  Recent infrared observations at 1.5-4.0 $\mu m$ 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 $\mu m$, 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 ($\delta
\lambda$ ~ 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 no
clear absorption features at ~1.2 $\mu$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$O on Europa are 17% (<10% for most) at the
3$\sigma$ 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.
