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Cassini image of Saturn's |
Cassini view of Dione showing whispy white ice fractures |
Cassini image of Dione showing craters and other features |
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Dione |
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Mother of Aphrodite
Tethys [TEE-this] is the ninth of Saturn's known moons and is the fourth largest. It was named after the Greek Titaness who was a sea goddess and both sister and wife of Oceanus. She was the personification of the fertile ocean, and her 3,000 children became the springs, lakes, and rivers of the world. Tethys was discovered by Giovanni Cassini in 1684. Most of what we now know about Tethys was learned during the Voyager missions.
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Cassini view of Dione with |
Cassini close-up image of Dione showing fractured terrain |
Cassini image of Dioneshowing craters and ice fractures |
A Desolate Ice world
Dione is the densest of Saturn's moons with the exception of Titan. It is composed mainly of water ice, but must contain a larger amount of rocky material than Saturn's other ice moons, Tethys and Rhea. Dione is very similar to Rhea in composition, although somewhat smaller. Dione is believed to have a rocky core with less ice coverage than Rhea.
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This Cassini spacecraft image shows Dione in orbit above the planet Saturn. Cloud bands and oval-shaped |
Features of Dione
Dione is locked in a synchronous orbit similar to that of Rhea. This causes the same face of the moon to point towards Saturn at all times. Dione has similar albedo features and terrain to that of its close cousin, Rhea. Dione's surface consists of heavily cratered areas, moderate and lightly cratered plains, and bright, wispy features. Most of the heavily cratered areas exist on the trailing hemisphere of the moon. Some of these craters exceed 62 miles (100 km) in diameter, while most of the craters in the plains areas are less than 18 miles (30 km) in diameter. The largest crater is called Amata and is 150 miles (241 km) in diameter. Like Rhea, these craters lack the high relief features seen on Mercury and the Moon. Heavy cratering would normally be expected on the leading edge of a tidally locked satellite. Since most of Dione's craters are located on the trailing hemisphere, astronomers believe that Rhea may have once been tidally locked with Saturn in the opposite orientation. Since Dione is relatively small, it would have only taken an impact leaving a 21-mile (35 km) crater to spin the moon around. With many craters on Dione exceeding 21 miles, it is possible that the moon has been spun around more than once throughout its long history. The origin of the bright, wispy streaks is not known. The streaks overlay many of the craters, which indicates that they are newer. They may have been formed by ice eruptions through cracks in the surface. This material may have fallen back to the surface as snow or ash. Dione has no detectable atmosphere.
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