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Recent infrared observations at 1.5-4.0 $\mu m$ using large ground-basedtelescopes have suggested that Cl-bearing salts are likely present on Europa'ssurface as non-ice materials. The chemical compositions of those Cl-bearingsalts are key to understanding Europa's ocean chemistry and habitability. Herewe report the results of ground-based telescope observations of Europa acrosstwo wavelength ranges, 1.0-1.5 and 1.5-1.8 $\mu m$, of which the former rangeincludes absorption features owing to some hydrated Cl-bearing salts. Weobtained spatially resolved reflectance spectra using the Subaru Telescope/IRCSand the adaptive optics system AO188 with high wavelength resolutions ($\delta\lambda$ ~ 2 nm for 1.0-1.5 $\mu m$ and $\delta \lambda$ ~ 0.9 nm for 1.5-1.8$\mu m$) and low noise levels (1$\sigma$ ~ 1-2 $\times$ 10$^{-3}$). We found noclear absorption features at ~1.2 $\mu$m caused by hydrated Cl-bearing salts.We estimated that conservative upper limits to the abundances ofMgCl$_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$O on Europa are 17% (<10% for most) at the3$\sigma$ noise level. These values are lower than the proposed abundance ofsome hydrated Cl-bearing salts (> ~20%) on Europa based on previousobservations. This supports the idea that Cl-bearing salts on Europa are likelyanhydrous 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 inEuropa's ocean, which would be possible if the oceanic pH is circumneutral oralkaline.

Spatially resolved observations of Europa's surface with Subaru/IRCS at 1.0-1.8 $\mu m$: Upper limits to the abundances of hydrated Cl-bearing salts