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 that it belongs to a group of meteorites rich in organic compounds.
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, leaving a cloud of smoke. About 30 seconds later, a tremor was heard. Many fragments were found over an area larger than 13 km², with individual mass up to 7 kg; one, weighing 680 g, broke through a roof and fell in hay. The total collected mass exceeds 100 kg.
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 before falling to Earth. CM chondrites, together with the CI group, are rich in carbon and are among the most chemically primitive meteorites. Like other CM chondrites, Murchison contains abundant CAIs. Over 15 amino acids (some of the basic components of life) have been identified in the meteorite. All the amino acids found in the Murchison meteorite have been synthesized in laboratory experiments by the action of electric discharge on a mixture of methane, nitrogen, and water with traces of ammonia.
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. 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.
More research found that some amino acids were present in enantiomeric excess, 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." 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. The list of organic materials identified in the meteorite was extended to polyols by 2001.
|Purines and pyrimidines
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 benzyloxyacetaldehyde in a sequential aldol type reaction in an organic solvent like DMF. In other words a small enantiomeric excess of left-handed amino acids may explain terrestrial life's preference for right-handed sugars.
Several lines of evidence indicate that the interior portions of well-preserved fragments from Murchison are pristine. A 2010 study using high resolution analytical tools including spectroscopy, identified 14,000 molecular compounds including 70 amino acids in a sample of the meteorite. The limited scope of the analysis by mass spectrometry provides for a potential 50,000 or more unique molecular compositions, with the team estimating the possibility of millions of distinct organic compounds in the meteorite.
Measured purine and pyrimidine compounds are indigenous components of the Murchison meteorite. Carbon isotope ratios for uracil and xanthine of δ13C = +44.5‰ and +37.7‰, respectively, indicate a non-terrestrial origin for these compounds. These 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.
- ^ a b c Meteoritical Bulletin Database: Murchison
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