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Mars meteorite
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A Mars meteorite is a meteorite that has landed on Earth and originated from Mars. This could have been the result of an impact of a celestial body on Mars, sending material from Mars into space. Of the many thousand meteorites that have been found on Earth, only 34 have been identified as originating from Mars, most of which have been found since 2000.
Note that this does not refer to meteorites actually found on Mars, such as Heat Shield Rock.
983 it was suggested by Smith et al that meteorites in the so called SNC group (Shergottites, Nakhlites, Chassignites) originated from Mars, from evidence from an instrumental and radiochemical neutron activation analysis of the meteorites.
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Encyclopedia
A Mars meteorite is a meteorite that has landed on Earth and originated from Mars. This could have been the result of an impact of a celestial body on Mars, sending material from Mars into space. Of the many thousand meteorites that have been found on Earth, only 34 have been identified as originating from Mars, most of which have been found since 2000.
Note that this does not refer to meteorites actually found on Mars, such as Heat Shield Rock.
History
In 1983 it was suggested by Smith et al that meteorites in the so called SNC group (Shergottites, Nakhlites, Chassignites) originated from Mars, from evidence from an instrumental and radiochemical neutron activation analysis of the meteorites. They found that the SNC meteorites possess chemical, isotopic, and petrologic features consistent with data available from Mars at the time, findings further confirmed by Trieman et al a few years later, by similar methods. Then in late 1983, Bogard et al showed that the isotopic concentrations of various noble gases of some of the shergottites were consistent with the observations of the atmosphere of Mars made by the Viking spacecraft in the mid-to-late 1970s.
In 2000, an article by Trieman, 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."
Composition
The 34 Mars meteorites are divided into three rare groups of achondritic (stony) meteorites: shergottites (25), nakhlites (7), and chassignites (2). Consequently, Mars 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.
Shergottites
The first shergottite, the Shergotty meteorite, was found in Sherghati, India in 1865.
Shergottites are igneous rocks. Lherzolitic shergottites (one from Antarctica, two from California) are identified by their Deuterium/Hydrogen ratios. The crystals appear to be 154-187 million years old and they appear, from cosmic ray analysis, to have spent 2.5 to 3.6 million years in space. There are also basaltic shergottites, some of which appear (from the presence of hydrated carbonates and sulfates) to have been exposed to liquid water prior to injection into space.
Nakhlites
There are 7 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. The most recent nakhlite was found in Antarctica on December 15, 2003.
Nakhlites are igneous rocks that are rich in augite and were formed from basaltic magma at about 1.3 Ga. 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.
It has been shown that the nakhlites were suffused with liquid water around 620 Ma, and that they were ejected from Mars around 10.75 Ma by an asteroid impact, and fell to Earth within the last 10,000 years.
Chassignites
The first chassignite, the Chassigny meteorite, was found in Chassigny, Haute-Marne, France in 1815. There has been only one other chassignite found , known as Diderot, or NWA 2737, since it was found in North-West Africa. NWA 2737 was found by meteorite hunters B. Fectay and C. Bidaut of meteorite dealer The Earth's Memory in August 2000, and it was shown by Beck et al that its "mineralogy, major and trace element chemistry as well as oxygen isotopes revealed an unambiguous Martian origin and strong affinities with Chassigny."
Other types
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). 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. however, most experts agree that the microfossils are not indicative of Martian life, but of contamination by earthly biofilms. It has not yet conclusively been shown how they formed. ALH 84001 is much older than the others in the SNC group — dating back to the original formation of Mars about 4.5 billion years ago. In this respect, it resembles a typical meteorite rather than the other SNCs, which all appear to be formed less than 1.3 billion years ago.
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. Because Phobos has similarities to chondritic asteroids and because the Kaidun meteorite is a chondrite, Kaidun is not a Martian meteorite sensu stricto. However, it may contain small fragments of material from the Martian surface.
Origin
The majority of SNC meteorites are quite young by geologic standards and seem to imply that volcanic activity was present on Mars only a few hundred million years ago. Cosmic ray traces in the meteorites indicate relatively short stays (3 to 3.5 million years) in space. It has been asserted that there are no large young craters on Mars that are candidates as sources for the SNC meteorites, but recent research claims to have a likely source for ALH84001 and a possible source for other shergottites.
Possible evidence of life
Possible evidence of life has been hypothesized in three meteorites.
- A 1.3 billion-year-old meteorite from near El-Nakhla, Egypt. Small structures that look vaguely like Earth bacteria. More like bacteria than those in the better-known Allan Hills meteorite.
- A 165-million-year-old meteorite from Sherghati, India. Still to be analyzed.
- A 4.5 billion-year-old meteorite found in the Allan Hills of Antarctica (ALH84001). 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 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 Antarctica 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 possibly be nanobacteria fossils in carbonate material in crack fills (investigators McKay, Gibson, Thomas-Keprta, Zare). 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.
In August 2002, a NASA team led by Thomas-Keptra published a study indicating that 25% of the magnetite in ALH 84001 occurs as small, uniform-sized crystals in a crystal form that, on Earth, is associated only with biologic activity. The remainder of the material appears to be normal inorganic magnetite. The extraction technique did not permit determination as to whether the possibly biologic magnetic was organized into chains as would be expected.
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