Hunten D M
Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona 85721, USA.
Nature. 2006 Oct 12;443(7112):669-70. doi: 10.1038/nature05157.
Saturn's largest satellite, Titan, has a dense atmosphere of nitrogen with a few per cent of methane. At visible wavelengths its surface is hidden by dense orange-brown smog, which is produced in the stratosphere by photochemical reactions following the dissociation of methane by solar ultraviolet light. The most abundant of the products of these reactions is ethane, and enough of it should have been generated over the life of the Solar System to form a satellite-wide ocean one kilometre deep. Radar observations have found specular reflections in 75 per cent of the surface spots observed, but optical searches for a sun-glint off an ocean have been negative. Here I explain the mysterious absence or rarity of liquid ethane: it condenses onto the smog particles, instead of into liquid drops, at the cold temperatures in Titan's atmosphere. This dusty combination of smog and ethane, forming deposits several kilometres thick on the surface, including the observed dunes and dark areas, could be named 'smust'. This satellite-wide deposit replaces the ocean long thought to be an important feature of Titan.
土星最大的卫星土卫六拥有浓厚的氮气大气层，其中含有百分之几的甲烷。在可见光波长下，其表面被浓密的橙褐色烟雾所遮蔽，这种烟雾是在平流层中由太阳紫外线使甲烷解离后发生光化学反应产生的。这些反应最丰富的产物是乙烷，在太阳系的寿命期间应该已经产生了足够多的乙烷，足以形成一个覆盖整个卫星、深达一公里的海洋。雷达观测在观测到的75%的表面区域发现了镜面反射，但对海洋太阳反光的光学搜索结果为阴性。在此我解释液态乙烷神秘缺失或稀少的原因：在土卫六大气的低温下，它凝结在烟雾颗粒上，而不是形成液滴。这种烟雾和乙烷的尘土混合物在表面形成了数公里厚的沉积物，包括观测到的沙丘和暗区，可将其命名为“烟雾土”。这种覆盖整个卫星的沉积物取代了长期以来被认为是土卫六重要特征的海洋。
