Radar Observations of Main-Belt M-class Asteroids

Using the S-band radar at Arecibo Observatory, we have observed 19 Tholen M-class asteroids. The mean radar albedo for all our targets is 0.28 ± 0.13, considerably higher than the mean radar albedo of every other class (Magri et al. 2007, Icarus 186, 126-151). We find approximately one-third (six) of these 19 objects have radar albedos suggesting a very high metal content (mean 0.43 ± 0.13), consistent with a remnant iron/nickel core interpretation (irons) or exotic high-metal meteorite types (such as CB), and we propose designating these objects Mm-types. A 3 micron spectral absorption feature (usually attributed to hydrated phases) is absent in the four Mm-types that have been observed at that wavelength. The remaining 13 asteroids have moderately high mean radar albedos (mean 0.21 ± 0.06) and are more likely enstatite chondrite (EH/EL), CH, CB, or CI/CM in composition, in order of preference. All of the non-Mm asteroids show a positive correlation between visual and radar albedo. This may indicate that all are of similar composition, with metal abundance being the primary variable affecting their albedo. Acknowledgements: This work was funded by NSF grant AST-0605903 to MKS and AST-0606704 to BEC