, pristine impact craters surrounded by beautiful outflows, and fractured, stretched, crumpled and split open by upwelling magma. Even hardened professional astronomers were inspired with a sense of wonder at these discoveries.
A smoothed-out world
Radar data from the Pioneer Venus Orbiter and Magellan showed that Venus is an extraordinarily smooth world, largely at one low level and quite different from the Earth. About 85 percent of the surface lies within one thousand meters of the average planetary radius, 6,051.9 thousand meters. A coating of lava has smoothed these vast low-lying plains. Without its water, the topography of the Earth occurs at two distinct elevations, which correspond to the continents and ocean floors.
Although most of Venus's terrain consists of smooth, low-lying, volcanic plains, about 15 percent of the planet's surface consists of highlands that tower above the plains, rising an average 4 thousand to 5 thousand meters above the mean planetary radius. There are two large-scale elevated regions that punctuate the smoothed-out surface; they are Aphrodite Terra in the equatorial region and Ishtar Terra in the far north.
Aphrodite Terra is over 10 million meters long and covers a quarter of the planet's circumference at the equator. It contains tall volcanoes, long lava flows and deep faults and fractures. Western Aphrodite is built from the massifs Ovda and Thetis Regiones; the eastern part of Aphrodite is occupied by Atla Regio. Ishtar Terra fills about half the planet's circumference at its high northern latitudes and is about the size of Australia. It consists of an elevated plateau encircled by narrow mountain belts.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University