Europa’s ice-related atmosphere: The sputter contribution
Europa’s ice-related atmosphere: The sputter contribution
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木卫二与冰相关的大气：溅射的贡献
DOI：
10.1016/j.icarus.2018.03.022
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发表时间：
2018
期刊：
影响因子：
3.2
通讯作者：
P. Wurz
中科院分区：
文献类型：
--
作者：
A. Vorburger;P. Wurz
木卫二是木星最里面的一颗冰冷的卫星，它很好地嵌入了木星的磁层等离子体中，磁层等离子体是一种强烈的离子和电子流，几乎与木星共旋转。等离子体可以被认为是由两个群体组成：冷的、热的等离子体包含带电粒子，能量范围从1ev到1kev，热的、高能的等离子体包含带电粒子，能量范围从10kev到100mev。当带电粒子与木卫二表面相互作用时，它们不仅在化学和物理上改变了木卫二冰冷的表面，而且还通过一种叫做溅射的过程从表面释放出物质，从而形成了稀薄的大气层。在本文中，我们通过从头开始模拟木卫二冰溅射大气的形成来计算溅射对大气的贡献。我们考虑H、H2、O、OH、H2O、O2、HO2、H2O2和O3，它们都与水冰表面有关。虽然H2O、H2和O2的冰溅射产率已经很好地确定，但H、O、OH、ho2和o3的冰溅射产率（以及由此产生的密度分布）很小，而且很大程度上是未知的。我们使用可用的等离子体离子和电子能谱以及可用的水冰溅射量作为模型输入。基于第一性原理，即不对观测数据进行任何标度，我们从头开始计算大气密度。我们的结果与现有的观测数据和先前发表的建模工作很好地匹配。木卫二的外逸层主要由靠近表面（低于100公里）的热容纳的o2和高海拔的轻得多的h2分子组成。在冷等离子体和热等离子体溅射中释放出的与水冰相关的物质粘附在表面（被冻结），数量大致相同。此外，在H2、O2和H2O2的情况下，电子对溅射产率的贡献几乎与离子一样显著。
Europa, Jupiter’s innermost icy satellite, is embedded well within Jupiter’s magnetospheric plasma, an intense flux of ions and electrons that approximately co-rotate with Jupiter. The plasma can be thought of as consisting of two populations: The cold, thermal plasma containing charged particles with energies ranging from 1 eV to 1 keV, and the hot, energetic plasma containing charged particles with energies ranging from 10 keV to 100 MeV. When the charged particles interact with Europa’s surface, they not only chemically and physically alter the icy surface, but also liberate material from the surface through a process called sputtering, which in turn forms a tenuous atmosphere.In this paper we calculate the sputter contribution to the atmosphere by modeling the formation of Europa’s ice-sputtered atmosphere ab initio. We consider the species H, H2, O, OH, H2O, O2, HO2, H2O2, and O3, all of which are related to the water–ice surface. Whereas the ice sputter yields of H2O, H2, and O2 have been well established, the ice sputter yields (and the resulting density profiles) of H, O, OH, HO2and O3are small and largely unknown. As model input we use available plasma ion and electron energy spectra as well as available water-ice sputter yields. Based on first principles, i.e., without applying any scaling to observed data, we calculate atmospheric densities ab initio.Our results match available observational data and previously published modeling efforts well. Europa’s exosphere is dominated by thermally accommodated O2close to the surface (below a few 100 km), and the much lighter H2molecules at higher altitudes. The water-ice related species that stick to the surface (freeze out) are liberated by cold and hot plasma sputtering in about equal amounts. In addition, in the case of H2, O2, and H2O2, electrons contribute almost as significantly to the sputter yield as ions do.