Radar observations of asteroids 1 Ceres, 2 Pallas, and 4 Vestal

Asteroids 1 Ceres, 2 Pallas, and 4 Vesta were observed with the 13-cm Arecibo radar and the 3.5-cm Goldstone radar during several apparitions between 1981 and 1995, These observations help to characterize the objects' surface properties. Echoes from Cere;: and Pallas are similar to 95% polarized (mu(C) = sigma(SC/)sigma(OC) approximate to 0.05) in the sense expected for specular (mirror) reflection yet broadly distributed in Doppler frequency, thus revealing surfaces that are smoother than the Moon at decimeter scales but much rougher (rms slopes > 20 degrees) on larger scales. Slopes on Ceres appear to be somewhat higher when viewed with the 3.5-cm wavelength, a trend that is observed for the terrestrial planets and the Moon, In contrast, echoes from Vesta are significantly depolarized, indicating substantial near-surface complexity at scales near 13 cm (mu(C) = 0.24 +/- 0.04) and 3.5 cm (mu(C) = 0.32 +/- 0.04), which is probably a consequence of Vesta's relatively strong basaltic surface material and may be a signature of large impact features inferred to be present on the surface. The low radar albedos of Ceres (<(sigma)over cap>(OC) = 0.042 +/- 0.006) and Pallas (<(sigma)over cap>(OC) = 0.075 +/- 0.011) are in the range expected for surfaces with a carbonaceous chondrite mineralogy. Pallas' distinctly higher albedo implies a similar to 35% higher surface density, which could result from a lower regolith porosity and/or a higher specific gravity (zero-porosity density), Given a porosity of 45%, the specific gravities of the surface materials on Ceres and Pallas would be similar to 2.3 and similar to 3.0 g cm(-3), respectively, which would be consistent with (1) the presence of an additional silicate component on Pallas' surface (as inferred from spectroscopic observations) and (2) recent mass estimates, which suggest a higher mean (volume-averaged) density for Pallas than for Ceres. (C) 1996 Academic Press, Inc.