Observations of high-velocity, weakly shocked ejecta from experimental impacts

Abstract

A SMALL proportion of meteorites found on Earth are thought to come from planet-sized bodies^1,2. The 'lunar meteorites' are now well established as having come from the Moon^3–6 on the basis of direct comparison with lunar samples. The SNC meteorites (shergottites, nakhlites and chassignites) — seven achondrite meteorites distinguished by extremely young formation ages (<1.3 Gyr), high volatile contents, distinctive oxygen isotopic ratios and rare earth compositions — are igneous rocks, believed² to have formed on a planet, probably Mars. But it is hard to 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 modelling^7–10 of impacts has yet to resolve this issue, although it has been proposed^9,10 that surface rarefaction near an impact can produce high-velocity, weakly shocked ejecta. Here we present the results of a cratering experiment which separates and captures the ejecta from diffe-rent regions around the impact site. We recover high-velocity, weakly shocked material as predicted^9,10, lending additional support both to our understanding of cratering mechanics and to a planetary or lunar origin for the SNC and lunar meteorites.