What is space weathering?
Space weathering is the slow change of airless surfaces, such as the Moon and asteroids, caused by solar wind, cosmic rays and micrometeorite impacts. It typically darkens and reddens their surfaces.
On Earth, weathering means rain, frost and wind breaking down rocks. Airless bodies have no weather of that kind, yet their surfaces still change over time. Without an atmosphere or magnetic field for protection, they are exposed to the solar wind, a stream of charged particles from the Sun, as well as to cosmic rays and a constant rain of tiny micrometeorites striking at many kilometres per second.
These processes alter the outer layer of each grain. Micrometeorite impacts melt and vaporise tiny amounts of material, and the solar wind knocks atoms loose. Some vapour recondenses as thin coatings on grain surfaces containing extremely small particles of metallic iron, known as nanophase iron. These specks, only nanometres across, darken the surface and change the way it reflects light, typically making it redder and weakening the absorption features that identify minerals.
This matters for linking meteorites to asteroids. For decades astronomers puzzled over why the most common meteorites, the ordinary chondrites, did not seem to match the spectra of the most common asteroids of similar composition, the S-type asteroids. Space weathering was a leading explanation, and it was confirmed when Japan's Hayabusa mission returned dust from the S-type asteroid Itokawa in 2010. The grains matched LL ordinary chondrites in composition and showed thin weathered rims, proving that space weathering had disguised the true composition.
The effect is also visible on the Moon. Fresh craters, such as Tycho, are surrounded by bright rays because the excavated material has not had time to weather, while older surfaces are darker. Samples from Ryugu and Bennu are now revealing how space weathering affects carbon-rich surfaces too, which behave somewhat differently. In meteorites, gases implanted by the solar wind show that some fragments once sat on an asteroid's surface.