MARS ROCK / Martian meteorite NWA 6963 / Météorite martienne / Marsmeteorit

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Martian meteorite NWA 6963 weight: 11 mg size: about 2 mm x 2mm x 2 mm size of the box: 56 mm x 36 mm A martian meteorite is a rock that formed on the planet Mars, was ejected from Mars by the impact of an asteroid or comet, and landed on the Earth. Of over 53000 meteorites that have been found on Earth, 99 are martian (as of 2011-07-30).These meteorites are thought to be from Mars because they have elemental and isotopic compositions that are similar to rocks and atmosphere gases analyzed by spacecraft on Mars. By the early 1980s, it was obvious that the SNC group of meteorites (Shergottites, Nakhlites, Chassignites) were significantly different from most other meteorite types. Among these differences were younger formation ages, a different oxygen isotopic composition, the presence of aqueous weathering products, and some similarity in chemical composition to analyses of the martian surface rocks in 1976 by the Viking landers. Several workers suggested these characteristics implied the origin of SNC meteorites from a relatively large parent body, possibly Mars (e.g., Smith et al. and Treiman et al ). Then in 1983, various trapped gases were reported in impact-formed glass of the EET79001 shergottite, gases which closely resembled those in the martian atmosphere as analyzed by Viking.These trapped gases provided direct evidence for a martian origin. In 2000, an article by Treiman, Gleason and Bogard gave a survey of all the arguments used to conclude the SNC meteorites (of which 14 had been found at the time) were from Mars. They wrote, "There seems little likelihood that the SNCs are not from Mars. If they were from another planetary body, it would have to be substantially identical to Mars as it now is understood. As of 2011-07-30, 98 of the 99 Martian meteorites are divided into three rare groups of achondritic (stony) meteorites: shergottites (83), nakhlites (13), and chassignites (2), with the oddball meteorite ALH 84001 usually placed within a specific "OPX group". Consequently, Martian meteorites as a whole are sometimes referred to as the SNC group . They have isotope ratios that are said to be consistent with each other and inconsistent with the Earth. The names derive from the location of where the first meteorite of their type was discovered. Roughly three-quarters of all Martian meteorites can be classified as Shergottites. They are named after the Shergotty meteorite, which fell at Sherghati, India in 1865.Shergottites are igneous rocks of mafic to ultramafic lithology. They fall into three main groups, the Basaltic, Olivine-Phyric and Lherzolitic Shergottites, based on their crystal size and mineral content. They can be categorised alternatively into three or four groups based on their Rare-Earth Element content. These two classification systems do not line up with each other, hinting at complex relationships between the various source rocks and magmas that the Shergottites formed from. The shergottites appear to have crystallised as recently as 180 million years ago,which is a surprisingly young age considering how ancient the majority of the surface of Mars appears to be, and the small size of Mars itself. Because of this, some have advocated the idea that the Shergottites are much older than this.This "Shergottite Age Paradox" remains unsolved and is still an area of active research and debate. 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 s... read more

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