Callisto (moon of Jupiter)
Callisto's surface, the darkest of any of the Galilean moons (although still twice as bright as our own Moon), is the most heavily cratered of any object in the solar system, testifying to an almost complete absence of geological activity over the past 4 billion years. Indeed, Callisto is the only body greater than 1,000 km in diameter which shows no signs of having undergone any significant resurfacing since the end of the late bombardment phase of the solar system about 3.8 billion years ago. Its surface features are dominated by shallow impact craters and rings. Two large features, Valhalla and Asgard, which resemble bullseyes, are believed to be the remains of massive impacts. Seven chains of impact craters have been mapped and are thought to have been formed when comets were broken up by Jupiter's gravity and collided with Callisto.
In February 1999, the discovery was announced, based on measurements taken by Galileo's near-infrared mapping spectrometer, of carbon dioxide ice on Callisto's surface together with a very tenuous atmosphere of carbon dioxide. Since this gas must constantly leak into space under the action of ultraviolet rays from the Sun, it must be continuously replenished, possibly by venting of carbon dioxide from the interior. This discovery means that all four Galilean moons are now known to have extremely tenuous atmospheres.
Related entry Jupiter, moons
Related category PLANETS AND MOONS
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