Carbonaceous chondrite
Carbonaceous chondrite | |
---|---|
— chondrules. | |
Type | Chondrite |
Alternative names | C chondrites |
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 8 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites. The C chondrites represent only a small proportion (4.6%)[1] of meteorite falls.
Some famous carbonaceous chondrites are:
.General description
C chondrites contain a high proportion of carbon (up to 3%), which is in the form of graphite, carbonates and organic compounds, including amino 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 the Allende meteorite, contain calcium-aluminum-rich inclusions (CAIs). These are compounds that emerged early from the primeval solar nebula, condensed out and represent the oldest minerals formed in the Solar System.[3][4]
Some primitive carbonaceous chondrites, such as the CM chondrite
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]
Composition and classification
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-letter CX designation, where C stands for "carbonaceous" (other types of
Several groups of carbonaceous chondrites, notably the
CI group
This group, named after the
CI chondrites typically contain a high proportion of water (up to 22%),
Five CI chondrites have been observed to fall:
. 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.CV group
This group takes its name from Vigarano (Italy). Most of these chondrites belong to the petrologic type 3.
CV chondrites observed falls:
CM group
The group takes its name from
CR group
The group takes its name from Renazzo (Italy). The best parent body candidate is 2 Pallas.[11]
CR chondrites observed falls:
Other famous CR chondrites:
CH group
"H" stands for "high metal" because CH chondrites may contain up to as much as 40% of metal.[16] 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 was ALH 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.[11]
CB group
The group takes its name from the most representative member: Bencubbin (Australia). Although these chondrites contain over 50% nickel-iron metal, they are not classified as mesosiderites because their mineralogical and chemical properties are strongly associated with CR chondrites.[11]
CK group
This group takes its name from Karoonda (Australia). These chondrites are closely related to the CO and CV groups.[11]
CO group
The group takes its name from
Famous CO chondrite falls:
Famous finds:
CL group
Officially recognized in 2022[17] after minimum specimens (five) described.[18] CL chondrites, named after type specimen(s) Loongana, are chondrite-rich, metal-rich, and volatile-poor.
C ungrouped
The most famous members:
- Tagish Lake
- Tarda
Organic matter
Most of the
The CM meteorite
.Extraterrestrial amino acids
Amino acids in carbonaceous chondrites have important implications for theories describing the delivery of organic compounds to the early Earth and the subsequent development of life. Shortly after its fall and recovery in Australia in 1969, the Murchison meteorite was found to host five protein amino acids (glycine, alanine, valine, proline, and glutamic acid) in addition to 12 non-proteinogenic amino acids including α-aminoisobutyric acid and isovaline, which are rare on Earth.[19] 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.[20] 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.[21]
Amino acids may be structurally chiral, meaning that they have two possible non-superimposable mirror image structures, termed enantiomers. Conventionally, these are referred to as left-handed (L) and right-handed (D) by analogy with glyceraldehyde. 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.[22] In contrast with terrestrial biology, early laboratory studies, including the famous Miller-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.[23] 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.[19] 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 the Strecker synthesis which produces racemic mixtures of enantiomers.[24]
Ehrenfreund et al. (2001)
Enantiomeric excesses observed in extraterrestrial amino acids
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.
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.[30] 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).[31] 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.
Implications for extraterrestrial biosignatures
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)[30] emphasize three criteria that must be met: chiral asymmetry, light 13C isotopic composition, and simplified distribution of structural isomers. If a distribution of amino acids in an extraterrestrial sample is found to be chirally asymmetric, display structural isomeric preference, and carry 13C, 15N, and D depletions relative to associated inorganic material, a compelling case may be made for its biological origin. With the current interest in sample 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 our Solar System.
See also
References
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- ^ Addi Bischof et al.: The old, unique C1 chondrite Flensburg – Insight into the first processes of aqueous alteration, brecciation, and the diversity of water-bearing parent bodies and lithologies. In: Geochimica et Cosmochimica Acta, Vol. 293, 15 January 2021, pages 142-186
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External links
- Carbonaceous Chondrite Images from Meteorites Australia - Meteorites.com.au