Endogenic and Exogenic Contributions to Visible-wavelength Spectra of Europa's Trailing Hemisphere

TL;DR

This study uses Hubble Space Telescope spectra to map and analyze the surface composition of Europa's trailing hemisphere, distinguishing between endogenous and exogenous sulfur radiolysis effects and their spatial distributions.

Contribution

It provides the first spatially resolved spectral analysis of Europa's trailing hemisphere, linking spectral features to geological and radiolytic processes and suggesting irradiated sulfate salts as key materials.

Findings

Spectral features at 360 nm and red slope are linked to endogenous material altered by sulfur radiolysis.

Absorptions at near-UV and 530 nm are consistent with sulfur allotropes from radiolysis.

Spatial distributions of spectral features reveal complex interactions between endogenous and exogenous processes.

Abstract

The composition of Europa's trailing hemisphere reflects the combined influences of endogenous geologic resurfacing and exogenous sulfur radiolysis. Using spatially resolved visible-wavelength spectra of Europa obtained with the Hubble Space Telescope, we map multiple spectral features across the trailing hemisphere and compare their geographies with the distributions of large-scale geology, magnetospheric bombardment, and surface color. Based on such comparisons, we interpret some aspects of our spectra as indicative of purely exogenous sulfur radiolysis products and other aspects as indicative of radiolysis products formed from a mixture of endogenous material and magnetospheric sulfur. The spatial distributions of two of the absorptions seen in our spectra -- a widespread downturn toward the near-UV and a distinct feature at 530 nm -- appear consistent with sulfur allotropes previously suggested from ground-based spectrophotometry. However, the geographies of two additional features -- an absorption feature at 360 nm and the spectral slope at red wavelengths -- are more consistent with endogenous material that has been altered by sulfur radiolysis. We suggest irradiated sulfate salts as potential candidates for this material, but we are unable to identify particular species with the available data.