JWST Reveals Spectral Tracers of Recent Surface Modification on Europa

Abstract Europa has been modified by a variety of geologic processes, exposing internally derived materials that are heavily irradiated by charged particles trapped in Jupiter’s magnetosphere. Prior spectral analysis of H 2 O ice on Europa relied on low signal-to-noise data at wavelengths >2.5 μ m, limiting assessment of a 3.1 μ m Fresnel peak that is diagnostic of exposed crystalline ice. We report new measurements of H 2 O ice spectral features using high signal-to-noise data collected by the NIRSpec spectrograph (1.48–5.35 μ m) on the James Webb Space Telescope. These data reveal a narrow 3.1 μ m crystalline H 2 O ice Fresnel peak, which is primarily located at southern latitudes in Tara and Powys Regiones. Our analysis indicates that crystalline ice exposed in these low-latitude regiones is likely sustained by ongoing thermal (re)crystallization outpacing charged particle amorphization of the top ∼10 μ m of Europa’s regolith over short timescales (<15 days). We also measured H 2 O ice features centered near 1.5, 1.65, and 2.0 μ m, and a broad 3.6 μ m H 2 O continuum peak, which are all stronger at northern latitudes, in contrast to the 3.1 μ m Fresnel peak identified at southern latitudes. These results support the hypothesis that H 2 O ice in Europa’s regolith is vertically stratified, with amorphous ice grains dominating its exposed surface, except in Tara and Powys Regiones. We also find that a previously detected 4.38 μ m 13 CO 2 feature is present almost exclusively at southern latitudes in Tara and Powys Regiones, likely derived from an internal source of carbon-bearing material.