COSMIC SECRETS
The Enigmas on Mars 69
Martian Meteorites
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Martian meteorite EETA79001
 
Martian Meteorite
 
A Martian 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 over 50000 meteorites that have been found on Earth, currently 95 have been identified as originating from Mars, most of which have been found since 2000.[1] Chemical analysis of the martian meteorites suggests that the ambient near-surface temperature of Mars has most likely been below 0 C° for the last four billion years.[2]

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

History

In 1983 it was suggested by Smith et al.[3] 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 Treiman et al.[4] a few years later, by similar methods. Then in late 1983, Bogard et al.[5] 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 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."[6]

Composition

33 of the 34 Martian meteorites are divided into three rare groups of achondritic (stony) meteorites: shergottites (24), nakhlites (7), and chassignites (2), with the oddball meteorite ALH 84001 not usually placed in a group.[7] 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.

Shergottites

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Martian meteorite ALH84001

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.[8] Shergottites are igneous rocks of mafic to ultramaficlithology. 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.[9] 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,[10] 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.[11] This "Shergottite Age Paradox" remains unsolved and is still an area of active research and debate.

Nakhlites

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Nakhla meteorite's two sides and its inner surfaces after breaking it in 1998

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 basalticmagma about 1.3 billion years ago. They contain augite and olivinecrystals. 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.[12]

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.[12]

Chassignites

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.[13] 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. As of 2005, 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.[14] 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.[15] Because Phobos has similarities to chondritic asteroids and because the Kaidun meteorite is a chondrite, Kaidun is not a Martian meteorite in the strict sense. 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[16] and a possible source for other shergottites.[17]

Possible evidence of life

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 (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 (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 nanobacteriafossils in carbonate material in crack fills (investigators McKay, Gibson, Thomas-Keprta, Zare).[18] 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.

Many studies disputed the validity of the fossils.[19][20] For example, it was found that most of the organic matter in the meteorite was of terrestrial origin.[22] 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.[14][22]

References:

  1. Meteoritical Bulletin Database
  2. Shuster, David L.; Weiss, Benjamin P. (July 22, 2005). "Martian Surface Paleotemperatures from Thermochronology of Meteorites". Science 309 (5734): 594�600. . doi:10.1126/science.1113077. PMID16040703
  3. 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.
  4. 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).
  5. 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).
  6. Treiman, A.H.; et al. (October 2000). "The SNC meteorites are from Mars". Planetary and Space Science 48 (12–14): 1213–1230. doi:10.1016/S0032-0633(00)00105-7. Bibcode: 2000P&SS...48.1213T
  7. Mars Meteorites - A complete list of Martian meteorites from NASA's Jet Propulsion Laboratory. URL accessed September 6, 2006.
  8. Shergotty Meteorite - JPL, NASA
  9. The SNC meteorites: basaltic igneous processes on Mars, Bridges & Warren 2006
  10. Ages and geologic histories of Martian meteorites, Nyquist 2001
  11. Bouvier, A. et al., 2009. Martian meteorite chronology and the evolution of the interior of Mars. Earth and Planetary Science Letters Volume 280, pp. 285-295. "doi:10.1016/j.epsl.2009.01.042"
  12. 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.
  13. 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.
  14. Ancient Martians were carried to Earth, November 26, 2009, By PAUL SUTHERLAND, Skymania.com, Scientific American
  15. Zolensky, M. and Ivanov A. (2003). "The Kaidun Microbreccia Meteorite: A Harvest from the Inner and Outer Asteroid Belt". Chemie der Erde/Geochemistry 63: 185–246. doi:10.1078/0009-2819-00038
  16. Birthplace of famous Mars meteorite pinpointed - September 16, 2005 New Scientist article. URL accessed September 8, 2006.
  17. McEwen, A.S. et al. (2005). "The rayed crater Zunil and interpretations of small impact craters on Mars". Icarus176: 351–381. doi:10.1016/j.icarus.2005.02.009
  18. McKay, D. et al. 1996. Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite AL84001. Science: 273. 924-930.
  19. Gibbs, W. and C. Powell. In Focus Bugs in the Data? 1996. Scientific American. October. 20-22
  20. Controversy Continues: Mars Meteorite Clings to Life - Or Does It? By Leonard David - Senior Space Writer, posted: 20 March 2002, Space.com
  21. Bada, J. et al. 1998. A Search for Endogenous Amino Acids in Martian Meteorite AL84001. Science: 279. 362-365
  22. Thomas-Keprta, K., S. Clemett, D. McKay, E. Gibson and S. Wentworth 2009. Origin of Magnetite Nanocrystals in Martian Meteorite ALH84001 journal Geochimica et Cosmochimica Acta: 73. 6631-6677.
Related Links
SOURCE: Wikipedia Shergottites
Shergotty Meteorite
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Image of the Shergotty meteorite.

