Zahnle K, Pollack J B, Grinspoon D, Dones L
NASA Ames Research Center, Moffett Field, California 94035, USA.
Icarus. 1992 Jan;95(1):1-23. doi: 10.1016/0019-1035(92)90187-c.
The competition between impact erosion and impact supply of volatiles to planetary atmospheres can determine whether a planet or satellite accumulates an atmosphere. In the absence of other processes (e.g., outgassing), we find either that a planetary atmosphere should be thick, or that there should be no atmosphere at all. The boundary between the two extreme cases is set by the mass and velocity distributions and intrinsic volatile content of the impactors. We apply our model specifically to Titan, Callisto, and Ganymede. The impacting population is identified with comets, either in the form of stray Uranus-Neptune planetesimals or as dislodged Kuiper belt comets. Systematically lower impact velocities on Titan allow it to retain a thick atmosphere, while Callisto and Ganymede get nothing. Titan's atmosphere may therefore be an expression of a late-accreting, volatile-rich veneer. An impact origin for Titan's atmosphere naturally accounts for the high D/H ratio it shares with Earth, the carbonaceous meteorites, and Halley. It also accounts for the general similarity of Titan's atmosphere to those of Triton and Pluto, which is otherwise puzzling in view of the radically different histories and bulk compositions of these objects.
撞击侵蚀与向行星大气中供应挥发性物质之间的竞争，能够决定一颗行星或卫星是否会积累起大气层。在没有其他过程（例如排气作用）的情况下，我们发现要么行星大气层应该很厚，要么根本就不应该有大气层。这两种极端情况之间的界限，是由撞击体的质量和速度分布以及其固有的挥发性物质含量所决定的。我们将我们的模型具体应用于土卫六、木卫四和木卫三。撞击群体被认定为彗星，其形式要么是游离的天王星 - 海王星微行星，要么是被逐出的柯伊伯带彗星。土卫六上系统地较低的撞击速度使其能够保留浓厚的大气层，而木卫四和木卫三则什么都留不住。因此，土卫六的大气层可能是后期吸积的、富含挥发性物质的表层的一种表现。土卫六大气层的撞击起源自然地解释了它与地球、碳质陨石以及哈雷彗星所共有的高D/H比。这也解释了土卫六大气层与海卫一和冥王星大气层的总体相似性，否则鉴于这些天体截然不同的历史和整体组成，这种相似性会令人困惑。
