Thermal metamorphism of the C, G, B, and F asteroids seen from the 0.7 μm, 3 μm, and UV absorption strengths in comparison with carbonaceous chondrites

Abstract

Abstract Thermal metamorphism study of the C, G, B, and F asteroids has been revisited using their UV, visible, NIR, and 3 μm reflectance spectra. High‐quality reflectance spectra of seven selected C, G, B, and F asteroids have been compared with spectra for 29 carbonaceous chondrites, including thermally‐metamorphosed CI/CM meteorites. There are three sets of spectral counterparts, among which 511 Davida and B‐7904 are the most similar to each other in terms of both spectral shape and brightness. By comparing the 0.7 μm and 3 μm absorption strengths of 21 C, G, B, and F asteroids and heated Murchison samples, these asteroids have been grouped into three heating‐temperature ranges. These correspond to (1) <400 °C: phyllosilicate‐rich; (2) 400–600 °C: phyllosilicates transformed to anhydrous silicates; and (3) >600 °C: fully anhydrous. A good correlation between the UV and 3 μm absorption strengths has been confirmed for the C, G, B, and F asteroids and the CI, CM, and CR meteorites. A plot of the UV absorption strength vs. the IRAS diameter for 142 C, G, B, and F asteroids shows that the maximum UV absorption strength decreases as the diameter increases for the asteroids >60 km, with a notable exception, Ceres. These relationships suggest that some of the larger asteroids may be the heated inner portions of once larger bodies and that common CI/CM meteorites may have come from the lost outer portions, which escaped extensive late‐stage heating events.