Venus Surface Features
Venus rotates very slowly on its axis, and the direction is retrograde (opposite to that of Earth). Curiously, the periods of Venus's orbit and rotation cause the same side of the planet to always face Earth when the two planets are closest. At such times, the side facing Earth can be viewed and mapped by Earth-based radar.
In contrast to the very large antenna needed for Earth-based radar mapping of Venus, a modest instrument on Pioneer Venus 1 was able to conduct a nearly global survey. Combined with data from the Soviet probes, the Magellan orbiter, and Earth-based radar, the survey shows that the surface of Venus is primarily a rolling plain interrupted by two continent-sized highland areas, which have been named Ishtar Terra and Aphrodite Terra after two manifestations of the goddess Venus. Aphrodite Terra, although not as high as Ishtar Terra, extends nearly halfway around the equatorial region; it occupies the planet's far side as viewed from Earth at closest approach.
The more powerful radar aboard the Magellan spacecraft has revealed huge volcanoes, large solidified lava flows, and a large array of meteorite craters. The largest impact crater is almost 300 km (190 mi) across—the smallest about 5 km (3 mi). Although the probe's radar could resolve even smaller craters, if any were present, Venus's dense atmosphere apparently prevents smaller meteorites from impacting the surface of the planet. It is believed that all craters older than about 500 million years have been obliterated, while the more recent ones show little sign of modification.
The global survey and other probes have also revealed evidence that a great deal of tectonic activity has taken place on Venus, at least in the past. Such evidence includes ridges, canyons, a troughlike depression that extends across 1400 km (870 mi) of the surface, and a gigantic volcanic cone whose base is more than 700 km (435 mi) wide. The Soviet probes sent back photographs of the areas in which they set down, and also measured the natural radioactivity of the rocks. The radioactivity resembles that of granite and strongly suggests that the material of Venus is differentiated, or chemically separated, by volcanic activity. Angular rocks that are visible in the Soviet pictures also suggest the existence of geologic activity that would counteract the forces of erosion.
In contrast to the very large antenna needed for Earth-based radar mapping of Venus, a modest instrument on Pioneer Venus 1 was able to conduct a nearly global survey. Combined with data from the Soviet probes, the Magellan orbiter, and Earth-based radar, the survey shows that the surface of Venus is primarily a rolling plain interrupted by two continent-sized highland areas, which have been named Ishtar Terra and Aphrodite Terra after two manifestations of the goddess Venus. Aphrodite Terra, although not as high as Ishtar Terra, extends nearly halfway around the equatorial region; it occupies the planet's far side as viewed from Earth at closest approach.
The more powerful radar aboard the Magellan spacecraft has revealed huge volcanoes, large solidified lava flows, and a large array of meteorite craters. The largest impact crater is almost 300 km (190 mi) across—the smallest about 5 km (3 mi). Although the probe's radar could resolve even smaller craters, if any were present, Venus's dense atmosphere apparently prevents smaller meteorites from impacting the surface of the planet. It is believed that all craters older than about 500 million years have been obliterated, while the more recent ones show little sign of modification.
The global survey and other probes have also revealed evidence that a great deal of tectonic activity has taken place on Venus, at least in the past. Such evidence includes ridges, canyons, a troughlike depression that extends across 1400 km (870 mi) of the surface, and a gigantic volcanic cone whose base is more than 700 km (435 mi) wide. The Soviet probes sent back photographs of the areas in which they set down, and also measured the natural radioactivity of the rocks. The radioactivity resembles that of granite and strongly suggests that the material of Venus is differentiated, or chemically separated, by volcanic activity. Angular rocks that are visible in the Soviet pictures also suggest the existence of geologic activity that would counteract the forces of erosion.
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