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Venus craters.
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| THE CRATERS OF VENUS | |||
| Venus has about 900 impact craters. Compared to heavily cratered bodies like the Moon and Mercury, the crater population of Venus is low. Such a small number of craters suggests that the present surface of Venus is relatively young. On the Moon the high density of craters indicates that parts of the surface have not changed for 3.8 thousand million years; when cratering rates in the Solar System were higher. The small number of craters on Venus is thought to indicate a far younger surface than 0.5 thousand million years. The young age of the Venusian craters is also indicated by their pristine condition. Their surfaces appear crisp and undegraded. Most larger craters have central peaks and some have terraces on their inner slopes. They differ from lunar and Mercurian craters in that the material thrown outward by the impact, the ejecta, is confined to a small area; equivalent to three crater radii. This is due to the greater gravitational influence Venus has on the ejected material. | |||
| Although there are no very large basins that can be identified with certainty (some have suggested that Atalanta Planitia is an ancient impact basin), there are several multi ringed craters on Venus, including Mead Crater and Barton, just 50 kilometres across. Mead Crater, at 280 kilometres in diameter, is the largest crater on Venus. | |||
 Mead crater |
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Mead
Crater |
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Barton
Crater |
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 Barton Crater. |
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| There are few big craters on Venus, because most of those from the early history of the solar system have been obliterated by lavas that erupted to form the plains. Really small craters are also absent from the surface of Venus. If small ones were there we would be able to see them because Magellan radar images show objects a few hundred metres across. The thick atmosphere though, causes the small meteorites to burn up. For this reason Venus is missing the usual range of smaller craters, as found on the other rocky bodies of the Solar System. | |||
| Because radar is good at detecting differences in surface roughness, the lava flows, caused by impact melting the surface can be seen. These rough and therefore radar bright lava fields continue for hundreds of kilometres show up well against the smooth dark plains. It also means it is possible to see how loose surface materials are re-distributed by the blast. | |||
 Isabella crater and lava flows |
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| Impacts do not just make craters and melt rocks, they also disturb the atmosphere. On Venus these effects can be seen in the re-distribution of loose surface materials near the crater. Many craters on Venus have a dark oval shaped halo hundreds of kilometres wide and up to 1500 kilometres long. These are thought to be created by winds which follow the impact picking up fine material like dust and small sand grains, depositing it down-wind of the impact. This fine debris fills in gaps in the craggy surface, making the surface appear smoother. Nearer the crater, it is common to see a bright ring. This is thought to be caused by the strong winds scouring, winnowing material from cracks and niches in the rock, making the surface appear rougher. | |||
 Magellan radar image of Adivar Crater, Venus |
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