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NASA 美国宇航局的每日一图

发布: 2009-1-12 15:16 | 作者: webmaster | 来源: 本站原创 | 查看: 292次

2008 May 13

Ancient Craters of Southern Rhea

Explanation: Saturn's ragged moon Rhea has one of the oldest surfaces known. Estimated as changing little in the past billion years, Rhea shows craters so old they no longer appear round – their edges have become compromised by more recent cratering. Like Earth's Moon, Rhea's rotation is locked on Saturn, and the above image shows part of Rhea's surface that always faces Saturn. Rhea's leading surface is more highly cratered than its trailing surface. Rhea is composed mostly of water-ice but is thought to include about 25 percent rock and metal. The above image was taken by the robot Cassini spacecraft now orbiting Saturn. Cassini swooped past Rhea last month and captured the above image from about 350,000 kilometers away. Rhea spans 1,500 kilometers making it Saturn's second largest moon after Titan. Several surface features on Rhea remain unexplained including large light patches like those seen near the image top.

土卫五的古老陨石坑

影像版权: Cassini Imaging Team, SSI, JPL, ESA, NASA

说明: 土星凹凸不平的土卫五 (Rhea )拥有所知最古老的表面之一。据估计土卫五过去数十亿年之间几乎没有改变，它的陨石坑是如此古老，坑缘在较近期形成的陨石坑改造下，陨石坑的外观不再是圆形的。就像地球的月亮一样，土卫五的自转受到土星的锁定，上面影像呈现了永远面向土星的那一面。土卫五领头表面的陨石坑数量远高于坠后的那面。土卫五的主要组是是水冰，不过它可能含有百分之25的岩石和金属。上面影像是由正绕行土星的卡西尼号探测船所拍摄的，它在上个月飞掠过土卫五，并在三十五万公里距离时拍下这幅影像。土卫五的大小约有1,500公里，排行在泰坦卫星之后，是土星的第二大卫星。数个土卫五的表面特征成因仍未明，其中包括影像上方的数个大型的淡色区块。