The Shergotty meteorite is the first example of the shergottite Mars meteorite family. It was a 5 kilograms (11 lb) Martian meteorite which fell to Earth at Shergotty (now Sherghati), in the Gaya district, Bihar, India on 25 August 1865, and was retrieved by witnesses almost immediately.[1] This meteorite is relatively young; radiometric dating indicates that it solidified from a volcanic magma about 4.1 billion years ago.[2] It is composed mostly of pyroxene and is thought to have undergone preterrestrial aqueous alteration for several centuries. Certain features within its interior are suggestive of being remnants of biofilm and their associated microbial communities.[3]

References:

  1. Shergotty Meteorite - JPL, NASA
  2. Bouvier, A.; et al. (2009). "Martian meteorite chronology and the evolution of the interior of Mars.". Earth and Planetary Science Letters 280: 285�295. doi:10.1016/j.epsl.2009.01.042
  3. Evidence for ancient Martian life. E. K. Gibson Jr., F. Westall, D. S. McKay, K. Thomas-Keprta, S. Wentworth, and C. S. Romanek, Mail Code SN2, NASA Johnson Space Center, Houston TX, USA.
SOURCE: Wikipedia Shergotty Meteorite
Nakhla Meteorite
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Image of the Nakhla meteorite

Nakhla meteorite

The Nakhla meteorite, the prototypical example of a Nakhlite type meteorite of the SNC Group of Mars meteorites, fell to Earth on June 28, 1911, at approximately 09:00, in the Nakhla region of Abu Hommos, Alexandria, Egypt.[1][2] Many people witnessed its explosion in the upper atmosphere before the meteorite fell to Earth in about forty pieces; the fragments were buried in the ground up to a meter deep. From an estimated original weight of 10 kg (22 pounds), recovered fragments ranged in weight from 20g to 1813g.[1]

Beloe: Scanning electron microscope (SEM) image of the surface of a grain of the meteorite showing small pits filled with material. On Earth, such pits are carved by bacteria;

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This is a scanning electron microscope (SEM) image of a series of partly filled pits on the surface of a mineral grain from Nakhla. Similar pits are sometimes found on Earth minerals which have been exposed to microbial attack. They are interpreted as etch features caused by organic acid generated by microbes and their biofilms. The pits in the Martian minerals are partly filled with debris, possibly the clay-like remains of organic microbes.
Date: November 2009(2009-11) Credit: NASA/David McKay - Source: NASA

The Nakhla dog

One fragment of the meteorite was said to have landed on a dog, as observed by a farmer named Mohammed Ali Effendi Hakim in the village of Denshal, near Nakhla, supposedly vaporizing the animal instantly. Since no remains of the dog were recovered and there were no other eyewitness to the dog's demise, this story remains apocryphal.[1] However, the story of the Nakhla dog has become something of a legend among astronomers.

Martian origins

Thirty-four meteorites thought to have originated from Mars have been catalogued from around the world[3], including the Nakhla meteorite. These are considered to have been ejected by the impact of another large body colliding with the Martian surface. They then travelled through the solar system for an unknown period of time before penetrating the Earth's atmosphere.

SEM image of a chip of the Naklha meteorite, depicting biomorph material: the larger, broad knife-like and the small, donut-shaped features.

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Complex biomorphs appear on another Nakhla chip shown in this scanning electron microscope (SEM) frame. This image contains three basic forms: Broad smooth knife-shaped features, elongated features with rounded endcaps and transverse compartments or dividers, and donut shaped small features, each about 1 micrometer in diameter. One possibility is the donut-shaped features are derived from the compartments present in the elongated features.
Date: November 2009(2009-11) Credit: NASA/David McKay - Source: NASA

Signs of water

Nahklah is the first Martian meteorite to show signs of aqueous processes on Mars. The rock contains carbonates and hydrous minerals, formed by chemical reactions in water. In addition, the rock was exposed to water after it formed, which caused secondary accumulations of minerals. The carbonates contain more 13C than rocks formed on Earth, indicating Martian origin.[4]

Signs of life

In March 1999, a team from NASA's Johnson Space Center examined the Nakhla meteorite using an optical microscope and a powerful scanning electron microscope (SEM), revealing biomorphic forms of a limited size range, among other features.[5] London's Natural History Museum, which holds several intact fragments of the meteorite, allowed NASA researchers to break one open in 2006, providing fresh samples, relatively free from Earth-sourced contamination. These scientists found an abundance of complex carbonaceous material occupying dendritic pores and channels in the rock, reminiscent of the effects of bacteria observed in rocks on Earth[6]

A debate was opened by some at the 37th Lunar and Planetary Science Conference in March 2006 in Houston, Texas, postulating that the carbon-rich content within the pores of the rocks hinted at the remains of living matter. However, because carbon is the fourth most abundant element in the universe (after hydrogen, helium, and oxygen), the presence of shapes reminiscent of living organisms has itself proved insufficient to convince all scientists that bacteria once lived on Mars.[7]

Amino acids within the meteorite

In 1999, various amino acids were isolated from the meteorite fragment at Johnson Space Center. Among them were aspartic acid, glutamic acid, glycine, alanines, and butyric acid. However, it is not clear whether they were originally from the meteorite or the product of terrestrial contamination.[4]

References:

  1. The Nakhla Meteorite - From NASA's Jet Propulsion Laboratory
  2. Nakhla meteorite fragment - From the Natural History Museum. Rotatable image of a fragment of the meteorite. URL accessed September 6, 2006.
  3. Mars Meteorites - NASA
  4. Glavin, Daniel P. Amino acids in the Martian meteorite Naklah
  5. Space rock re-opens Mars debate - February 8, 2006 BBC News article. URL accessed September 6, 2006
  6. BBC News - Space rock re-opens Mars debate
  7. BBC News - Life on Mars - new claims
SOURCE: Wikipedia Nakhla Meteorite
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