The Shergotty Consortium and SNC meteorites: An overview

The Shergotty meteorite has a multi-phase (magmatic and shock) history. While the Shergotty picture is complex, consortium studies have advanced our knowledge and understanding of Shergotty and shergottites, nakhlites and Chassigny (SNC) meteorites. Martian origin for the SNC meteorites is strongly favored by several workers from the evidence of trapped noble gases and nitrogen compositions in glasses (lithology C) of the EETA 79001 meteorite, which compare well with the Martian atmosphere analysis made by the Viking Spacecraft. The parent body is about 2 to 4 times richer in volatiles (Cl, Br, Na, K, Rb, Zn, F, Pb, etc.) than the Earth. Consortium studies on Shergotty show very low thermoluminescence, no deformation of tracks, cosmic ray exposure age of about 2.5 million years (m.y.), a pre-atmospheric size of about 12 cm radius, and apparently one shock event at 30 GPa pressure that converted plagioclase to maskelynite. The crystallization age of Shergotty by the Sm-Nd method is 360 ± 16 m.y. The Rb-Sr age for Shergotty is reported as 166 m.y. and the Pb-U age as about 200 m.y. Interpretations of age-dating and exposure scenarios are controversial and may require further studies.At least two scenarios for the ejection of SNC meteorites are possible: 1) ejection as a large body (>6 m size) by a single impact on Mars and then multiple breakup in the asteroidal belt at about 11 m.y. for Chassigny and nakhlites, at 2.5 m.y. for Shergotty, Zagami and ALHA 77005, and at 0.6 m.y. for EETA 79001; and 2) ejection of small objects (<0.5 m size) by multiple impacts on the Martian terrain at 11, 2.5 and 0.6 m.y. with no breakup in space.