A spectral slope vs. Sun-distance correlation for S-type Main Belt and Near Earth asteroids.

The space weathering affects the optical properties of S-type asteroid surfaces. The space weathering depends mainly on the age (older asteroids are expected to be in mean redder and darker), but it may depend also on the distance from the Sun, whenever the Sun-dependent effects (for example, ion bombardment and sputtering) dominate the evolution of the surface properties. The mean distance (in the past) from the Sun is easily estimated for Main Belt asteroids, for which it is often possible to assume a reasonable orbital stability. We obtain a significant anti-correlation between the spectral slope and the mean distance from the Sun for Main Belt asteroids. The extension to near Earth bodies is not straightforward. A direct application of the method to the present orbits leads to opposite results (positive slope-distance correlation). However, the present orbital status is usually very recent, and the past exposure of NEAs to space weathering depends mainly on their orbits BEFORE the injection into the near Earth region. With the aid of a theoretical estimate of the probability, for every individual NEA, to have been injected through each of the main dynamical channels, we computed the past distance from the Sun for NEAs in our sample. Again, we obtained a significant anti-correlation slope-distance, with a strong similarity to that obtained for MB bodies