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).[1] 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.[2]
Note that this does not refer to meteorites found on Mars, such as Heat Shield Rock.
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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.[3] and Treiman et al.[4]). 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.[5] 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."[2]
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".[1] 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.[6] 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.[7] 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,[8] 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.[9] This "Shergottite Age Paradox" remains unsolved and is still an area of active research and debate.
There are 13 known nakhlites, the first of which, the Nakhla meteorite, fell in El-Nakhla, Alexandria, Egypt in 1911 and had an estimated weight of 10 kg.
Nakhlites are igneous rocks that are rich in augite and were formed from basaltic magma about 1.3 billion years ago. They contain augite and olivine crystals. Their crystallization ages, compared to a crater count chronology of different regions on Mars, suggest the nakhlites formed on the large volcanic construct of either Tharsis, Elysium, or Syrtis Major Planum.[10]
It has been shown that the nakhlites were suffused with liquid water around 620 million years ago and that they were ejected from Mars around 10.75 million years ago by an asteroid impact. They fell to Earth within the last 10,000 years.[10]
The first chassignite, the Chassigny meteorite, fell at Chassigny, Haute-Marne, France in 1815. There has been only one other chassignite recovered, named Northwest Africa (NWA) 2737. NWA 2737 was found in Morocco or Western Sahara in August 2000 by meteorite hunters Bruno Fectay and Carine Bidaut, who gave it the temporary name "Diderot." It was shown by Beck et al.[11] that its "mineralogy, major and trace element chemistry as well as oxygen isotopes revealed an unambiguous Martian origin and strong affinities with Chassigny."
The famous specimen ALH 84001 has a different rock type than other martian meteorites: it is an orthopyroxenite (an igneous rock dominantly composed of orthopyroxene). For this reason it is classified within its own group, the "OPX martian meteorites". This meteorite received a lot of attention after an electron microscope revealed structures that were considered to be the fossilized remains of bacteria-like lifeforms. As of 2005[update], scientific consensus was that the microfossils were not indicative of Martian life, but of contamination by earthly biofilms. However, in 2009, new analyses ruled out earthly and non-biological origins, presenting strong evidence of life on Mars.[12] ALH 84001 is as old as the basaltic and intermediate shergottite groups — i.e., 4.1 billion years old.
In March 2004 it was suggested that the unique Kaidun meteorite, which landed in Yemen on March 12, 1980, may have originated on the Martian moon of Phobos.[13] Because Phobos has similarities to C-type asteroids and because the Kaidun meteorite is a carbonaceous chondrite, Kaidun is not a Martian meteorite in the strict sense. However, it may contain small fragments of material from the Martian surface.
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.[8][14] Formation ages determined for many shergottites are variable and much younger, mostly ~150-575 Ma.[8][15][16][17] The chronological history of shergottties 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,[18] 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.[8] Some of the shergottites also seem to form distinct groups according to their CRE ages and formation ages, again indicating ejection of several different shergottties from Mars by a single impact. However, CRE ages of shergottites vary considerably (~0.5-19 Ma),[8] 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[19] and a possible source for other shergottites.[20]
Several Martian meteorites have been found to contain what some think is evidence for fossilized Martian life forms. The most significant of these is a meteorite found in the Allan Hills of Antarctica (ALH 84001). Ejection from Mars seems to have taken place about 16 million years ago. Arrival on Earth was about 13 000 years ago. Cracks in the rock appear to have filled with carbonate materials (implying groundwater was present) between 4 and 3.6 billion-years-ago. Evidence of polycyclic aromatic hydrocarbons (PAHs) have been identified with the levels increasing away from the surface. Other Antarctic meteorites do not contain PAHs. Earthly contamination should presumably be highest at the surface. Several minerals in the crack fill are deposited in phases, specifically, iron deposited as magnetite, that are claimed to be typical of biodepositation on Earth. There are also small ovoid and tubular structures that might be nanobacteria fossils in carbonate material in crack fills (investigators McKay, Gibson, Thomas-Keprta, Zare).[21] Micropaleontologist Schopf, who described several important terrestrial bacterial assemblages, examined ALH 84001 and opined that the structures are too small to be Earthly bacteria and don't look especially like lifeforms to him. The size of the objects is consistent with Earthly "nanobacteria", but the existence of nanobacteria itself is controversial.
Many studies disputed the validity of the fossils.[22][23] For example, it was found that most of the organic matter in the meteorite was of terrestrial origin.[24] But, a recent study suggests that magnetite in the meteorite could have been produced by Martian microbes. The study, published in the journal of the Geochemical and Meteoritic Society, used more advanced high resolution electron microscopy than was possible in 1996.[12][25]
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