The porosity of dark meteorites and the structure of low-albedo asteroids

We report new measurements of the densities of dark CI, CM, CV, and CO carbonaceous chondrite and black ordinary chondrite meteorites. These meteorites are suggested as candidate materials for some low-albedo asteroids. The reported low bulk densities of Phobos, Deimos, and 253 Mathilde are substantially below all measured meteorites except for one sample of a water-rich CI meteorite. However, the absence of water features in the spectra of these objects rules out hydrated materials in their surface regions. Our bulk density measurements of anhydrous carbonaceous chondrites and shock-blackened ordinary chondrites show that these low-albedo meteorites have similar bulk densities that are substantially greater than those of the hydrated carbonaceous chondrites. For anhydrous objects, bulk density alone is not a diagnostic parameter for distinguishing between these possible analogues to C-asteroid compositions. However, these results strongly imply that anhydrous dark asteroids must be very porous. To match the observed bulk density of 234 Mathilde with any type of meteorite, the asteroid would have to have porosities greater than 40%. (C) 2000 Academic Press.