We present reflectance spectra of Ganymede's leading and trailing hemispheres in the wavelength range 138–215 nm, obtained by the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) in 2014. The most notable feature of both spectra is the absence of a sharp water absorption edge at ~165 nm, seen in laboratory measurements of ice reflectivity and in previous observations of Saturn's icy moons and rings. Rather than displaying a sharp change in the reflectivity at the wavelength of the water ice absorption edge, Ganymede's reflectance gradually increases with wavelength at λ > 165 nm. We show that the observed shape of Ganymede's UV reflectance is inconsistent with intimate mixture models of pure ice with UV‐dark materials including tholins, amorphous carbon, graphite, and silicates. However, we find that intraparticle models, in which a small proportion of a UV‐absorbing contaminant is trapped as inclusions within the ice matrix, are able to suppress the 165 nm feature at contaminant concentrations of <1%. We show that models of ice with inclusions of silicates, Triton‐type tholin, or H₂O₂ are able to produce the observed gradual increase in reflectivity at λ > 165 nm, but additional absorbing surface materials are required to produce a good fit to Ganymede's previously observed near‐UV and visible reflectance.

中文翻译：

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木卫三的远紫外反射率：约束地表冰中的杂质

我们提供了哈尼太空望远镜宇宙起源光谱仪（HST / COS）在2014年获得的波长为138-215 nm的盖尼米德前半球和后半球的反射光谱。这两个光谱最显着的特征是没有尖锐的水在实验室对冰反射率的测量以及之前对土星冰冷的月球和环的观测中都可以看到〜165 nm处的吸收边缘。而不是在水冰吸收边缘的波长处反射率出现急剧变化，木卫三的反射率随λ> 165 nm处的波长逐渐增加。我们显示，观察到的木卫三的紫外线反射形状与纯冰与紫外线暗物质（包括索林，无定形碳，石墨和硅酸盐）的紧密混合模型不一致。然而，我们发现，在颗粒内部模型中，一小部分吸收紫外线的污染物被夹杂在冰基质中，能够在污染物浓度<1％时抑制165 nm的特征。我们证明了含有硅酸盐，Triton型索林或H夹杂物的冰模型₂ O ₂能够在λ> 165 nm处产生观察到的反射率逐渐增加，但是需要额外的吸收性表面材料才能很好地适合Ganymede先前观察到的近紫外和可见反射率。