Europa’s Sub-Surface Ice Observed With The Juno Microwave Radiometer

Observations with Juno’s Microwave Radiometer (MWR), taken in late 2022 and covering a longitude range from 70^oW to 50^oE and a latitude range from ~20^oS to ~50^oN at frequencies of 0.6, 1.2, 2.5, 5.2, 10, and 22 GHz, allow us to constrain the depth and subsurface structure of Europa’s ice shell.  The observed temperature gradient constrains the thickness of the thermally conductive part of the ice shell, and pores or cracks beneath the surface scatter microwaves, enabling us to characterize the size and distribution of the scatterers.  Assuming pure water ice, our best-fit model has conductive ice shell thickness 29±10 km, negligible surface reflectivity, volume fraction of scatterers 0.045, scale height of scatterers 219 m, and scatterer size distribution power law index -3.96.  The size, depth, and volume fraction of the scatterers suggest that they alone are likely not capable of carrying nutrients between the ocean and the surface.  Ice salinity of 15 mg/kg would reduce our estimate of the thickness by about 5 km.  A thermally convective layer would increase the total ice shell thickness but only slightly decrease our estimate of the conductive layer.  We will discuss these and other complications, as well as next steps.