Non-linear radar response to the radial structure of Europa plume fallout deposits

Abstract

Observations of water vapor plumes emanating from Europa's surface provide direct evidence of on-going activity within the ice shell that may have important implications for the potential habitability of Europa's putative global ocean. However, the scarcity of repeated observations suggest that Europa plumes are sporadic and may prove elusive when attempting to sample them directly with the instruments on-board either NASA's Europa Clipper or ESA's JUpiter ICy moons Explorer (JUICE) spacecraft. An alternative approach is then to search for and characterize the remnant plume fallout deposits preserved on Europa's surface. To this end, in this study we investigate the radar response of surficial plume fallout deposits when illuminated by both the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument on-board Europa Clipper as well as the Radar for Icy Moons Exploration (RIME) instrument on-board JUICE. Our results reveal distinct patterns in radial coherent surface reflectivity associated with different families of plume heights. Uniform fallout depositional rates surrounding low (≤25 km) altitude plumes result in a radially-constant coherent surface reflectivity that oscillates as a function of plume duration. In contrast, fallout deposits from high (≥50 km) altitude plumes exhibit radially variable, ripple-like patterns in coherent surface reflectivity as the velocity at which a particle is ejected from the plume is size dependent. The amplitude of the reflectivity oscillations as a function of fallout deposit thickness is directly dependent on porosity, with greater oscillation amplitudes being associated with older fallout deposits that have undergone some degree of densification (as opposed to the young, porous deposits of very recently active plumes). Finally, the results highlight that in the presence of near-surface structure, there is no direct proportionality between observed radar surface reflectivity and near-surface bulk density.