Murchison meteorite

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Murchison
A Murchison meteorite specimen at the National Museum of Natural History (Washington)
Type	Chondrite
Class	Carbonaceous chondrite
Group	CM2
Composition	22.13% total iron, 12% water
Shock stage	S1-2
Country	Australia
Region	Victoria
Coordinates	36°37′S 145°12′E / 36.617°S 145.2°E / -36.617; 145.2Coordinates: 36°37′S 145°12′E / 36.617°S 145.2°E / -36.617; 145.2[1]
Observed fall	Yes
Fall date	28 September 1969
Total Known Weight (TKW)	100 kg
Pair of grains from the Murchison meteorite

The Murchison meteorite is named after Murchison, Victoria, in Australia. It is one of the most studied meteorites due to its large mass (>100 kg), the fact that it was an observed fall, and it belongs to a group of meteorites rich in organic compounds.

[edit] History

On 28 September 1969 at about 10:58 AM, near the town of Murchison, Victoria in Australia, a bright fireball was observed to separate into three fragments before disappearing^[1], leaving a cloud of smoke. About 30 seconds later, a tremor was heard. Many specimens were found over an area larger than 13 km², with individual masses up to 7 kg; one, weighing 680 g, broke through a roof and fell in hay^[1]. The total collected mass exceeds 100 kg.

[edit] Classification and composition

The meteorite belongs to the CM group of carbonaceous chondrites (see meteorite classification). Like most CM chondrites, Murchison is petrologic type 2, which means that it experienced extensive alteration by water-rich fluids on its parent body^[2] before falling to Earth. CM chondrites, together with the CI group, are rich in carbon and are among the most chemically primitive meteorites in our collections. Like other CM chondrites, Murchison contains abundant CAIs. Over 100 amino acids (some of the basic components of life) have been identified in the meteorite.

[edit] Organic matter

Fragment of the Murchison meteorite (at right) and isolated individual particles (shown in the test tube).

Murchison contains common amino acids such as glycine, alanine and glutamic acid as well as unusual ones like isovaline and pseudoleucine.^[3] The initial report stated that the amino acids were racemic (that is, the chirality of their enantiomers are equally left- and right-handed), indicating that they are not present due to terrestrial contamination. A complex mixture of alkanes was isolated as well, similar to that found in the Miller-Urey experiment. Serine and threonine, usually considered to be earthly contaminants, were conspicuously absent in the samples. A specific family of amino acids called diamino acids was identified in the Murchison meteorite as well.^[4]

More research found that some amino acids were present in enantiomeric excess,^[5] leading some to suspect terrestrial contamination, since it would be "unusual for an abiotic stereoselective decomposition or synthesis of amino acids to occur with protein amino acids but not with non-protein amino acids."^[6] In 1997 research showed that individual amino-acid enantiomers from Murchison were enriched in the nitrogen isotope ¹⁵N relative to their terrestrial counterparts, which confirmed an extraterrestrial source for an L-enantiomer excess in the Solar System.^[7] The list of organic materials identified in the meteorite was extended to polyols by 2001.^[8]

Building on the idea that homochirality (existence of only left handed amino acids and right handed sugars) is triggered by deposition of chiral molecules on meteorites, research in 2005 demonstrated that an amino acid like L-proline is capable of catalyzing the formation of chiral sugars. The catalysis is non-linear, that is proline with an enantiomeric excess of 20% yields an allose with enantiomeric excess of 55% starting from a benzyloxy acetaldehyde in a sequential aldol type reaction in an organic solvent like DMF.^[9] In other words a small enantiomeric excess of left-handed amino acids may explain terrestrial life's preference for right-handed sugars.

According to Engel, several lines of evidence indicate that the interior portions of well-preserved fragments from Murchison are pristine. Engel points to the array of amino acids Murchison contains and to isotope studies to bolster his position. Other scientists are equally convinced that the evidence proves the opposite: that Murchison is now thoroughly contaminated by terrestrial organic material.^{[citation needed]}

A 2008 study showed that the Murchison meteorite contains nucleobases. Measured carbon isotope ratios indicate a non-terrestrial origin for these compounds.^[11]

[edit] Nucleobases

Measured purine and pyrimidine compounds are indigenous components of the Murchison meteorite. Carbon isotope ratios for uracil and xanthine of 44.5‰ and +37.7‰, respectively, indicate a non-terrestrial origin for these compounds. These new results demonstrate that many organic compounds which are components of life on Earth, were already present in the early solar system and may have played a key role in life's origin.^[12]

[edit] References

[edit] External links

• Rosenthal, Anne M. (2003-02-12). "Murchison's Amino Acids: Tainted Evidence?". Astrobiology Magazine. http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=375.