Materials exposed to the harsh space environment are altered over time in both physical and compositional properties. The general definition of space weathering simply refers to this alteration. There are multiple processes, however, that can and do alter materials on the surface of an airless body: comminution, melting, vaporization by energetic impacts; gardening of a particulate soil and mixing with local and foreign materials by repeated impacts; sputtering at the atomic/molecular scale by solar wind; crystal damaging by UV radiation and energetic particles; structural fatigue from diurnal thermal cycling; mobility or loss of volatile species by radiant heating and sublimation; etc. Thus, space weathering takes many forms and its alteration products are a direct function of a) location in the solar system, b) composition and texture of the surface, c) mass and magmatic evolution of the body, and d) length of time exposed.
The regolith of only two extraterrestrial bodies has been directly sampled (The Moon and Itokawa). These samples and the ongoing detailed analyses in Earth-based laboratories provide a cornerstone for recognizing and understanding some aspects of space weathering effects. A handful of additional bodies have been visited by spacecraft and studied in detail for extended periods with remote sensors (Mercury, Eros, Vesta, and most recently the dwarf planet Ceres and comet Churyumov-Gerasimenko). Seven additional asteroids have been observed with instruments during a spacecraft flyby, while the satellites of Mars, Jupiter, and Saturn, have been studied to different degrees through multiple flybys. A few of the various forms of space weathering that are now recognized (or hypothesized) will be discussed and illustrated.