Asteroid surface alteration by space weathering processes

Micrometeorite bombardment and irradiation by solar wind and cosmic-ray ions cause variations in the optical properties of small solar system bodies surfaces, affecting efforts to draw connections between specific meteorites and asteroid types. These space weathering processes have been widely studied for the Moon and S- and V-type asteroids, and they are currently being investigated for other asteroid types. Here we review the laboratory studies performed by several groups on meteorites and asteroid surface analogs, aimed at simulating space weathering by using ion irradiation and laser ablation. Together with direct evidence of weathering of particles from asteroid Itokawa acquired by the Hayabusa mission, these results have provided a fundamental contribution to the spectral interpretation of asteroid observations, to establish a solid asteroids-meteorites link, and to understand the energetic processes affecting the surfaces of minor bodies. A general scheme for asteroid optical maturation is thus emerging. Slope trends from large surveys and in particular of young asteroid families have confirmed that solar wind is the main source of rapid (10⁴-10⁶ yr) weathering, and that a number of rejuvenating processes (impacts by small meteorites, planetary encounters, regolith shaking, etc.) efficiently counterbalance the fast weathering timescales.