The Spatial Distribution of the Unidentified 2.07 \textmu m Absorption Feature on Europa and Implications for its Origin

TL;DR

This study investigates the spatial distribution of a 2.07 μm absorption feature on Europa to understand its origin, finding it correlates with irradiation patterns but not with endogenous geological features, suggesting a radiolytic process.

Contribution

The paper provides new spatial analysis of the 2.07 μm feature on Europa, challenging the hypothesis of an endogenous salt origin and proposing a radiolytic process as its source.

Findings

The 2.07 μm feature correlates with irradiation patterns.

No association between the feature and chaos terrains.

The feature is absent in Pwyll crater ejecta, indicating recent formation.

Abstract

A weak absorption feature at 2.07 \textmu m on Europa's trailing hemisphere has been suggested to arise from radiolytic processing of an endogenic salt, possibly sourced from the interior ocean. However, if the genesis of this feature requires endogenic material to be present, one might expect to find a correlation between its spatial distribution and the recently disrupted chaos terrains. Using archived near-infrared observations from Very Large Telescope/SINFONI with a $\sim$1 nm spectral resolution and a linear spatial resolution $\sim$130 km, we examine the spatial distribution of this feature in an effort to explore this endogenic formation hypothesis. We find that while the presence of the 2.07 \textmu m feature is strongly associated with the irradiation pattern on Europa's trailing hemisphere, there is no apparent association between the presence or depth of the absorption feature and Europa's large-scale chaos terrain. This spatial distribution suggests that the formation pathway of the 2.07 \textmu m feature on Europa is independent of any endogenous salts within the recent geology. Instead, we propose that the source of this feature may simply be a product of the radiolytic sulfur cycle or arise from some unidentified parallel irradiation process. Notably, the 2.07 \textmu m absorption band is absent from the Pwyll crater ejecta blanket, suggesting that radiolytic processing has not had enough time to form the species responsible and placing a lower limit on the irradiation timescale. We are unable to find a plausible spectral match to the 2.07 \textmu m feature within the available laboratory data.