1 Introduction

The origin of most meteorite types is still a matter of debate (asteroidal, cometary or planetary origin).

In this work we attempted a comparison between hydrated asteroids and CM2 carbonaceous chondrites as they exhibit similar spectroscopic behaviour.

The distribution of asteroidal classes is dependent on heliocentric distance: objects closer to the Sun (belonging above all to S-type), appear to have been strongly heated and differentiated, while asteroids at greater distance seem to have undergone little or no heating and differentiation. These ones belong to C, P and D taxonomic types: they are low-albedo ($\le$ 0.05) objects, darkened by carbonaceous and organic materials on their surfaces (Gradie & Tedesco 1982). So the small bodies at great heliocentric distances ($d_{\odot} \ge 3$ AU) may have preserved materials which witnessed the condensation of the Solar nebula and the early phases of the formation of the Solar System. Vilas (1994), has revealed that a particular zone of the outer main belt seems dominated by objects which have undergone aqueous alteration process, that is the low temperature chemical alteration of materials by liquid water which acts as a solvent and produces materials like phyllosilicates, sulfates, oxides, carbonates, and hydroxides. The study of this process can give important information on the evolution of the earliest Solar System. It is now believed that hydrous minerals could not have condensed directly from the solar nebula, but that they have been produced by hydration of pristine anhydrous silicates. This means that water was present in the primordial asteroids, probably in the form of ice condensed together with the lithic material. Successively, an heat source, probably electric induction by the solar wind during the T-Tauri phase of our Sun (as suggested by Shimazu & Teresawa 1995; Herbert 1989) melted the ice and produced the liquid water necessary to alter superficial minerals. As liquid water can exist only under particular conditions of temperature and pressure, the study of aqueous alteration processes can help to reconstruct the different evolutionary stages of the chemical and thermal history of our Solar System.

The asteroids that show hydrated minerals on their surfaces seem to dominate the region of the main belt between 2.6 and 3.5 AU, called also "aqueous alteration zone'' (Vilas 1994), and belong essentially to the C, G, F and B taxonomic classes.

  

Table 1: Observational and physical characteristics of the observed asteroids: circumstances of the observations (date and site), visual magnitude of the asteroids, the solar analog stars used for reduction, semimajor axis (AU), diameter (km) and albedo derived from IRAS observations, and taxonomic type (Tholen taxonomy)

Hydrated materials produce characteristic absorption features on the spectra of the asteroids: in the infrared region the 3 $\mu$m band (Lebofsky 1980; Jones et al. 1990), associated to "free'' water molecules, and to OH ion bounded in the mineral crystal lattice; in the visible range there are several bands centered around 0.43 $\mu$m, 0.60-0.65 $\mu$m, 0.70 $\mu$m and 0.80-0.90 $\mu$m, attributed to Fe$^{2+}\rightarrow$ Fe³⁺ charge transfer transitions in oxidized iron (Vilas et al. 1993, 1994; Vilas 1994; Barucci et al. 1998).

In this context we have started a spectroscopic survey of asteroids located between about 2 and 4 AU. Moreover, we attempted a comparison of the spectra of hydrated asteroids with those of CM2 carbonaceous chondrites which show features due to aqueous altered materials, in order to obtain information about the origin of CM2 chondrites.