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Sodium Chloride on the Surface of Europa

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Sodium Chloride on the Surface of Europa

Samantha K Trumbo et al. Sci Adv.

Abstract

The potential habitability of Europa's subsurface ocean depends on its chemical composition, which may be reflected in that of Europa's geologically young surface. Investigations using Galileo Near-Infrared Mapping Spectrometer data led to the prevailing view that Europa's endogenous units are rich in sulfate salts. However, recent ground-based infrared observations have suggested that, while regions experiencing sulfur radiolysis may contain sulfate salts, Europa's more pristine endogenous material may reflect a chloride-dominated composition. Chlorides have no identifying spectral features at infrared wavelengths, but develop distinct visible-wavelength absorptions under irradiation, like that experienced on the surface of Europa. Using spectra obtained with the Hubble Space Telescope, we present the detection of a 450-nm absorption indicative of irradiated sodium chloride on the surface. The feature correlates with geologically disrupted chaos terrain, suggesting an interior source. The presence of endogenous sodium chloride on the surface of Europa has important implications for our understanding of its subsurface chemistry.

Figures

Fig. 1
Fig. 1. HST/STIS spectra showing a distinct 450nm spectral feature, consistent with an NaCl F-center absorption, and a clear lack of a 720nm NaCl M-center absorption.
(A) Single spectrum from within Tara Regio, which exhibits a particularly strong 450-nm absorption. The dashed line is a third-order polynomial continuum fit. The continuum-removed feature is included underneath the spectrum. Overlain in red is a continuum-removed laboratory spectrum of irradiated NaCl at 100 K, taken from figure 2 of (24). This spectrum corresponds to an NaCl F-center absorption that has evolved in the absence of unrealistic laboratory radiation fluxes. The laboratory F-center absorption has been scaled to match the depth of the observed feature. (B) High signal-to-noise spectrum produced by averaging all spectra from locations exhibiting a 450-nm feature, weighted by the strength of that feature in each location. The weighted average is divided by the average of all spectra from locations in which the 450-nm feature is absent and rescaled to approximate the known Europa continuum. A continuum-removed version is shown underneath the spectrum, where the vertical dashed line indicates the anticipated band center at 720 nm.
Fig. 2
Fig. 2. A map of the strength of the 450nm absorption.
The observed feature maps solely to the leading hemisphere. Black outlines correspond to large-scale chaos regions, mapped approximately from (44). The largest absorptions fall within the chaos region Tara Regio (~85°W), with additional concentration in eastern Powys Regio (~125°W). This distribution is separate from the geography of sulfur radiolysis and suggests a subsurface source, consistent with the chloride hypothesis for Europa’s endogenous material. The spatial resolution of the mapped data is ~150 km at the sub-observer point. Background image credit: NASA/JPL/Björn Jónsson/Steve Albers.

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