| Carbonaceous chondrite | |
|---|---|
| — Class — | |
 A slice of theAllende meteorite showing circularchondrules. | |
| Type | Chondrite |
| Alternative names | C chondrites |
Carbonaceous chondrites orC chondrites are a class ofchondritic meteorites comprising at least 8 known groups and many ungroupedmeteorites. They include some of the most primitive known meteorites. The C chondrites represent only a small proportion (4.6%)[1] ofmeteorite falls.
Some famous carbonaceous chondrites are:Allende,Murchison,Orgueil,Ivuna,Murray,Tagish Lake,Sutter's Mill, andWinchcombe.
C chondrites contain a relatively high proportion of carbon (up to 3%), which is in the form ofgraphite,carbonates and organic compounds, includingamino acids. In addition, they contain water and minerals that have been modified by the influence of water.[2]
The carbonaceous chondrites were not exposed to higher temperatures, so that they are hardly changed by thermal processes. Some carbonaceous chondrites, such as theAllende meteorite, contain calcium-aluminum-rich inclusions (CAIs). These are compounds that emerged early from the primevalsolar nebula, condensed out and represent the oldest minerals formed in theSolar System.[3][4]
Some primitive carbonaceous chondrites, such as the CM chondriteMurchison, contain presolar minerals, includingmoissanite (naturalsilicon carbide) and tiny nanometer-sized diamonds that apparently were not formed in the Solar System. These presolar minerals were probably formed during the explosion of a nearbysupernova or in the vicinity of a pulsatingred giant (more precisely: a so-calledAGB star) before they got into the cloud of matter from which the Solar System was formed. Such star explosions release pressure waves that can condense clouds of matter in their surroundings, leading to the formation of new ones, stars andplanetary systems.[5]
Another carbonaceous chondrite, the Flensburg meteorite (2019), provides evidence of the earliest known occurrence of liquid water in the young Solar System to date.[6][7]
Carbonaceous chondrites are grouped according to distinctive compositions thought to reflect the type of parent body from which they originated. These C chondrite groups are now each named with a standard two-letterCX designation, whereC stands for "carbonaceous" (other types ofchondrites do not begin with this letter) plus a capital letter in the spotX, which is very often the first letter of the name of a prominent meteorite—often the first to be discovered—in the group. Such meteorites are often named for the place where they fell, thus giving no clue as to the physical nature of the group. GroupCH, where H is for "high metal" is so far the only exception. See below for name derivations of each group.
Several groups of carbonaceous chondrites, notably theCM andCI groups, contain high percentages (3% to 22%) ofwater,[8] as well asorganic compounds. They are composed mainly ofsilicates,oxides andsulfides, with the mineralsolivine andserpentine being characteristic. The presence of volatile organic chemicals and water indicates that they have not undergone significant heating (>200 °C) since they were formed, and their compositions are considered to be close to that of thesolar nebula from which theSolar System condensed. Other groups of C chondrites, e.g., CO, CV, and CK chondrites, are relatively poor in volatile compounds, and some of these have experienced significant heating on their parent asteroids.
This group, named after theIvuna meteorite (Tanzania), have chemical compositions that are close to that measured in the solar photosphere (aside from gaseous elements,and elements such as lithium which are underrepresented in the Sun's photosphere by comparison to their abundance in CI chondrites). In this sense, they are chemically the most primitive known meteorites.[citation needed]
CI chondrites typically contain a high proportion of water (up to 22%),[8] and organic matter in the form ofamino acids[9] andPAHs.[10] Aqueous alteration promotes a composition of hydrousphyllosilicates,magnetite, andolivine crystals occurring in a black matrix, and a possible lack ofchondrules. It is thought they have not been heated above 50 °C (122 °F), indicating that they condensed in the cooler outer portion of the solar nebula.
Five CI chondrites have been observed to fall:Ivuna,Orgueil,Alais,Tonk, andRevelstoke. Four others have been found by Japanese field parties in Antarctica. In general, the extreme fragility of CI chondrites causes them to be highly susceptible to terrestrial weathering, and they do not survive on Earth's surface for long after they fall.
This group takes its name fromVigarano (Italy). Most of these chondrites belong to thepetrologic type 3.
CV chondrites observed falls:
The group takes its name fromMighei (Ukraine), but the most famous member is the extensively studiedMurchison meteorite. Many falls of this type have been observed and CM chondrites are known to contain a rich mix of complex organic compounds such as amino-acids and purine/pyrimidine nucleobases.[17][18][19]CM chondrite famous falls:
The group takes its name fromRenazzo (Italy). The bestparent body candidate is2 Pallas.[17]
CR chondrites observed falls:
Other famous CR chondrites:
"H" stands for "high metal" because CH chondrites may contain up to as much as 40% of metal.[23] That makes them one of the most metal-rich of any of the chondrite groups, second only to the CB chondrites and some ungrouped chondrites such as NWA 12273. The first meteorite discovered wasALH 85085. Chemically, these chondrites are closely related to CR and CB groups. All specimens of this group belong only to petrologic types 2 or 3.[17]
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The group takes its name from the most representative member: Bencubbin (Australia).[24] Although these chondrites contain over 50% nickel-iron metal, they are not classified asmesosiderites because their mineralogical and chemical properties are strongly associated with CR chondrites.[17]
This group takes its name fromKaroonda (Australia). These chondrites are closely related to the CO and CV groups.[17]
The group takes its name fromOrnans (France). The chondrule size is only about 0.15 mm on average. They are all of petrologic type 3.
