Search for Aqueous Altered Materials on Asteroids

The thermal metamorphism of asteroids is still intriguing and debated. In this context we have investigated on the problem of aqueous alteration on asteroid surfaces. The study of the aqueous alteration materials detected on the composition of dark asteroids can give important constraints on the understanding of the early Solar System. Objects located in the outer part of the main asteroid belt seem to have undergone little or no chemical alteration since the formation of the Solar System. Vilas et al. (1994, Icarus, 109,274) have revealed that a particular zone of the outer main belt seems characterized by objects which have undergone aqueous alteration process, that is the low chemical alteration of materials by liquid water which acts as a solvent and produces materials like phillosilicates, sulfates, oxides, carbonates and hydroxides. This region has been identified between 2.6 and 3.5 AU and the asteroids which have shown the presence of aqueous altered materials (hydrated silicates or clays), are essentially the C-types. This may indicate the presence of water ice in the original asteroids that has been heated during the primordial phases of our Sun. To confirm the existence of the ``alteration zone'', we performed a CCD spectroscopic survey of C asteroids to obtain high S/N spectra to search for the presence of features due to aqueous alteration products. About 70% of the 29 observed objects show absorption features produced by aqueous alteration. A broad band centered around 0.7 mu m which is believed to arise from Fe(2+) ->Fe(3+) charge transfer transitions in phyllosillicate minerals, has been revealed on 13 obtained spectra. Some weaker features have been found around 0.6, 0.8 and 0.9 mu m on the spectra of other 7 asteroids.