Mercury's seasonal sodium exosphere: MESSENGER orbital observations
Mercury's seasonal sodium exosphere: MESSENGER orbital observations
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DOI：
10.1016/j.icarus.2014.10.037
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发表时间：
2015-03-01
期刊：
影响因子：
3.2
通讯作者：
Vervack, Ronald J., Jr.
中科院分区：
文献类型：
--
作者：
Cassidy, Timothy A.;Merkel, Aimee W.;Vervack, Ronald J., Jr.
水星表面、空间环境、地球化学和测距（信使号）航天器上的水星大气和表面成分光谱仪（MASCS）紫外和可见光光谱仪（UVVS）首次近距离观察了水星的钠外大气层。UVVS几乎每天都在轨道上观测外逸层，已经有超过10个水星年了。在本文中，我们描述和分析了这些数据的一个子集：低纬度日面和南极以上的高度剖面图。观测结果显示出时空变化，但年际变化很小或没有变化；我们没有看到地面观测者报告的偶发性变率。我们用这些海拔剖面来估计钠的密度和温度。外逸层的大部分温度约为1200k，比水星表面的温度高得多。这一数值与一些地面测量结果一致，表明光子刺激的解吸是主要的喷射过程。我们还观察到微弱的能量成分，但在水星表面温度附近没有发现预测的热化（或部分热化）钠的证据。总的来说，我们没有看到大多数外逸层蒙特卡罗模型所预测的温度的可变混合。(C) 2014年由Elsevier Inc.出版。
The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) Ultraviolet and Visible Spectrometer (UVVS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft now orbiting Mercury provides the first close-up look at the planet's sodium exosphere. UVVS has observed the exosphere from orbit almost daily for over 10 Mercury years. In this paper we describe and analyze a subset of these data: altitude profiles taken above the low-latitude dayside and south pole. The observations show spatial and temporal variation but there is little or no year-to-year variation; we do not see the episodic variability reported by ground-based observers. We used these altitude profiles to make estimates of sodium density and temperature. The bulk of the exosphere is about 1200 K, much warmer than Mercury's surface. This value is consistent with some ground-based measurements and suggests that photon-stimulated desorption is the primary ejection process. We also observe a tenuous energetic component but do not see evidence of the predicted thermalized (or partially thermalized) sodium near Mercury's surface temperature. Overall we do not see the variable mixture of temperatures predicted by most Monte Carlo models of the exosphere. (C) 2014 Published by Elsevier Inc.