木卫二表面水冰结晶度：观测与热物理和粒子通量模拟之间的差异
Europa's surface water ice crystallinity: Discrepancy between observations and thermophysical and particle flux modeling
摘要：
通过热松弛、带电粒子轰击和可能的低温火山活动对欧罗巴表面水冰进行的物理处理，可以改变欧罗巴表面水冰的结晶形式与水冰的无定形形式（“结晶度”）的百分比。在欧罗巴地面温度下，无定形水冰热转化为结晶水冰的时间尺度表明，那里的水冰应该主要以结晶形式存在，然而，木星磁场引起的带电粒子的表面轰击，以及欧罗巴羽流中水冰的气相沉积，可以在短时间尺度上产生无定形水冰表面沉积物。这项调查的目的是确定是否欧罗巴表面水冰结晶度来自地面的光谱测量是在协议的基础上预期的温度和辐射模型的结晶度。使用1D热物理模型的木卫二的表面，我们计算了一个完整的磁盘结晶度木卫二的领先半球，将热松弛的无定形结晶水冰和降解的结晶无定形水冰的照射。与此同时，我们使用Grundy等人（1999），Busarev等人（2018）和Apache Point天文台的近红外地面光谱观测与Mastrapa等人的实验室光谱进行比较得出欧罗巴领先半球的全圆盘结晶度。我们计算出一个模型的结晶度显着高于堆叠来自地面观测和实验室光谱。这种差异可能是地球物理过程的结果，例如气相沉积的羽流材料，也可能是结晶度计算中的假设和不确定性造成的。
英文摘要：
Physical processing of Europan surface water ice by thermal relaxation, charged particle bombardment, and possible cryovolcanic activity can alter the percentage of the crystalline form of water ice compared to that of the amorphous form of water ice (the ''crystallinity'') on Europa's surface. The timescales over which amorphous water ice is thermally transformed to crystalline water ice at Europan surface temperatures suggests that the water ice there should be primarily in the crystalline form, however, surface bombardment by charged particles induced by Jupiter's magnetic field, and vapor deposition of water ice from Europan plumes, can produce amorphous water ice surface deposits on short timescales. The purpose of this investigation is to determine whether the Europan surface water ice crystallinity derived from ground-based spectroscopic measurements is in agreement with the crystallinity expected based on temperature and radiation modeling. Using a 1D thermophysical model of Europa's surface, we calculate a full-disk crystallinity of Europa's leading hemisphere by incorporating the thermal relaxation of amorphous to crystalline water ice and the degradation of crystalline to amorphous water ice by irradiation. Concurrently, we derive the full-disk crystallinity of Europa's leading hemisphere using a comparison of near-infrared ground-based spectral observations from Grundy et al. (1999), Busarev et al. (2018), and the Apache Point Observatory with laboratory spectra from Mastrapa et al. (2018) and the Ice Spectroscopy Lab at the Jet Propulsion Laboratory. We calculate a modeled crystallinity significantly higher than crystallinities derived from ground-based observations and laboratory spectra. This discrepancy may be a result of geophysical processes, such as by vapor-deposited plume material, or it may arise from assumptions and uncertainties in the crystallinity calculations.
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