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.[1]

Note that this does not refer to meteorites actually found on Mars, such as Heat Shield Rock.

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[edit] History

In 1983 it was suggested by Smith et al, [2] that meteorites in the so called SNC group 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[3] a few years later, by similar methods. Then in late 1983, Bogard et al[4] 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[5] gives a survey of all the arguments used to conclude the SNC meteorites (of which 14 had been found at the time of the article) were from Mars. They write "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."

[edit] Composition

The 34 Mars meteorites are divided into three rare groups of achondritic (stony) meteorites: shergottites (25), nakhlites (7), and chassignites (2).[1] 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.

[edit] Shergottites

The first shergottite, the Shergotty meteorite, was found in Sherghati, India in 1865.

The most famous shergottite, known as ALH84001, 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 however, most experts agree that the microfossils are not indicative of life, but of contamination by earthly biofilms. It has not yet conclusively been shown how they formed. ALH84001 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 SNC's, which all appear to be formed less than 1.3 billion years ago.[6]

All the meteorites are igneous rocks. Lherzolitic shergottites (one from Antarctica, 2 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.

[edit] 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.[7]

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.[7]

[edit] 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 in August 2000, and it was shown by Beck et al[8] that its "mineralogy, major and trace element chemistry as well as oxygen isotopes revealed an unambiguous Martian origin and strong affinities with Chassigny."

In March 2004, it was suggested that the unique Kaidun meteorite landed on March 12, 1980 in Yemen, may have originated on the Martian moon of Phobos.

[edit] 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[9] and a possible source for other shergottites.[10]

[edit] 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 Allen 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 Allen 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.

[edit] References

  1. ^ a b "Mars Meteorites" - A complete list of the Mars meteorites from NASA's Jet Propulsion Laboratory. URL accessed September 6, 2006.
  2. ^ Smith, M.R. et al, "Petrogenesis of the SNC (Shergottites, Nakhlites, Chassignites) Meteorites: Implications for Their Origin From a Large Dynamic Planet, Possibly Mars" - (PDF) Proceedings of the fourteenth Lunar and Planetary Science Conference, Part 2, Journal of Geophysical Research, Vol. 89, Supplement, pp. B612-B630, February 15, 1984.
  3. ^ Treiman et al, "Core formation in the Earth and Shergottite Parent Body (SPB):Chemical evidence from basalts" - (PDF) Geochemica et Cosnochimica Acta Vol. 50, pp. 1071-1091 (1986).
  4. ^ Bogard, D.D. et al, "Noble gas contents of shergottites and implications for the Martian origin of SNC meteorites" - (PDF) Geochimica et Cosnocimica Acta Vol. 48, pp. 1723-1739 (1984).
  5. ^ Treiman, A.H. et al, "The SNC meteorites are from Mars" - Planetary and Space Science, Vol. 48, Iss. 12-14, October 2000, pp. 1213-1230.
  6. ^ - Mars Meteorite ALH 84001 – An Illustrated History. URL accessed March 18, 2006.
  7. ^ a b Treiman, A.H., "The nakhlite meteorites: Augite-rich igneous rocks from Mars" - (PDF) Chemie der Erde 65, p. 203-270, (2005). URL accessed September 8, 2006.
  8. ^ Beck, P et al, "The Diderot meteorite: The second chassignite" - (PDF) 36th Annual Lunar and Planetary Science Conference, March 14-18, 2005, in League City, Texas, abstract no.1326. URL accessed September 8, 2006.
  9. ^ "Birthplace of famous Mars meteorite pinpointed" - September 16, 2005 New Scientist article. URL accessed September 8, 2006.
  10. ^ McEwan, A.S. et al, "The rayed crater Zunil and interpretations of small impact craters on Mars" - (PDF) Icarus, 176, pp.351-381, 2005. URL accessed September 8, 2006.

[edit] External links

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