Juno Microwave Radiometer Observations of Europa's Subsurface Ice

On September 9, 2022, the Juno spacecraft flew within a few hundred kilometers of Jupiter's moon Europa, and the Microwave Radiometer (MWR) collected passive microwave brightness data over the course of several 30-second rotations of the spacecraft. MWR's 6 channels range from 0.6 GHz to 22 GHz, and receive thermal emission from as deep as several kilometers beneath Europa's icy surface, as well as emission from the galaxy and Jovian synchrotron radiation (JSR) which partially reflects from discontinuities in the ice. The observations span a longitude range from 70˚W to 50˚E and a latitude range from ~20˚S to ~50˚N. The observed microwave brightness temperature in each channel is sensitive to the temperature, opacity, and reflectivity of the ice. The combination of multiple frequencies and varying viewing geometry, along with the angular dependence of the JSR and the known temperature of the surface ice, allow us to place constraints on the reflective properties of the subsurface ice and temperature gradient, which in turn constrains the thickness of the ice shell that covers Europa's subsurface water ocean. In the region observed, we find that the conductive part of Europa’s ice shell, if pure water ice, is greater than 20 km thick, with scatterers predominantly less than a few cm in radius extending hundreds of meters below the surface. We will present our latest results and how we interpret the data.