Europa’s H₂O₂: Temperature Insensitivity and a Correlation with CO₂

Abstract H 2 O 2 is part of Europa’s water-ice radiolytic cycle and a potential source of oxidants to Europa’s subsurface ocean. However, factors controlling the concentration of this critical surface species remain unclear. Though laboratory experiments suggest that Europa’s H 2 O 2 should be concentrated in the coldest, most ice-rich regions toward the poles, Keck adaptive optics observations have shown the strongest H 2 O 2 signatures in comparatively warm, salt-bearing terrain at low latitudes. As a result, it was suggested that the local non-ice composition of these terrains—particularly hypothesized enrichments of CO 2 —may be a more dominant control on H 2 O 2 than temperature or water-ice abundance. Here we use observations of Europa from the NASA Infrared Telescope Facility, Keck Observatory, and JWST to disentangle the potential effects of temperature and composition. In order to isolate the effect of temperature on Europa’s H 2 O 2 , we use the ground-based observations to assess its response to temperature changes over timescales associated with Europa’s daily eclipse and diurnal cycle. We use JWST Cycle 1 data to look for any geographic correlation between Europa’s H 2 O 2 and CO 2 . Changes in Europa’s 3.5 μ m H 2 O 2 absorption band both from pre- to post-eclipse and across a local day suggest minimal effects of the local temperature on these timescales. In contrast, the JWST observations show a strong positive correlation between Europa’s H 2 O 2 and CO 2 bands, supporting the previously suggested possibility that the presence of CO 2 in the ice may enhance H 2 O 2 concentrations via electron scavenging.