Radar Characterization of Salt Layers in Europa's Ice Shell as a Window Into Critical Ice‐Ocean Exchange Processes

The potential habitability of Jupiter's moon Europa has motivated two missions: NASA's Europa Clipper and ESA's JUpiter ICy moons Explorer (JUICE). Both missions are equipped with ice‐penetrating radars which will transmit radio waves into the subsurface, recording reflections from interfaces defined by contrasts in ice shell dielectric properties. Assuming an MgSO4 ocean, we show that salt layers, formed through the freezing of subsurface liquid water reservoirs, can be detected by ice‐penetrating radar instruments on Europa Clipper and JUICE. Furthermore, because these features are thermodynamically stable within the minimally attenuating portion of Europa's ice shell, referred to here as the “pellucid region,” they could produce brighter reflections than deeper liquid water interfaces. We demonstrate how ice‐penetrating radar measurements of salt layer thickness could establish lower bounds on the parameter space of possible initial reservoir thickness and salinity, constrain the origin of reservoirs (ice shell melt vs. ocean injection), and—if sourced through ocean injection—the ocean salinity. Plain Language Summary Europa, one of Jupiter's moons, is thought to have the conditions necessary to support life as we know it. Europa is going to be explored by two missions: NASA's Europa Clipper and ESA's JUpiter ICy moons Explorer (JUICE). Both missions will use radar to look beneath the icy surface to search for liquid water and structures formed from liquid water freezing. In this work we show that salt layers, mixtures of salt and ice formed when salty water freezes into a solid, can be seen by radars on Europa Clipper and JUICE. These layers could give us clues about the water's original size and saltiness, and how these underground reservoirs formed—either from melting ice or ocean water pushing up into the ice shell. This research could help us understand more about Europa's ocean and its potential for life. Key Points Salt layers formed through the process of cryoconcentration represent radar‐detectable structure in Europa's ice shell Ice‐penetrating radar measurements of salt layer thickness can help to determine if ice shell reservoirs are sourced through injection of ocean water The salinity of Europa's ocean can be bounded through combined constraints on maximum initial reservoir thickness and salt layer thickness