Timescales of shock processes in chondritic and martian meteorites

Messages from Mars About 35 of the thousands of meteorites so far found on Earth are recognized as being from Mars, probably thrown up by the impacts of large bodies such as asteroids on the martian surface. The trace-element distribution between high-pressure minerals formed by intense shock in these meteorites is a measure of the duration of the events that formed them, and the brief (10 ms) duration suggests that the impacting bodies were of the order of 100 metres in diameter. In contrast, stony meteorites (chondrites) formed by collisions much earlier in the life of the Solar System record the presence of much larger colliding bodies, around 5 km in size and causing a 1-second shock on impact. The accretion of the terrestrial planets from asteroid collisions and the delivery to the Earth of martian and lunar meteorites has been modelled extensively 1 , 2 . Meteorites that have experienced shock waves from such collisions can potentially be used to reveal the accretion process at different stages of evolution within the Solar System. Here we have determined the peak pressure experienced and the duration of impact in a chondrite and a martian meteorite, and have combined the data with impact scaling laws to infer the sizes of the impactors and the associated craters on the meteorite parent bodies. The duration of shock events is inferred from trace element distributions between coexisting high-pressure minerals in the shear melt veins of the meteorites. The shock duration and the associated sizes of the impactor are found to be much greater in the chondrite (∼1 s and 5 km, respectively) than in the martian meteorite (∼10 ms and 100 m). The latter result compares well with numerical modelling studies of cratering on Mars, and we suggest that martian meteorites with similar, recent ejection ages (10 5 to 10 7 years ago) 3 may have originated from the same few square kilometres on Mars.