Radar observations of Europa in 2011-2024: New insights into radar scattering properties
Three of Jupiter's Galilean satellites - Europa, Ganymede, and Callisto - are of particular scientific interest due to their icy shells and suspected subsurface oceans. However, the radar properties of the icy satellites have not been measured since observations in 1987-1991. Because radio waves can penetrate pure ice to considerable depths, radar observations provide a powerful means of characterizing the subsurface properties of the icy shells of these satellites, offering key insights into planetary evolution. We have observed Europa using the Goldstone 3.5-cm Solar System Radar and the Green Bank Telescope (GBT) in 2011-2024 in order to address a longstanding gap in the radar studies of these moons. In this paper, we present the most longitudinally comprehensive set of radar measurements of Europa to date and describe its disk-integrated radar properties. On the basis of monostatic Goldstone data, we find radar albedo values in two circular polarizations of $\hatσ_{\rm OC}$ = 0.92 $\pm$ 0.11 and $\hatσ_{\rm SC}$ = 1.35 $\pm$ 0.13 (unweighted mean and root-mean-square dispersion), with a circular polarization ratio of $μ_c$ = 1.44 $\pm$ 0.12 (weighted mean and root-mean-square dispersion). Values obtained bistatically at the GBT are similar. The $μ_c$ values suggest a leading-vs-trailing side dichotomy in radar scattering properties on Europa. Our results support the existence of the coherent backscatter opposition effect (CBOE), currently the most widely accepted physical mechanism that explains the unusual radar scattering properties of the icy Galilean satellites. Because we observed Europa with a bistatic configuration, we can place a lower bound on the width of Europa's CBOE peak equal to 36 arcsec, which provides an upper bound of 32 m ($\sim$1000 wavelengths) on the penetrating depth of X-band radar waves at Europa.