中性粒子从木卫二表面释放
Neutral particle release from Europa's surface
摘要：
在本文中，我们将研究木星卫星欧罗巴冰冷表面的空间风化过程。木星等离子体中的高能重离子（H+、O+、S+、C+）可以通过离子溅射（IS）侵蚀木卫二的表面，每个离子可以喷射出多达1000个H2O分子。撞击欧罗巴表面的紫外光子也可以通过光子刺激解吸（PSD）和化学变化（光解）导致中性原子释放。在这项工作中，我们研究了喷射水分子的IS和PSD过程的效率，模拟产生的中性H2O密度。我们还估计了离子背散射（IBS）过程对总中性原子释放的贡献。此外，我们估计检测溅射的高能原子的可能性，以区分从其他表面释放机制的IS过程的行动。我们的主要结果是：（1）入射S+离子对中性H_2O ~-密度的贡献最大，且最显著的粒子通量来自入射S+离子，（2）PSD产生的H_2O ~-密度比溅射产生的H_2O ~-密度低约1.5个数量级; 3）在低于1 keV的能量范围内，IBS对于中性粒子的产生可以被认为是可忽略的，4）木卫二的总溅射速率为2.0 \cdot × 10 ~（27）H_2 O s ~（-1）; 5）通过IS逸出的H_2O占总溅射粒子数的22%，而PSD产生的H_2O逸出占总PSD粒子数的30%。由于PSD散逸层低于IS散逸层，因此木卫二表面侵蚀的主要因素是非照明侧和照明侧的IS。最后，外逸层中性密度，估计从伽利略电子密度测量似乎高于计算的H2O单独，这有利于存在的情况下，产生的O2的辐解和光解。
英文摘要：
In this paper, we look at space weathering processes on the icy surface of Jupiter's moon Europa. The heavy energetic ions of the Jovian plasma (H+, O+, S+, C+) can erode the surface of Europa via ion sputtering (IS), ejecting up to 1000 H2O molecules per ion. UV Photons impinging the Europa's surface can also result in neutral atom release via photon-stimulated desorption (PSD) and chemical change (photolysis). In this work, we study the efficiency of the IS and PSD processes for ejecting water molecules, simulating the resulting neutral H2O density. We also estimate the contribution to the total neutral atom release by the Ion Backscattering (IBS) process. Moreover, we estimate the possibility of detecting the sputtered high energy atoms, in order to distinguish the action of the IS process from other surface release mechanisms. Our main results are: 1) The most significant sputtered-particle flux and the largest contribution to the neutral H2O-density come from the incident S+ ions; 2) The H2O density produced via PSD is lower than that due to sputtering by ~1.5 orders of magnitude; 3) In the energy range below 1 keV, the IBS can be considered negligible for the production of neutrals, whereas in the higher energy range it becomes the dominant neutral emission mechanism; 4) the total sputtering rate for Europa is 2.0\cdot 1027 H2O s-1; 5) the fraction of escaping H2O via IS is 22% of the total sputtered population, while the escape fraction for H2O produced by PSD is 30% of the total PSD population. Since the PSD exosphere is lower than the IS one, the major agent for Europa's surface erosion is IS on both the non-illuminated and illuminated side. Lastly, the exospheric neutral density, estimated from the Galileo electron density measurements appears to be higher than that calculated for H2O alone; this favours the scenario of the presence of O2 produced by radiolysis and photolysis.
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