In January 2020,cosmochemists reported that theoldest material found on Earth to date are thesilicon carbide particles from the Murchison meteorite, which have been determined to be 7 billion years old, about 2.5 billion yearsolder than the 4.54-billion-yearage of the Earth and theSolar System.[a] The published study noted that "dust lifetime estimates mainly rely on sophisticated theoretical models. These models, however, focus on the more common small dust grains and are based on assumptions with large uncertainties."[3]
On 28 September 1969 at approximately 10:58 a.m. local time, nearMurchison, Victoria, in Australia, a brightfireball was observed to separate into three fragments before disappearing,[1] leaving a cloud of smoke. About 30 seconds later, a tremor was heard. Many fragments were found scattered over an area larger than 13 km2 (5.0 mi2; 3,200 acres), with individual mass up to 7 kilograms (15.4 lb); one, weighing 680 grams (1.5 lb), broke through a roof and fell in hay.[1] The total collected mass of the meteorite exceeds 100 kilograms (220 lb).[4]
The meteorite belongs to theCM group ofcarbonaceous chondrites. Like most CM chondrites, Murchison ispetrologic type 2, which means that it experienced extensive alteration by water-rich fluids on itsparent body[5] before falling to Earth. CM chondrites, together with the CI group, are rich incarbon and are among the most chemically primitive meteorites.[6] Like other CM chondrites, Murchison contains abundantcalcium-aluminium-rich inclusions. More than 15amino acids, some of the basic components of life, have been identified during multiple studies of this meteorite.[7]
In January 2020, astronomers reported thatsilicon carbide grains from the Murchison meteorite had been determined to bepresolar material. The oldest of these grains was found to be 3 ± 2 billion years older than the 4.54 billion yearsage of the Earth andSolar System, making it theoldest material found on Earth to date.[3][8]
The initial report in 1970 stated that the amino acids wereracemic and therefore formed in an abiotic manner, because amino acids of terrestrialproteins are all of the L-configuration ofchirality. Later, in 1982, it was reported that the amino acidalanine had an excess of the L-configuration,[11] but this is a protein amino acid which led several scientists to suspect terrestrial contamination according to the argument that it would be "unusual for an abioticstereoselective decomposition or synthesis of amino acids to occur with protein amino acids but not with non-protein amino acids".[12] But in 1997, L-excesses were also reported for several non-protein amino acids,[13] suggesting an extraterrestrial source for molecular asymmetry in theSolar System. Some amino acids were found to be racemic (equal quantities of right-handed and left-handed). Around the same time, an enrichment in theisotope15N was reported,[14] however this result and the non-racemicity of alanine (but not of the others) were explained as possibly due to analysis error.[15]
By 2001, the list of organic materials identified in the meteorite was extended topolyols.[16]
The meteorite contained a mixture of left-handed and right-handed amino acids; most amino acids used by living organisms are left-handed inchirality, and most sugars used are right-handed. A team of chemists in Sweden demonstrated in 2005 that thishomochirality could have been triggered orcatalyzed by the action of a left-handed amino acid such asproline.[18]
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 includingspectroscopy, identified 14,000 molecular compounds, including 70 amino acids, in a sample of the meteorite.[19][20] The limited scope of the analysis bymass 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.[21]
^Bada, Jeffrey L.; Cronin, John R.; Ho, Ming-Shan; Kvenvolden, Keith A.; Lawless, James G.; Miller, Stanley L.; Oro, J.; Steinberg, Spencer (10 February 1983). "On the reported optical activity of amino acids in the Murchison meteorite".Nature.301 (5900):494–496.Bibcode:1983Natur.301..494B.doi:10.1038/301494a0.S2CID4338550.
^Córdova, Armando; Engqvist, Magnus; Ibrahem, Ismail; Casas, Jesús; Sundén, Henrik (2005). "Plausible origins of homochirality in the amino acid catalyzed neogenesis of carbohydrates".Chemical Communications (15):2047–2049.doi:10.1039/b500589b.PMID15834501.
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This article incorporatespublic domain material from websites or documents of theNational Aeronautics and Space Administration.
