The variability of Mercury's exosphere by particle and radiation induced surface release processes
The variability of Mercury's exosphere by particle and radiation induced surface release processes
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粒子和辐射引起的表面释放过程导致水星外逸层的变化
DOI：
10.1016/j.icarus.2003.08.012
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发表时间：
2003
期刊：
影响因子：
3.2
通讯作者：
A. Milillo
中科院分区：
文献类型：
--
作者：
H. Lammer;P. Wurz;Manish R. Patel;R. Killen;C. Kolb;S. Massetti;S. Orsini;A. Milillo
水星绕太阳的轨道很近，其微弱的固有磁场和没有大气层（p表面<1×10−8Pa）导致其表面直接暴露在高能离子、电子和紫外线辐射下。热过程和粒子-表面碰撞主导了导致表面化学和物理的表面相互作用过程，包括形成一个外逸层（N≤1014cm−2），其中重力是影响外逸层原子轨迹的主要力量。美国宇航局的水手10号宇宙飞船在水星的外逸层中观测到氢、氦和氧的存在。此外，外逸层的挥发性组分Na、K和Ca已被地面仪器观测到。我们通过计算停止截面、溅射产量和外逸层源率来研究几种粒子表面释放过程的效率。我们的研究表明，当Na+离子为溅射剂时，在500 eV至2 keV的能量范围内，Na的表面溅射产率约为0.27 ~ 0.35；当H+离子为溅射剂时，在500 eV至2 keV的能量范围内，表面结合能约为2 eV ~ 2.65 eV， Na的表面溅射产率约为0.037 ~ 0.082。在相同能量范围内，Ca原子的溅射产率约为0.032 ~ 0.06，K原子的溅射产率约为0.054 ~ 0.1。我们发现，当粒子能量在500 eV到2 keV质子之间时，O原子的溅射产额在0.025到0.04之间。利用Massetti等人（2003,Icarus, in press）计算的开放磁场线面积约为4×108cm−2s−1处的平均太阳风质子表面通量，根据假设的Na结合能、表土含量、溅射剂和太阳活动，得到O的平均溅射通量约为0.8-1.0×107cm−2s−1，Na的平均溅射通量约为1.3-1.6×105cm−2s−1。通过使用月球风化层的K值，我们得到了大约1.0-1.4×104cm−2s−1的溅射通量。Massetti等人（2003,Icarus, in press）取开放磁场线的平均面积约为2.8×1016cm2modelled，我们得出Na的平均表面溅射率约为4.2×1021s−1，O的平均表面溅射率约为2.5×1023s−1。假设月球风化层中含有K，则K原子的粒子溅射率约为3.0×1020s−1。溅射率取决于风化层中的粒子含量和水星表面的开放磁场线面积。此外，表层可能会在碱中耗尽。一个紫外线模型已经被开发出来，以产生一个水星年在任何时间和纬度的地表紫外线辐照度。计算了季节和日变化，并评估了沿水星轨道的光子激发解吸（PSD）通量。在近日点方向形成了一个太阳紫外线热点，具有显著的PSD粒子平均释放率和Na通量，约为3.0×106cm−2s−1。PSD释放Na粒子的平均源速率约为1×1024s−1。利用Madey et al.（1998）实验室获得的数据，J. Geophys。参考Res. 103, 5873-5887)计算K原子的PSD通量，我们得到沿水星轨道的通量约为104cm−2s−1。然而，这些值可能过高，因为它们是基于实验室中理想的光滑表面条件，而不包括水星风化层的粗糙度和孔隙度。此外，缺乏电离层和水星的小，时间和空间高度可变的磁层可能导致大量和快速增加的外逸层粒子，特别是Na在水星的外逸层。我们的研究表明，在太阳静止状态下，太阳粒子和辐射诱导的表面过程对外逸层的平均总源率可能是相同的。
Mercury's close orbit around the Sun, its weak intrinsic magnetic field and the absence of an atmosphere (Psurface<1×10−8Pa) results in a strong direct exposure of the surface to energetic ions, electrons and UV radiation. Thermal processes and particle-surface-collisions dominate the surface interaction processes leading to surface chemistry and physics, including the formation of an exosphere (N⩽1014cm−2) in which gravity is the dominant force affecting the trajectories of exospheric atoms. NASA's Mariner 10 spacecraft observed the existence of H, He, and O in Mercury's exosphere. In addition, the volatile components Na, K, and Ca have been observed by ground based instrumentation in the exosphere. We study the efficiency of several particle surface release processes by calculating stopping cross-sections, sputter yields and exospheric source rates. Our study indicates surface sputter yields for Na between values of about 0.27 and 0.35 in an energy range from 500 eV up to 2 keV if Na+ions are the sputter agents, and about 0.037 and 0.082 at an energy range between 500 eV up to 2 keV when H+are the sputter agents and a surface binding energy of about 2 eV to 2.65 eV. The sputter yields for Ca are about 0.032 to 0.06 and for K atoms between 0.054 to 0.1 in the same energy range. We found a sputter yield for O atoms between 0.025 and 0.04 for a particle energy range between 500 eV up to 2 keV protons. By taking the average solar wind proton surface flux at the open magnetic field line area of about 4×108cm−2s−1calculated by Massetti et al. (2003, Icarus, in press) the resulting average sputtering flux for O is about 0.8–1.0×107cm−2s−1and for Na approximately 1.3–1.6×105cm−2s−1depending on the assumed Na binding energies, regolith content, sputtering agents and solar activity. By using lunar regolith values for K we obtain a sputtering flux of about 1.0–1.4×104cm−2s−1. By taking an average open magnetic field line area of about 2.8×1016cm2modelled by Massetti et al. (2003, Icarus, in press) we derive an average surface sputter rate for Na of about 4.2×1021s−1and for O of about 2.5×1023s−1. The particle sputter rate for K atoms is about 3.0×1020s−1assuming lunar regolith composition for K. The sputter rates depend on the particle content in the regolith and the open magnetic field line area on Mercury's surface. Further, the surface layer could be depleted in alkali. A UV model has been developed to yield the surface UV irradiance at any time and latitude over a Mercury year. Seasonal and diurnal variations are calculated, and Photon Stimulated Desorption (PSD) fluxes along Mercury's orbit are evaluated. A solar UV hotspot is created towards perihelion, with significant average PSD particle release rates and Na fluxes of about 3.0×106cm−2s−1. The average source rates for Na particles released by PSD are about 1×1024s−1. By using the laboratory obtained data of Madey et al. (1998, J. Geophys. Res. 103, 5873–5887) for the calculation of the PSD flux of K atoms we get fluxes in the order of about 104cm−2s−1along Mercury's orbit. However, these values may be to high since they are based on idealized smooth surface conditions in the laboratory and do not include the roughness and porosity of Mercury's regolith. Further, the lack of an ionosphere and Mercury's small, temporally and spatially highly variable magnetosphere can result in a large and rapid increase of exospheric particles, especially Na in Mercury's exosphere. Our study suggests that the average total source rates for the exosphere from solar particle and radiation induced surface processes during quiet solar conditions may be of the same …