从哈勃Space Telescope观测探测到木卫三上的升华水大气
A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations
摘要：
木卫三的大气层是由带电粒子溅射和其冰冷表面的升华产生的。先前对O{ \small I\，}1356-氘和O{ \small I\，}1304-氘氧发射的远紫外观测被用来推断溅射分子氧（O $_2$）是大气成分，但预期的升华水（H $_2$ O）成分仍未被检测到。在这里，我们提出了一个高灵敏度的光谱和光谱图像的哈勃太空望远镜获得的分析揭示H $_2$ O在木卫三的大气。氧排放的相对强度需要来自水蒸气的解离激发的贡献，这表明在太阳下点附近H $_2$ O比O $_2$更丰富。远离太阳下区域，排放物与纯O $_2$大气一致。日食观测限制原子氧比这些其他物种少至少两个数量级。较高的H $_2$ O/O $_2$比以上的较温暖的后半球相比，较冷的领先半球，空间浓度太阳下的区域，和估计的丰度$\sim$ 10 $^{15}$ H $_2$ O/cm $^{2}$是一致的升华的冰面作为源。
英文摘要：
Ganymede's atmosphere is produced by charged particle sputtering and sublimation of its icy surface. Previous far-ultraviolet observations of the O{\small I\,}1356-Å and O{\small I\,}1304-Å oxygen emissions were used to infer sputtered molecular oxygen (O$_2$) as an atmospheric constituent, but an expected sublimated water (H$_2$O) component remained undetected. Here we present an analysis of high-sensitivity spectra and spectral images acquired by the Hubble Space Telescope revealing H$_2$O in Ganymede's atmosphere. The relative intensity of the oxygen emissions requires contributions from dissociative excitation of water vapor, indicating that H$_2$O is more abundant than O$_2$ around the sub-solar point. Away from the sub-solar region, the emissions are consistent with a pure O$_2$ atmosphere. Eclipse observations constrain atomic oxygen to be at least two orders of magnitude less abundant than these other species. The higher H$_2$O/O$_2$ ratio above the warmer trailing hemisphere compared to the colder leading hemisphere, the spatial concentration to the sub-solar region, and the estimated abundance of $\sim$10$^{15}$ H$_2$O/cm$^{2}$ are consistent with sublimation of the icy surface as source.
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