A Critical Analysis of the 2.07μm Absorption Feature on Europa

The near-infrared H+K band spectrum of Europa’s trailing hemisphere is mostly characterized by distorted water ice absorptions, hydrated sulfuric acid constrained to a bullseye like feature centered at 0°N, 270°W, and an additional non-icy, non-sulfuric acid component which has historically been attributed to endogenous sulfate salts. Ground based, high resolution spectra have revealed the presence of a small absorption feature at 2.07μm. Brown et al (2013) suggest that this feature may arise from Mg-sulfate salts or brine (e.g. epsomite) produced by radiolytic processing of endogenous magnesium chloride. Indeed, epsomite has an absorption feature at 2.07μm and reproduces this region of Europa’s spectrum quite nicely. However, spectral models including these sulfates offer a poor fit to the distorted water-ice absorptions in the 1.5-1.8μm region. Ligier et al (2016) suggest that the inclusion of Mg-sulfates is not necessary and reproduce the 2.07μm feature with a combination of Mg-chloride and Mg-perchlorate. In fact, with the addition of Mg-chlorate, they produce a better fit to the H-band distorted water ice features. We find, however, that in linear spectral models containing water ice, sulfuric acid, and chlorinated Mg species, those models which include Mg-chloride in quantities necessary to reproduce the 2.07μm feature also present a small unobserved absorption near 1.99μm. While we cannot confidently rule out the presence of either epsomite or Mg-chloride on Europa, we show that the evidence for the detection of either species is not compelling.

With no clear resolution of the genesis of the 2.07μm feature from spectral modeling, we turn to its spatial distribution in an attempt to gain insight. Using the high spatial and spectral resolution dataset from VLT/Sinfoni, we find that the presence of the 2.07μm feature is strongly correlated with the irradiation bombardment and sulfuric acid bullseye on Europa’s trailing hemisphere. However, we also find that the depth of the 2.07μm feature appears to be uncorrelated with Europa’s large scale geology. For locations at a given irradiation flux, the average depth is constant between chaos and non-chaos terrains. This suggests that the formation of the 2.07μm feature on Europa is independent of the endogenous salts thought to be present within the chaos terrains. Instead, we propose that the source of this feature may be a product of the radiolytic sulfur cycle, or something formed during the bombardment of water ice by electrons or the remaining (non-sulfur) ion species. Laboratory experiments are underway to test this hypothesis.