Famous CO chondrite falls:
Famous finds:
Officially recognized in 2022[25] after minimum specimens (five) described.[26] CL chondrites, named after type specimen(s) Loongana, are chondrite-rich, metal-rich, and volatile-poor.
The most famous members:
Most of theorganiccarbon in CI and CM carbonaceous chondrites is an insoluble complex material. That is similar to the description forkerogen. A kerogen-like material is also in theALH84001Martian meteorite (anachondrite).
The CM meteoriteMurchison has over 96 extraterrestrialamino acids and other compounds includingcarboxylic acids, hydroxy carboxylic acids, sulphonic and phosphonic acids, aliphatic, aromatic and polarhydrocarbons,fullerenes,heterocycles,carbonyl compounds,alcohols,amines andamides.
Amino acids in carbonaceous chondrites have important implications for theories describing the delivery of organic compounds to the early Earth and the subsequentdevelopment of life. Shortly after its fall and recovery in Australia in 1969, theMurchison meteorite was found to host five protein amino acids (glycine,alanine,valine,proline, andglutamic acid) in addition to 12 non-proteinogenic amino acids includingα-aminoisobutyric acid andisovaline, which are rare on Earth.[27] Since then, the number of characterized amino acids in the Murchison meteorite has risen to 96, including 12 of the 20 common biological amino acids, along with hundreds more that have been detected, but remain uncharacterized.[28] While the abundance of amino acids present in terrestrial soils presents a potential source of contamination, most of the amino acids characterized in Murchison are terrestrially rare or absent.[29]
Amino acids may be structurallychiral, meaning that they have two possible non-superimposable mirror image structures, termedenantiomers. Conventionally, these are referred to as left-handed (L) and right-handed (D) by analogy withglyceraldehyde. Living beings use L-amino acids, although there is no apparent reason why one enantiomer is favoured over the other as they behave equivalently in biological systems.[30] In contrast with terrestrial biology, early laboratory studies, including the famousMiller-Urey Experiment, have shown that amino acids may form under a range of possible abiotic conditions with equal (racemic) mixtures of D- and L-enantiomers.[31] Thus, the ratios between enantiomers for a given amino acid may discriminate between biotic and abiotic formation mechanisms. In the first characterization of amino acids in Murchison, all chiral examples were present in racemic mixtures indicating an abiotic origin.[27] This is consistent with proposed sythetic pathways, as the formation of isovaline and other α-dialkyl amino acids in CM chondrites has been attributed to theStrecker synthesis which produces racemic mixtures of enantiomers.[32]
Ehrenfreund et al. (2001)[9] found that amino acids in CI chondrites Ivuna and Orgueil were present at much lower concentrations than in CM chondrites (~30%), and that they had a distinct composition high in β-alanine,glycine, γ-ABA, andβ-ABA but low inα-aminoisobutyric acid (AIB) andisovaline. This implies that they had formed by a different synthetic pathway, and on a different parent body from the CM chondrites.
More recently, amino acids from several carbonaceous chondrites have been identified with significant L-enantiomeric excesses. L-excesses from 3 – 15% in several non-protein α-dialkyl amino acids have been found in the Murchison and Murray meteorites.[33] Their extraterrestrial origin is indicated by their absence in biological systems and significant heavyisotope enrichments in13C and deuterium compared to terrestrial values.[34] Further characterization of L-isovaline excesses up to 20.5% in a range of carbonaceous chondrite groups have supported a hypothesis that increasinghydrothermal alteration of the host meteorite correlates with increasing observed L-enantiomeric excess.[35] Large L-excesses for α-H amino acids have also been reported, but these are more problematic due to the potential for terrestrial contamination.[36] The ungrouped C2 chondriteTagish Lake has L-aspartic acid excesses up to ~60%, with carbon isotope measurements indicating an extraterrestrial origin due to significant enrichments in13C.[37] In Tagish Lake, proteinogenic amino acids show both significant L-excesses, and racemic mixtures: glutamic acid, serine, and threonine were found to have ~50 – 99% L-excesses, while alanine was racemic.[37]
It has been proposed that extraterrestrial amino acid L-excesses observed in carbonaceous chondrites are a result of differences in the crystallization behaviour of the enantiomers.[38]Circularly polarized ultraviolet light has been shown to generate L-excesses in crystallizing amino acids for experimental conditions mimicking alteration on asteroids, and this is thought to be the dominant extraterrestrial source of chiral symmetry breaking (i.e., the favouring of one enantiomer over another).[39] It is notable that only excesses of the L-enantiomer have been observed in extraterrestrial amino acids, suggesting that the abiotic process responsible for enantiomeric enrichments may be the original source of the L-amino acid selectivity currently observed in terrestrial life.
NASA have proposed a "Ladder of Life Detection" threshold of >20%enantiomeric excess in amino acids to distinguish extraterrestrial biosignatures. But, as previously mentioned, recent studies of carbonaceous chondrites and complementary experimental investigations have demonstrated that even larger enantiomeric excesses may be produced by abiotic pathways. To identify chiral asymmetry (enantiomeric excess) of biological origin, Glavin et al. (2020)[38] emphasize three criteria that must be met: chiral asymmetry, light13C isotopic composition, and simplified distribution ofstructural isomers. If a distribution of amino acids in an extraterrestrial sample is found to be chirally asymmetric, display structural isomeric preference, and carry13C,15N, and D depletions relative to associated inorganic material, a compelling case may be made for its biological origin. With the current interest insample return missions from carbonaceous asteroids (e.g.,OSIRIS-REx) and Mars headed by NASA and other space agencies, the subsequent analysis of returned samples devoid of terrestrial contamination will provide the best opportunity to discover potential biosignatures in the Solar System.
