Evidence for asteroid space weathering from the Sloan Digital Sky Survey

By studying color variations between young and old asteroid families we find evidence for processes that modify colors of asteroids over time. We show that colors of aging surfaces of S-type asteroids become increasingly 'redder' and measure the rate of these spectral changes. We estimate that the mean spectral slope between 0.35 and 0.9 mum increases with time t (given in My) as approximate to 0.01 mum(-1) x log(10)t. This empirical fit is valid only for 2.5 less than or equal to t less than or equal to 3000 My (the time interval where we have data) and for the mean spectral slope determined from wide-wavelength filter photometry obtained by the Sloan Digital Sky Survey. We also find that Gy-old terrains of S-type asteroids reflect about 15% more light at similar to 1-mum wavelengths than an similar to 5-My-old S-type asteroid surface when the flux is normalized by the reflected light at 0.55 mum. We attribute these effects to space weathering. This result has important implications for asteroid geology and the origin of meteorites that reach the Earth. Our results also suggest that surfaces of C-type asteroids exhibit color alterations opposite to those of the S-type asteroids. (C) 2004 Elsevier Inc. All rights reserved.