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Observations of the icy Galilean satellites, conducted during 1987-1991 with the Arccibo 13-cm system and the Goldstone 3.5-cm 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 •T or the circular polarization ratio bt c . For Europa, Ganymede, and Callisto our 13-cm estimates of mean values and rms dispersions are •T = 2.60 + 0.22, 1.39 + 0.14, and 0.69 + 0.06; and bl.c = 1.53 + 0.03, 1.43 + 0.06, and 1.17 + 0.04. Radar albedo features arc seen on each satellite. Evidence for btc featurcs is lacking, except for indications of a weak hemispheric asymmetry for Callisto. That intersatellite and intrasatellite fractional variations in albedo greatly exceed variations in bl.c is consistent with prcdictions 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 featurcs are tentatively identified with Galileo Regio and the Valhalla basin. Estimates of echo Doppler frcquencies show Callisto to be lagging its ephemeris by 200 + 50 km. INTRODUCTIONThe radar echoes from Europa, Ganymede, and Callisto are extraordinary. It has been known for 15 years that these objects' radar reflectivities dwarf values reported for comets, the Moon, the inner planets, and nonmetallic asteroids. When the radar transmission is circularly polarized, the icy satellites return echoes with the incident handedness preserved, in contrast with the behavior of other targets. At the principal Arecibo wavelength of 13 cm, the circular polarization ratio Rc, of echo power in the same sense of circular polarization as transmitted (the SC sense) to that in the opposite (OC) sense, exceeds unity for each of the icy Galilean satellites but is only ~0.1 for the Moon and less than 0.4 for most other planetary radar targets. The linear polarization ratio (gL = OL/SL) is about one half for all three satellites, again considerably larger than for other targets. The satellites' 13-cm radar albedos increase from Callism to Ganymede to Europa, whose OC radar reflectivity is the same as that of a metal sphere. Observations of Ganymede at Goldstone in 1977 [Goldstein and Green, 1980] indicated that this object's exotic radar behavior is preserved at 3.5 cm. (Articles reporting radar observations of the satellites are listed in Table 1.) Most efforts to understand the satellites' radar signatures have focused on the search for an electromagnetic scattering mechanism capable of yielding strong echoes with Ix c greater than unity. The 1978-1989 literature suggested that the satellites' signatures might be understood as being due to Eshleman, 1986a] and/or mode-decoupled, multiple, total-internal reflection [Goldstein and Green, 1980; Eshleman, 1986b] from subsurface variations in refractive index. Ostro and Shoemaker [1990] approached the problem from a geologic perspective and outlined explanations for the satellites' ...

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Observations of the Galilean satellites with the radar system at the Arecibo Observatory, Puerto Rico, show that their surfaces are highly diffuse scatterers of radio waves of length 12.6 centimeters; spectra of the radar echoes are asymmetric and broad. The geometric radar albedos for the outer three satellites-0.42 +/- 0.10, 0.20 +/- 0.05, and 0.09 +/- 0.02 for Europa, Ganymede, and Callisto, respectively-show about the same relative decreases as do the optical albedos, although the latter presumably bear only on material much nearer the surface. Radii of 1420 +/- 30, 2640 +/- 80, and 2360 +/- 70 kilometers for Europa, Ganymede, and Callisto were determined from the radar data and are in good agreement with the corresponding optically derived values. Io, observed successfully only once, appears to have an albedo comparable to Ganymede's, but no radius was estimated for it.