Asteroid Space Weathering and Regolith Evolution

Over time, exposure of airless bodies to the space environment results in optical changes to their surfaces. These optical changes are functions of the porosity, grain size distribution, and composition of the surface, and they depend on the relative rates of surface modification processes. Collectively, surface modification processes (such as impacts, solar wind ion implantation, sputtering, and micrometeorite bombardment) and their resulting optical effects have come to be known as "space weathering." Studies of lunar rocks and soils are the most important foundation we have on which to build an understanding of space weathering on asteroids. We cannot directly measure asteroid surfaces in a laboratory environment; therefore, we describe the lunar case, and compare it with the evidence for asteroids. In this chapter we review the evidence for space weathering on asteroids, including spectroscopy of optical effects, microscopy of physical effects, simulations of processes, lunar soils, meteorite breccias, spacecraft observations, and theoretical modeling. An understanding of space weathering is important to all remote-sensing studies of asteroid surfaces.