Optical properties of carbonaceous chondrites and their relationship to asteroids

We report diffuse reflectance measurements of nine samples of carbonaceous chondrites (one C1, three C2, four C3, and one C4) and one iron meteorite, Odessa. Measurements were also made of mineral mixtures in an attempt to understand the cause of some features of the meteorite reflectances. The results of these measurements are as follows: (1) The reflectances of the C1 and C2 samples are in the 5% range, and those of the C3 samples are in the 5–15% range. (2) Despite the large amounts of water known to be contained by carbonaceous chondrites the water absorptions near 1.4 and 1.9 μm, characteristic of hydrated minerals, are not evident in the meteorite reflectances. Our simulation studies indicate that this is due to the high degree of dispersal of the opaque carbonaceous component in the groundmass. (3) The simulation studies indicate that mixtures of layer-lattice silicate (in this case, montmorillonite) and small percentages of carbon black qualitatively match many features of the C1 reflectances: (4) Changes in the meteorite's spectral reflectance with particle size were noted, particularly for the C2 and C3 samples. In each case the near-infrared reflectance (1.0–2.5 μm) increases considerably for the smallest size fraction (<74 μm), and, for the C3 samples, broad absorptions centered near 1.0 and 2.0 μm become more prominent. These changes appear to result from greater contribution to the spectra from olivine and pyroxene in chondrules as they are more finely crushed. (5) The reflectance for the iron meteorite is a smooth curve, increasing from 0.3 to 2.5 μm. Comparison of these findings with current photometry of asteroids indicates the following results: (1) The Cl and C2 carbonaceous chondrites have reflectances low enough to match the anomalously low albedos of some asteroids. (2) Some asteroids have spectral reflectances similar to carbonaceous chondrites, whereas others with flat or F-type curves appear to match simulated mineral mixtures with slightly greater amounts of the carbonaceous component than those found in the meteorites. This observation suggests either that surface physical processes such as melting may be enhancing the optical effect of opaque carbonaceous material on some asteroid surfaces and/or that these asteroids contain even more carbonaceous material than the C1 meteorites. (3) The iron meteorite curve is similar to the reflectance of some asteroids but not those with flat or F-type curves. Neither does the reflectance of glass-rich lunar soils match any of these unusual asteroid reflectances. Thus, among materials of known cosmic importance, carbonaceous material appears to provide the best match to the optical properties of these asteroids.