Cosmic-ray exposure ages of meteorites and lunar rocks and their significance

Meteorites and lunar surface rocks experienced at least one, and possibly more, exposures to cosmic-ray particles. The nuclear effects within the extraterrestrial materials allow us to determine how long a meteorite existed as a meter-sized object before capture by Earth and how long a lunar rock resided on the top of the lunar surface until collection. In this review I discuss the cosmic-ray exposure (CRE) ages of the different meteorite classes that originated from the asteroid belt, Mars, and Moon. From mineralogical and isotopic characteristics it appears that there are impact produced fragments from about 100 asteroids in our collections. A large fraction of the meteorites from a specific class originated from a small number of ejection events. The stone meteorites representing crust and mantle materials of planetary bodies traveled for less than or equal to 120 Main space. The CRE ages of iron meteorites, the fragments of the cores of differentiated asteroids, are up to 1500 Ma, probably due to stronger resistance against crushing in space compared to silicate material. Rocks blasted off the Moon and Mars by asteroidal or cometary impacts are particularly valuable as they represent surface areas that were not previously sampled by manned or automated missions. The CRE ages of lunar and martian meteorites indicate that up to eight sites on the Moon and the same number of sites on Mars were sampled by the presently studied meteorites. They represent a dramatic increase of the sampling of the lunar and martian crust. The CRE ages of rocks collected by the Apollo missions on the rims of lunar craters were used to date the impact events exposing these rocks to the cosmic particle irradiation. When martian material will be available for laboratory studies, this dating method can also be applied to craters and young lava flows on Mars.