Icy Satellites of Saturn: Impact Cratering and Age Determination
Icy Satellites of Saturn: Impact Cratering and Age Determination
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DOI：
10.1007/978-1-4020-9217-6_19
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发表时间：
2009-01-01
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影响因子：
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通讯作者：
Zahnle, Kevin J.
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文献类型：
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作者：
Dones, Luke;Chapman, Clark R.;Zahnle, Kevin J.
土星是第一颗被轨道航天器访问的巨行星，可以向地球传输大量数据。从菲比向内的卫星到常规卫星和环卫星的陨石坑计数，为了解撞击人口的起源和时间历史提供了前所未有的见解。许多“旅行者”时代的科学家得出结论，这些卫星至少受到了两类撞击物的撞击。在这种观点中，星族1的撞击物，通常被认为是绕太阳运行的“彗星”，形成了大多数较大和较老的陨石坑，而星族2的撞击物，被解释为土星轨道上的卫星撞击产生的抛射物，产生了大多数较小和较年轻的陨石坑。旅行者号的数据还暗示，所有的“环卫星”，可能还有一些中等大小的传统卫星，自形成以来都遭到了灾难性的破坏和重新聚集。我们研究了土星系统中主要撞击体种群的模型。目前，“黄道彗星”可能起源于柯伊伯带/散射盘，预计将主导对常规卫星和环状卫星的影响，但模型需要外推大小（从观测到的柯伊伯带天体到产生陨石坑的小得多的天体）或距离（从已知的活跃木星家族彗星到9.5 AU）。土卫九、土卫八，甚至离土星更近的卫星也曾被不规则卫星撞击过。我们描述了尼斯模型，它提供了一种合理的机制，通过这种机制，整个太阳系可能在行星形成很久之后经历了一个猛烈轰击的时代。然后我们讨论了三种陨石坑年表，其中一个是基于尼斯模型的，它被用来从土星卫星上的陨石坑密度推断表面年龄。在回顾了撞击物的性质和它们产生的陨石坑之间的比例关系之后，我们提供了彗星撞击和灾难性破坏土星卫星的当前速率的模型估计。最后，我们展示了来自两组不同卫星的陨石坑计数。许多布满陨石坑的地形似乎接近饱和，因此仅从陨石坑的数量来推断撞击物的来源是困难的。在土卫八上发现的大型陨石坑比在其他卫星上发现的都多。土卫二地表年龄差异很大，从古老的中纬度平原到极其年轻的南极地形。卡西尼号的图像为“第二星族”的存在提供了一些证据。大多数观测到的陨石坑可能是在一次或多次“大灾难”中形成的，但要确定日心说和以行星为中心的天体在形成陨石坑中的作用还需要做更多的工作。
Saturn is the first giant planet to be visited by an orbiting spacecraft that can transmit large amounts of data to Earth. Crater counts on satellites from Phoebe inward to the regular satellites and ring moons are providing unprecedented insights into the origin and time histories of the impacting populations. Many Voyager-era scientists concluded that the satellites had been struck by at least two populations of impactors. In this view, the Population I impactors, which were generally judged to be "comets" orbiting the Sun, formed most of the larger and older craters, while Population II impactors, interpreted as Saturn-orbiting ejecta from impacts on satellites, produced most of the smaller and younger craters. Voyager data also implied that all of the "ring moons;" and probably some of the mid-sized classical moons, had been catastrophically disrupted and reaccreted since they formed. We examine models of the primary impactor populations in the Saturn system. At the present time, "ecliptic comets," which likely originate in the Kuiper Belt/Scattered Disk, are predicted to dominate impacts on the regular satellites and ring moons, but the models require extrapolations in size (from the observed Kuiper Belt Objects to the much smaller bodies that produce the craters) or in distance (from the known active Jupiter family comets to 9.5 AU). Phoebe, Iapetus, and perhaps even moons closer to Saturn have been struck by irregular satellites as well. We describe the Nice model, which provides a plausible mechanism by which the entire Solar System might have experienced an era of heavy bombardment long after the planets formed. We then discuss the three cratering chronologies, including one based upon the Nice model, that have been used to infer surface ages from crater densities on the saturnian satellites. After reviewing scaling relations between the properties of impactors and the craters they produce, we provide model estimates of the present-day rate at which comets impact, and catastrophically disrupt, the saturnian moons. Finally, we present crater counts on the satellites from two different groups. Many of the heavily cratered terrains appear to be nearly saturated, so it is difficult to infer the provenance of the impactors from crater counts alone. More large craters have been found on Iapetus than on any other satellite. Enceladus displays an enormous range of surface ages, ranging from the old mid-latitude plains to the extremely young South Polar Terrain. Cassini images provide some evidence for the reality of "Population II". Most of the observed craters may have formed in one or more "cataclysms," but more work is needed to determine the roles of heliocentric and planetocentric bodies in creating the craters.