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Martian meteorite
A Martian meteorite is a rock that formed on Mars, was ejected from the planet by an impact event, and traversed interplanetary space before landing on Earth as a meteorite. As of September 2020[update], 277 meteorites had been classified as Martian, less than half a percent of the 72,000 meteorites that have been classified. The second largest complete, uncut Martian meteorite, Taoudenni 002, was recovered in Mali in early 2021. It weighs 14.5 kilograms (32 pounds) and is on display at the Maine Mineral and Gem Museum.
There are three groups of Martian meteorite: shergottites, nakhlites and chassignites, collectively known as SNC meteorites. Several other Martian meteorites are ungrouped. These meteorites are interpreted as Martian because they have elemental and isotopic compositions that are similar to rocks and atmospheric gases on Mars, which have been measured by orbiting spacecraft, surface landers and rovers. The term does not include meteorites found on Mars, such as Heat Shield Rock.
By the early 1980s, it was obvious that the SNC group of meteorites (Shergottites, Nakhlites, and 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 scientists suggested these characteristics implied the origin of SNC meteorites from a relatively large parent body, possibly Mars.
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 that 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 April 25, 2018, 192 of the 207 Martian meteorites are divided into three rare groups of achondritic (stony) meteorites: shergottites (169), nakhlites (20), chassignites (3), and ones otherwise (15) (containing the orthopyroxenite (OPX) Allan Hills 84001, as well as 10 basaltic breccia meteorites). Consequently, Martian meteorites as a whole are sometimes referred to as the SNC group (pronounced /snɪk/). They have isotope ratios that are consistent with each other and inconsistent with a terrestrial origin. 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 (such as the Tissint group found in Morocco in 2011) 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 from which the shergottites formed.
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.
It has been suggested the 3-million-year-old crater Mojave, 58.5 km in diameter, was a potential source of these meteorites. A paper published in 2021, however, disputes this, proposing instead the 28 km crater Tooting, or possibly the crater 09-000015 as the crater source of the depleted olivine-phyric shergottites ejected 1.1 Ma ago.
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Martian meteorite
A Martian meteorite is a rock that formed on Mars, was ejected from the planet by an impact event, and traversed interplanetary space before landing on Earth as a meteorite. As of September 2020[update], 277 meteorites had been classified as Martian, less than half a percent of the 72,000 meteorites that have been classified. The second largest complete, uncut Martian meteorite, Taoudenni 002, was recovered in Mali in early 2021. It weighs 14.5 kilograms (32 pounds) and is on display at the Maine Mineral and Gem Museum.
There are three groups of Martian meteorite: shergottites, nakhlites and chassignites, collectively known as SNC meteorites. Several other Martian meteorites are ungrouped. These meteorites are interpreted as Martian because they have elemental and isotopic compositions that are similar to rocks and atmospheric gases on Mars, which have been measured by orbiting spacecraft, surface landers and rovers. The term does not include meteorites found on Mars, such as Heat Shield Rock.
By the early 1980s, it was obvious that the SNC group of meteorites (Shergottites, Nakhlites, and 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 scientists suggested these characteristics implied the origin of SNC meteorites from a relatively large parent body, possibly Mars.
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 that 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 April 25, 2018, 192 of the 207 Martian meteorites are divided into three rare groups of achondritic (stony) meteorites: shergottites (169), nakhlites (20), chassignites (3), and ones otherwise (15) (containing the orthopyroxenite (OPX) Allan Hills 84001, as well as 10 basaltic breccia meteorites). Consequently, Martian meteorites as a whole are sometimes referred to as the SNC group (pronounced /snɪk/). They have isotope ratios that are consistent with each other and inconsistent with a terrestrial origin. 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 (such as the Tissint group found in Morocco in 2011) 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 from which the shergottites formed.
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.
It has been suggested the 3-million-year-old crater Mojave, 58.5 km in diameter, was a potential source of these meteorites. A paper published in 2021, however, disputes this, proposing instead the 28 km crater Tooting, or possibly the crater 09-000015 as the crater source of the depleted olivine-phyric shergottites ejected 1.1 Ma ago.