用Subaru/IRCS在1.0-1.8 $μm$空间分辨观测木卫二表面：含水含氯盐丰度的上限
Spatially resolved observations of Europa's surface with Subaru/IRCS at 1.0-1.8 $μm$: Upper limits to the abundances of hydrated Cl-bearing salts
摘要：
最近使用大型地面望远镜在1.5-4.0 $μm$进行的红外观测表明，木卫二表面可能存在含Cl盐的非冰物质。这些含氯盐的化学成分是了解木卫二海洋化学和可居住性的关键。在这里，我们报告的结果，地面望远镜观测木卫二在两个波长范围内，1.0-1.5和1.5-1.8 $μm$，其中前一个范围包括吸收功能，由于一些水合氯轴承盐。我们使用斯巴鲁望远镜/IRCS和自适应光学系统AO 188获得了高波长分辨率（$δλ$ ~ 2 nm，1.0-1.5 $μm$; $δλ$ ~ 0.9 nm，1.5-1.8 $μm$）和低噪声水平（1 $σ$ ~ 1-2 $\times$ 10 $^{-3}$）的空间分辨反射光谱。我们发现没有明确的吸收功能，在1.2 $μ$ m所造成的水合氯轴承盐。我们估计，在3 $σ$噪声水平下，欧罗巴上MgCl $_2$ $\cdot$ nH $_2$ O、NaClO $_4$ $\cdot$ 2 H $_2$ O、Mg（ClO $_3$）$_2$ $\cdot$ 6 H $_2$ O和Mg（ClO $_4$）$_2$ $\cdot$ 6 H $_2$ O丰度的保守上限为17%（大多数<10%）。这些数值低于基于先前观测的木卫二上某些含水含氯盐的丰度（> ~20%）。这支持了木卫二上的含Cl盐可能是NaCl和/或NaClO $_4$的无水Na盐或水合NaCl $\cdot$ 2 H $_2$ O的观点。Na盐的存在表明Na $^+$可能是木卫二海洋中的主要阳离子，如果海洋的pH值是中性或碱性，这是可能的。
英文摘要：
Recent infrared observations at 1.5-4.0 $μm$ using large ground-based telescopes have suggested that Cl-bearing salts are likely present on Europa's surface as non-ice materials. The chemical compositions of those Cl-bearing salts are key to understanding Europa's ocean chemistry and habitability. Here we report the results of ground-based telescope observations of Europa across two wavelength ranges, 1.0-1.5 and 1.5-1.8 $μm$, of which the former range includes absorption features owing to some hydrated Cl-bearing salts. We obtained spatially resolved reflectance spectra using the Subaru Telescope/IRCS and the adaptive optics system AO188 with high wavelength resolutions ($δλ$ ~ 2 nm for 1.0-1.5 $μm$ and $δλ$ ~ 0.9 nm for 1.5-1.8 $μm$) and low noise levels (1$σ$ ~ 1-2 $\times$ 10$^{-3}$). We found no clear absorption features at ~1.2 $μ$m caused by hydrated Cl-bearing salts. We estimated that conservative upper limits to the abundances of MgCl$_2$$\cdot$nH$_2$O, NaClO$_4$$\cdot$2H$_2$O, Mg(ClO$_3$)$_2$$\cdot$6H$_2$O, and Mg(ClO$_4$)$_2$$\cdot$6H$_2$O on Europa are 17% (<10% for most) at the 3$σ$ noise level. These values are lower than the proposed abundance of some hydrated Cl-bearing salts (> ~20%) on Europa based on previous observations. This supports the idea that Cl-bearing salts on Europa are likely anhydrous Na salts of NaCl and/or NaClO$_4$, or hydrated NaCl$\cdot$2H$_2$O. The presence of Na salts suggests that Na$^+$ could be the major cation in Europa's ocean, which would be possible if the oceanic pH is circumneutral or alkaline.
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