charles 2009-3-23 10:41
土衛五的古老隕石坑
Ancient Craters on Saturn's Rhea
[img]http://i3.tinypic.com/48oky6a.jpg[/img]
影像提供: Cassini Imaging Team, SSI, JPL, ESA, NASA
[color=navy]Explanation:[/color]
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 have a small rocky core. The above image was taken by the robot Cassini spacecraft now orbiting Saturn. Cassini swooped past Rhea two months ago and captured the above image from about 100,000 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.
[color=navy]說明:[/color]
土星凹凸不平的土衛五 (Rhea )擁有所知最古老的表面之一。據估計土衛五過去數十億年之間幾乎沒有改變，它的隕石坑是如此古老，坑緣在較近期形成的隕石坑之改造下，隕石坑的外觀不再是圓形的。就像地球的月亮一樣，土衛五的自轉受到土星的鎖定，上面影像呈現了永遠面向土星的那一面。土衛五領頭表面的隕石坑數量遠高於墜後的那面。土衛五的主要組是是水冰，不過它可能也有個石質核心。上面影像是由正繞行土星的卡西尼號探測船所拍攝的，它在二個月之前飛掠過土衛星，並在十萬公里距離時拍下這幅影像。土衛五的大小約有1,500公里，排行在泰坦衛星之後，是土星的第二大衛星。數個土衛五的表面特徵成因仍未明，其中包括數個大型的淡色區塊。
[size=2]資料來源: Scientific American
Department of Physics, NCKU[/size]