OBSERVATIONS OF HIGH-VELOCITY, WEAKLY SHOCKED EJECTA FROM EXPERIMENTAL IMPACTS

A SMALL proportion of meteorites found on Earth are thought to come fr om planet-sized bodies1,2. The 'lunar meteorites' are now well establi shed as having come from the Moon3-6 on the basis of direct comparison with lunar samples. The SNC meteorites (shergottites, nakhlites and c hassignites) - seven achondrite meteorites distinguished by extremely young formation ages (<1.3 Gyr), high volatile contents, distinctive o xygen isotopic ratios and rare earth compositions - are igneous rocks, believed2 to have formed on a planet, probably Mars. But it is hard t o reconcile the weakly shocked nature of many lunar and SNC meteorites with the strong shock metamorphism known to accompany impacts of the size required to eject material from a planet-sized body. Computer mod elling7-10 of impacts has yet to resolve this issue, although it has b een proposed9,10 that surface rarefaction near an impact can produce h igh-velocity, weakly shocked ejecta. Here we present the results of a cratering experiment which separates and captures the ejecta from diff erent regions around the impact site. We recover high-velocity, weakly shocked material as predicted9,10, lending additional support both to our understanding of cratering mechanics and to a planetary or lunar origin for the SNC and lunar meteorites.