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

Abstract

Abstract 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 μm and 1.5–1.8 μm, of which the former range includes absorption features due 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 (1σ ∼ (1–2) × 10−3). We found no clear absorption features at ∼1.2 μm caused by hydrated Cl-bearing salts. We estimated that conservative upper limits to the abundances of MgCl2 · nH2O, NaClO4 · 2H2O, Mg(ClO3)2 · 6H2O, and Mg(ClO4)2 · 6H2O on Europa are 17% (<10% for most) at the 3σ 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 NaClO4, or hydrated NaCl · 2H2O. 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.