EVIDENCE OF THERMAL METAMORPHISM ON THE C-ASTEROID, G-ASTEROID, B-ASTEROID, AND F-ASTEROID

Reflectance spectra (0.3 to 2.6 micrometers) of 14 C, G, B, and F aste roids and 21 carbonaceous chondrite powders are compared in detail. On ly three thermally metamorphosed CM-CI chondrites that have a weak ult raviolet absorption are shown to have close counterparts among those a steroids. Reflectance spectra of heated Murchison CM2 chondrite are co mpared with the average C and G type asteroid spectra. Murchison heate d at 600-degrees to 1000-degrees-C exhibits a similar weak ultraviolet absorption and provides the best analog for those spectra. Comparison of ultraviolet absorption strengths between 160 C, G, B, and F astero ids and carbonaceous chondrites suggests that surface minerals of most of those asteroids are thermally metamorphosed at temperatures around 600-degrees to 1000-degrees-C.