Radar Observations of Asteroids 1 Ceres, 2 Pallas, and 4 Vesta

Details Radar Observations of Asteroids 1 Ceres, 2 Pallas, and 4 Vesta Radar Observations of Asteroids 1 Ceres, 2 Pallas, and 4 Vesta

Nov 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996icar..124..113m&link_type=abstract

Icarus, Volume 124, Issue 1, pp. 113-133.

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Scientific paper

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 Ceres and Pallas are ~95% polarized (mu_C = sigma_SC/sigma_OC ~ 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 deg) 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_OC = 0.042 +/- 0.006) and Pallas (sigma_OC = 0.075 +/- 0.011) are in the range expected for surfaces with a carbonaceous chondrite mineralogy. Pallas' distinctly higher albedo implies a ~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 ~2.3 and ~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.

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