The surface of Europa could contain the compositional imprint of an underlying interior ocean, but competing hypotheses differ over whether spectral observations from the Galileo spacecraft show the signature of ocean evaporates or simply surface radiation products unrelated to the interior. Using adaptive optics at the W. M. Keck Observatory, we have obtained spatially resolved spectra of most of the disk of Europa at a spectral resolution ~40?times higher than seen by the Galileo spacecraft. These spectra show a previously undetected distinct signature of magnesium sulfate salts on Europa, but the magnesium sulfate is confined to the trailing hemisphere and spatially correlated with the presence of radiation products like sulfuric acid and SO2. On the leading, less irradiated, hemisphere, our observations rule out the presence of many of the proposed sulfate salts, but do show the presence of distorted water ice bands. Based on the association of the potential MgSO4 detection on the trailing side with other radiation products, we conclude that MgSO4 is also a radiation product, rather than a constituent of a Europa ocean brine. Based on ocean chemistry models, we hypothesize that, prior to irradiation, magnesium is primarily in the form of MgCl2, and we predict that NaCl and KCl are even more abundant, and, in fact, dominate the non-ice component of the leading hemisphere. We propose observational tests of this new hypothesis.
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机译：欧罗巴的表面可能包含潜在的内部海洋的成分烙印，但是在从伽利略号航天器进行的光谱观测表明海洋蒸发的特征还是与内部无关的表面辐射产物方面，竞争性假设有所不同。使用W. M. Keck天文台的自适应光学器件，我们获得了欧罗巴大部分盘的空间分辨光谱，其光谱分辨率比伽利略航天器高40倍。这些光谱显示出欧罗巴上以前未检测到的硫酸镁盐的独特特征，但是硫酸镁被限制在尾随的半球中，并且在空间上与诸如硫酸和SO2的辐射产物的存在相关。在主要的，辐射较少的半球上，我们的观察结果排除了许多拟议的硫酸盐的存在，但确实显示了扭曲的水冰带的存在。基于尾端潜在的MgSO4检测与其他辐射产物的关联，我们得出结论，MgSO4也是辐射产物，而不是欧罗巴海洋盐水的组成部分。基于海洋化学模型，我们假设在辐照之前，镁主要以MgCl2的形式存在，并且我们预测NaCl和KCl甚至更丰富，并且实际上占主导半球的非冰成分。我们提出了这一新假设的观察性检验。
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