Modeling of S-Type Asteroid Spectra Using Primitive Achondrites and Iron Meteorites

The stony iron model for the S asteroids has been tested using observed reflectance spectra of the S asteroids and modeled spectra of meteoritic components. Observed reflectance spectra of 40 S asteroids were fit with linear combinations of laboratory reflectance spectra of 6 representative primitive achondrites and an iron meteorite. Specular and diffuse reflection components were taken into account for metallic-iron reflectance. Even with this limited set of meteorite spectra, 16 of those 40 S asteroids were fit fairly well in terms of absorption-band features and spectral profiles. The number of well-fit S asteroids may be greater if the number and variety of metallic and stony domains are expanded. This initial result suggests that many of the S-asteroidal surfaces may be various kinds of regional mixtures of primitive achondrites and metallic iron. If true, the S asteroids could have been originally formed of chondrite-like source materials because mineral assemblages of primitive achondrites seem to have recrystallized from chondrites by segregation of metallic components and loss of partial melts.