Search for Aqueous Altered Materials on Asteroids
M.A. Barucci (Observatoire de Paris-France), A. Doressoundiram (Observatoire de Paris-France), M. Fulchignoni (Univ. Paris VII-France), M. Florczak (ON/CNPq, Rio de Janeiro-Brasil), M. Lazzarin (Dipartimento di Astronomia di Padova-Italy), C. Angeli (ON/CNPq, Rio de Janeiro-Brasil), D. Lazzaro (ON/CNPq, Rio de Janeiro-Brasil)
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 
m which is believed to arise from Fe 
Fe 
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 m on the spectra of other 7
asteroids.