A Unique Spectral Slope-Exposure Relation for Silicate-Rich Main Belt and Near-Earth Asteroids

Space environment acts on optical properties of silicate-rich asteroid surfaces: older asteroids are expected to be generally redder and darker than the younger ones. The space weathering depends mainly on the age, but it may depend also on the distance from the Sun, especially if the Sun-dependent effects (for example, ion bombardment) are assumed as dominant. With the aid of a new method to compute the asteroids ages, we obtain a general and statistically significant relation between the spectral slope of the silicate-rich Main Belt and near Earth asteroids (NEAs) and the ``exposure" to space weathering, computed taking also into account the distance from the Sun. The age estimates depend on various parameters and in particular, for near Earth asteroids, on the assumed rate of the Yarkovsky-driven orbital mobility. We show that the range of values of the Yarkovsky effect parameter allowing to obtain a unique slope-age or slope--exposure relation for Main Belt asteroids and NEAs is consistent with the recent experimental measurements.