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

TL;DR

This study used high-resolution ground-based infrared spectroscopy to search for hydrated chloride salts on Europa's surface, setting upper limits on their abundance and suggesting Na salts are more likely present, informing ocean chemistry and habitability.

Contribution

First spatially resolved high-resolution infrared observations of Europa's surface to constrain hydrated chloride salt abundances and implications for ocean composition.

Findings

No clear absorption features of hydrated Cl salts detected.

Upper limits of 17% for certain hydrated Cl salts on Europa's surface.

Results support the dominance of Na salts, indicating possible ocean pH conditions.

Abstract

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.