Icy Galilean satellites: 70 cm radar results from Arecibo

The radar scattering properties of Europa, Ganymede, and Callisto are unlik e those of any other object observed with planetary radars. At wavelengths of 3.5 cm and 13 cm most inner Solar System targets have low reffectivities on the order of 0.1, while the icy Galilean satellites are strongly backsc attering with specific radar cross sections that can exceed unity (Campbell et al. 1978, Icarus 34, 254-267, Ostro et al. 1992, J. Geophys. Res. 97, 1 8227-18244). Their polarization ratios are also high, similar to1.5, indica tive of multiple scattering, and the echoes follow a diffuse scattering law at all incident angles with no indication of the quasi-specular reflection s characteristic of terrestrial planets and the Moon. We present observatio ns that were made in 1988 and 1990 with the Arecibo radar at a much longer wavelength, 70 cm. The total cross sections measured at this wavelength are much lower than those measured at the shorter wavelengths. At 0.62 +/- 0.2 0 and 0.15 +/- 0.09, respectively, Ganymede's and Callisto's total normaliz ed cross sections are a factor of 3 lower than their values at the short wa velengths. However, their 70-cm polarization ratios are greater than unity and consistent with those at the shorter wavelengths. Europa was not reliab ly detected at 70 cm and hence an upper limit on its total cross section is placed at 0.34, which is almost a factor of 10 lower than at the short wav elengths. Although all the 70-cm echoes are fairly weak and carry relativel y large uncertainties, it appears unlikely that single reflections from the vacuum-surface interface are contributing significantly to the reflections , and hence the mechanism responsible for the radar scattering properties a t 3.5 cm and 13 cm is still active at 70 cm, but apparently not operating a s efficiently. (C) 2001 Academic Press.