4 Conclusions

We obtained visible spectra of 34 asteroids belonging to C, G, and P classes, localized between 2.2 and 3.9 AU. Our analysis reveals that more than 65% of the investigated asteroids show the presence of absorption bands due to aqueous alteration products (0.6, 0.7, 0.8-0.9 $\mu$m, depth between 2% and 6% with respect to the continuum), in particular of the 0.7 $\mu$m band, which is the most characteristic feature of the hydrated materials in the visible. Our data also confirm the existence of a relationship between the albedo of the objects and the aqueous alteration process: in fact the percentage of the observed hydrated asteroids grows as albedo increases. We also found hydration features on 4 asteroids located outside the "aqueous alteration zone'' as defined by Vilas (1994). Our results confirm those recently obtained by Barucci et al. (1998).

We compared the spectra of the observed hydrated asteroids with those of 7 CM2 carbonaceous chondrite meteorites. This comparison reveals a good match (differences $\le$ 3%) between the meteorites and most of the observed objects, suggesting that hydrated asteroids might be the parent bodies of CM2 meteorites.