Space weathering of asteroids: similarities and discrepancies between Main Belt asteroids and NEOs

In two recent papers (Marchi et al., AJ, 131, 1138 (2006) and Lazzarin et al., Ap.J. submitted) it has been shown that the mean reflection spectral slope of most Main Belt asteroids (S,C and X complexes) is positively correlated with the age and, more precisely, with the exposure to the weathering effects due to the Sun. These findings have relevant consequences for what concerns the chemistry, the evolutionary history and even the taxonomical classification of Main Belt asteroids. In particular the dominance of reddening effects within the C complex (resulting from the balance among opposite trends due to different components)is effective for what concerns the bulk of MB asteroids, but does not hold for some subgroups, such as some families or bodies in the Outer Belt. In spite of a poor statistics, the same seems to happen for what concerns C-type NEOs. This result is unexpected on the basis of the current ideas on the origin of NEOs (in particular the assumed percentage of bodies coming from the Main Belt, and the relevance of the various dynamical channels)