What is a regolith breccia and what can it tell us?
A regolith breccia is a rock made from the compacted surface soil of an asteroid, the Moon or another airless body, preserving a record of its surface environment.
Airless bodies such as asteroids and the Moon are constantly bombarded by micrometeorites and larger impacts. Over time, this grinds their surfaces into a layer of loose dust and rubble called regolith. When regolith is compacted, perhaps by later impacts, it becomes solid rock: a regolith breccia.
Regolith breccias can be recognised in several ways. They usually look like a mixture of light and dark fragments. They contain tiny glass beads and agglutinates formed by micrometeorite impacts. Most tellingly, they contain gases implanted from the solar wind, such as helium and neon with solar isotope ratios, showing that their grains lay at the very surface, exposed to the Sun. Some grains also show dense tracks of damage from solar flare particles.
Regolith breccias are found among many meteorite types: howardites from Vesta, lunar meteorites, ordinary chondrites and some carbonaceous chondrites. They often contain foreign fragments, such as pieces of carbonaceous chondrite in howardites, showing that material was exchanged between asteroids.
They tell scientists about surface processes on airless bodies, including space weathering, impact gardening and the delivery of foreign material. Comparing them with samples returned by spacecraft from Itokawa, Ryugu and Bennu helps connect meteorites to the actual surfaces of asteroids. They also show how long grains were exposed at the surface, which helps estimate how quickly asteroid surfaces are churned.