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C. Magri (U. Maine at Farmington), S. J. Ostro, L. A. M. Benner (JPL/Caltech), B. R. Beeney (U. Maine at Farmington)
We use radar albedos estimated for 36 main-belt asteroids (MBAs) and 9 near-Earth asteroids (NEAs) (433 Eros, 1862 Apollo, 2063 Bacchus, 2100 Ra-Shalom, 3199 Nefertiti, 4179 Toutatis, 4769 Castalia, 6178 1986 DA, and 6489 Golevka) to estimate near-surface solid densities in two ways. One method uses established relationships between radar albedo, Fresnel reflectivity, and surface bulk density, plus the assumption of lunar regolith porosity, to estimate solid density. These relationships, applied to MBAs by Magri et al. (1999, Icarus 140, 379-407), are based on data obtained for the Moon and inner planets and on experimental laboratory results. The second method uses the fact that NEA 433 Eros has recently been shown to have L or LL-chondritic composition (Kerr 2000, Science 288, 1714-1715). We use Eros as a calibrator by assuming that other asteroids differ from it only in Fresnel reflectivity, and that the radar scattering law and near-surface porosity are the same for all targets. The fact that Eros' circular polarization ratio falls within the +/- one-sigma interval for our sample gives us confidence that these assumptions are a reasonable first approximation.
Solid densities derived by these two methods for our C-class and M-class radar targets are consistent with those previously inferred for MBAs by Magri et al. Most of the B, F, G, and P-class targets have low densities; in particular, those inferred via the Eros calibration method are lower than the density of CI chondrites, possibly implying ultraprimitive composition. Most radar-detected S-class asteroids are likely to have ordinary chondritic composition, although a minority group of stony-iron analogs cannot be ruled out.