Radar observations of the icy Galilean satellites

Abstract We report 12.6-cm-wavelength radar observations of Europa, Ganymede, and Callisto made at the Arecibo Observatory in November 1977 and February 1979. When combined with previous observations, our results establish firmly the distinguishing radar properties of these satellites: (i) high geometric albedos, α; (ii) circular polarization ratios, μ C , which anomalously exceed unity; (iii) linear polarization ratios, μ L , which are approximately 0.5; and (iv) diffuse scattering which varies as cos n θ , where θ is angle of incidence and 1 ⪅ n ⪅ 2. We tabulate weighted-mean values of α, μ C , μ L , and n derived from observations between 1975 and 1979. The values of μ C for Ganymede and Europa are nearly identical and significantly larger than that for Callisto. The values of n for Ganymede and Callisto are nearly identical and significantly smaller than that for Europa. Although significant albedo and/or polarization features are common in the radar spectra, the fractional rms fluctuation in disk-integrated properties is only ∼10%. No time variation in the radar properties has been evident during 1976–1979.