Martian Meteorite - Origin

Origin

The majority of SNC meteorites are quite young compared to most other meteorites and seem to imply that volcanic activity was present on Mars only a few hundred million years ago. The young formation ages of martian meteorites was one of the early recognized characteristics that suggested their origin from a planetary body such as Mars. Among martian meteorites, only ALH 84001 has a radiometric age older than about 1400 Ma (Ma = million years). All nakhlites, as well as Chassigny and NWA 2737, give similar if not identical formation ages around 1300 Ma, as determined by various radiometric dating techniques. Formation ages determined for many shergottites are variable and much younger, mostly ~150-575 Ma. The chronological history of shergottites is not totally understood, and a few workers have suggested that some may actually have formed prior to the times given by their radiometric ages, a suggestion not accepted by most workers. Formation ages of SNC meteorites are often linked to their cosmic-ray exposure (CRE) ages, as measured from the nuclear products of interactions of the meteorite in space with energetic cosmic ray particles. Thus, all measured nakhlites give essentially identical CRE ages of approximately 11 Ma, which when combined with their possible identical formation ages indicates ejection of nakhlites into space from a single location on Mars by a single impact event. Some of the shergottites also seem to form distinct groups according to their CRE ages and formation ages, again indicating ejection of several different shergottites from Mars by a single impact. However, CRE ages of shergottites vary considerably (~0.5-19 Ma), and several impact events are required to eject all the known shergottites. It has been asserted that there are no large young craters on Mars that are candidates as sources for the martian meteorites, but recent research claims to have a likely source for ALH 84001 and a possible source for other shergottites.