Rethinking Europa’s 2.07 μm Absorption: Evidence Against a Subsurface Ocean Link
A small absorption feature near 2.07μm on Europa’s trailing hemisphere has been used as evidence for the presence of endogenic magnesium chloride (MgCl₂), possibly sourced from Europa’s subsurface salty ocean. While the spectrum of MgCl₂ does not have this specific absorption feature, it could be altered via irradiation on Europa’s trailing hemisphere into either more complex magnesium chlorate/perchlorate salts (Ligier et al. 2016) or if combined with Io-genic sulfur, into a sulfur-bearing salt such as epsomite (MgSO₄⋅10H₂O) (Brown et al. 2013).
If the 2.07 μm band arises from such irradiation products, we would expect it to be strongest in geologically young terrains—regions thought to expose ocean-derived material. Instead, using archival VLT/SINFONI H+K band spectra, we found that the feature is not tied to chaos terrains or other recent surface features. Rather, it follows a bullseye-shaped spatial pattern consistent with Iogenic sulfur implantation (see Davis et al. 2023).
This suggests the absorption is unlikely to result from irradiation processed MgCl₂ and neither epsomite nor magnesium chlorate/perchlorate salts appear consistent with the observed spatial distribution. Instead, we propose that the feature may originate from a radiolysis product unrelated to endogenic material. Unfortunately, no current laboratory spectra match the feature, highlighting the need for additional experiments to identify the responsible species and refine our understanding of Europa’s surface chemistry.