Explanation: What has happened to Saturn’s moon Iapetus? Vast sections of this strange world are dark brown, while others are as bright white. The composition of the dark material is unknown, but infrared spectra indicate that it possibly contains some dark form of carbon. Iapetus also has an unusual equatorial ridge that makes it appear like a walnut. To help better understand this seemingly painted moon, NASA directed the robotic Cassini spacecraft orbiting Saturn to swoop within 2,000 kilometers in 2007. Iapetus is pictured here in 3D. A huge impact crater seen in the south spans a tremendous 450 kilometers and appears superposed on an older crater of similar size. The dark material is seen increasingly coating the easternmost part of Iapetus, darkening craters and highlands alike. Close inspection indicates that the dark coating typically faces the moon’s equator and is less than a meter thick. A leading hypothesis is that the dark material is mostly dirt leftover when relatively warm but dirty ice sublimates. An initial coating of dark material may have been effectively painted on by the accretion of meteor-liberated debris from other moons.
Tomorrow’s picture: black hole eats
说明: 土星的卫星土卫八到底发生了什么事？因为这个奇特的天体有大片的表面是暗棕色，而其他的区域则是亮白色。这些黝黑物质的组成目前不明，但红外 光谱显示，它可能是色泽暗沉的某种碳。土卫八有奇特的赤道突脊，因此它的外观形似核桃。为了解这颗多彩的卫星提供助力，NASA让绕行土星的卡西尼号太空船，在2007年以不到2,000公里的间距掠过土卫八。在这幅土卫八的3D影像里，南半球可见到一个宽达450公里的庞大撞击坑，而且看似叠在另一大小相近的较古老撞击坑之上。此外，越靠近土卫星的东缘，不管是撞击坑或高原，皆披着越来越浓密的暗色物质。如仔细检视，可发现暗色的披覆通常与这颗卫星的赤道重叠，而且厚度不到1公尺。目前最可信的假说指出暗色的物质，可能是相对温暖的脏冰升华留下后的泥尘。而起初披覆的第一层暗色物质，可能是土卫八吸积了其他卫星被流星扬起的碎片。
明日的图片: black hole eats