EUROPA, GANYMEDE, AND CALLISTO - NEW RADAR RESULTS FROM ARECIBO AND GOLDSTONE

Observations of the icy Galilean satellites, conducted during 1987-1991 with the Arecibo 13-cm system and the Goldstone 3.5-an system, yield significant improvements in our knowledge of the satellites' radar properties. Hardly any wavelength dependence is seen for either the total power radar albedo sigma(T) or the circular polarization ratio mu(C). For Europa, Ganymede, and Callisto our 13-cm estimates of mean values and rms dispersions are sigma(T) = 2.60 +/- 0.22, 1.39 +/- 0.14, and 0.69 +/- 0.06; and mu(C) = 1.53 +/- 0.03, 1.43 +/- 0.06, and 1.17 +/- 0.04. Radar albedo features am seen on each satellite. Evidence for mu(C) features is lacking, except for indications of a weak hemispheric asymmetry for Callisto. That intersatellite and intrasatellite fractional variations in albedo greatly exceed variations in mu(C) is consistent with predictions of coherent backscatter theory and implies that albedo might be a crude indicator of relative silicate abundance. The satellites' albedo distributions overlap. The most prominent radar features are tentatively identified with Galileo Regio and the Valhalla basin. Estimates of echo Doppler frequencies show Callisto to be lagging its ephemeris by 200 +/- 50 km.