NASA 美国宇航局的每日一图 3月份

影像提供: Paul Shenk (LPI), USRA; Cassini Imaging Team, SSI, JPL, ESA, NASA    

说明: 生命可以在土卫二表面下生存吗？最近一次飞掠这颗土星的冰卫星支持了这个想法。两年前，由绕着土星运行的卡西尼号宇宙飞船所拍摄的照片使天文学家们发现，土星的卫星土卫二会由它虎纹状的表面爆裂处喷发出气体和冰晶的喷泉。上个月，卡西尼号朝着羽状烟俯冲而下，并侦测到它们含有水蒸气以及少量的甲烷和一些简单及复杂的有机分子。令人惊讶的是，土卫二所喷发出来的羽状烟，其成份与许多彗星相当类似。更甚者，由羽状烟的温度及密度显示它们可能是来自于表面之下较温暖的地方，源头可能是液态的。一个含有有机分子的海洋是寻找生命的好地方。上图是一个细长的虎纹状火山口的近照，其中垂直方向有经过拉长。计算机合成的影像是由最近一次卡西尼号飞越时所拍摄的影像和阴影部份产生的。卡西尼号预计将还有九次的机会飞掠土卫二。  

Explanation: Could life exist beneath Enceladus? A recent flyby of Saturn's icy moon has bolstered this fascinating idea. Two years ago, images from the robotic Cassini spacecraft orbiting Saturn led astronomers to the undeniable conclusion that Saturn's moon Enceladus was spewing fountains of gas and ice crystals through cracks in its surface dubbed tiger stripes. Last month, Cassini dove through some of these plumes and determined that they contained water vapor laced with small amounts of methane as well as simple and complex organic molecules. Surprisingly, the plumes of Enceladus appear similar in make-up to many comets. What's more, the temperature and density of the plumes indicate they might have originated from a warmer source -- possibly a liquid source -- beneath the surface. A liquid water sea containing organic molecules is a good place to look for life. Pictured above is a vertically exaggerated close-up of some long, venting tiger stripes. The computer composite was generated from images and shadows taken during recent Cassini flybys. Nine more flybys of Enceladus by Cassini are planned.