"Space weathering" refers to the slight erosion caused on unprotected planets, moons, and asteroids by the solar wind, cosmic rays, micrometeorites and larger meteors. Space weathering affects the physical and optical properties of the surfaces of planetary bodies, so understanding its specifics is important for interpreting remotely sensed data, such as space probe photographs of outer solar system moons.
The first form of space weathering that was recognized is that of agglutination -- tiny bits of material that are vaporized by tiny micrometeorites and scattered over the surface. Material covered in agglutination appears black to the human eye due to the presence of nanophase iron. Agglutination is common, in, for instance, lunar soils, where it makes up as much as 60 to 70% of mature lunar soils. Agglutination and space weathering is partially responsible for the dark appearance of the lunar maria. Because the average micrometeorite is tiny (just a few tens of nanometers in diameter), the weathering takes place on tiny surface levels, and only in the last couple decades have we had microscopes powerful enough to probe the specifics of its structure.
Another form of space weathering that occurs, partially on the Moon, where it has been studied, has to do with the solar wind. Over billions of years, solar wind hitting the Sun-facing surface of the Moon has deposited light elements, especially Helium-3, which is being eyed as a second-generation nuclear fusion power source. Helium-3 requires more energy to fuse than first-generation nuclear fusion fuels such as deuterium, but liberates more energy as well. Only 20 tonnes of Helium-3 could power the energy needs of the United States, if we successfully develop a fusion reactor that can get out more energy from Helium-3 than is required to fuse it.
The Helium-3 on the Moon caused by space weathering is being eyed as an economic resource by the Russian and Chinese governments. The Chinese government has cited Helium-3 as the primary reason for trying to reach the Moon, and a Russian energy company has put forth the goal of mining Helium-3 on the Moon by 2020.
