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{{Short description|Class of chondritic meteorites}}
{{Infobox meteorite subdivision
|Subdivision               = Class
|Name                      = Carbonaceous chondrite
|Alternative_names         = C chondrites
|Image                     = Allende meteorite.jpg
|Image_caption             = A slice of the [[Allende meteorite]] showing circular [[chondrules]].
|Image_alt_text            = 
|Compositional_type        = 
|Type                      = [[Chondrite]]
|Structural_classification = 
|Parent_body               = 
|Composition               = 
|Petrologic_type           =
|Number_of_specimens       = 
|TKW                       = 
|Image2                    = 
|Image2_caption            = 
|Image2_alt_text           = 
}}
'''Carbonaceous chondrites''' or '''C chondrites''' are a class of [[chondrite|chondritic]] [[meteorite]]s 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%)<ref>{{cite journal |last1=Bischoff |first1=A. |last2=Geiger |first2=T. |title=Meteorites for the Sahara: Find locations, shock classification, degree of weathering and pairing |journal=Meteoritics |issn=0026-1114 |volume=30 |issue=1 |pages=113–122 |year=1995 |bibcode=1995Metic..30..113B |doi=10.1111/j.1945-5100.1995.tb01219.x|doi-access=free }}</ref> of [[meteorite fall]]s.

Some famous carbonaceous chondrites are: [[Allende meteorite|Allende]], [[Murchison meteorite|Murchison]], [[Orgueil (meteorite)|Orgueil]], [[Ivuna meteorite|Ivuna]], [[Murray meteorite|Murray]], [[Tagish Lake (meteorite)|Tagish Lake]], [[Sutter's Mill meteorite|Sutter's Mill]] and [[Winchcombe meteorite|Winchcombe]].

== General description ==
C chondrites contain a high proportion of carbon (up to 3%), which is in the form of [[graphite]], [[carbonate]]s and organic compounds, including [[amino acid]]s. In addition, they contain water and minerals that have been modified by the influence of water.<ref>BÜHLER: . Springer-Verlag, 2013, {{ISBN|978-3-0348-6667-5}} , pp. 130 ( limited preview in Google Books search).</ref>

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 [[Formation and evolution of the Solar System|solar nebula]], condensed out and represent the oldest minerals formed in the [[Solar System]].<ref>Horst Rauchfuss:. Springer-Verlag, 2006, {{ISBN|978-3-540-27666-1}} , pp. 83 ( limited preview in Google Books search).</ref><ref>Gregor Markl:  . Springer-Verlag, 2014, {{ISBN|978-3-662-44628-7}} , pp. 420 ( limited preview in Google Book Search).</ref>

Some primitive carbonaceous chondrites, such as the CM chondrite [[Murchison meteorite|Murchison]], contain presolar minerals, including [[moissanite]] (natural [[silicon carbide]]) and tiny nanometer-sized diamonds that apparently were not formed in our solar system. These presolar minerals were probably formed during the explosion of a nearby [[supernova]] or in the vicinity of a pulsating [[red giant]] (more precisely: a so-called [[Asymptotic giant branch|AGB 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 and [[planetary systems]].<ref>Martin Vieweg: Ancient carbonates are evidence of water, on: Wissenschaft.de from January 22, 2021 The oldest carbonates in the solar system , on: EurekAlert! from January 20, 2021</ref>

Another carbonaceous chondrite, the Flensburg meteorite (2019), provides evidence of the earliest known occurrence of liquid water in the young Solar System to date.<ref>Addi Bischof et al.: ''[https://www.sciencedirect.com/science/article/pii/S0016703720306463?dgcid=rss_sd_all 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</ref><ref>Robert Hutchison: Cambridge University Press, 2006, {{ISBN|978-0-521-03539-2}}, pp. 42 (limited preview in Google Books search).</ref>

==Composition and classification==
[[File:carbonaceous chondrites.jpg|thumb|300px|Some carbonaceous chondrites. From left to right: Allende, Yukon and Murchison.]]
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 [[chondrites]] do not begin with this letter) plus a capital letter in the spot '''''X''''', 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. Group '''CH''', 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 the [[#CM group|CM]] and [[#CI group|CI]] groups, contain high percentages (3% to 22%) of [[Asteroidal water|water]],<ref name=norton>{{cite book |title=The Cambridge Encyclopedia of Meteorites |last=Norton |first=O. Richard |year=2002 |publisher=Cambridge University Press |location=Cambridge |isbn=978-0-521-62143-4 |pages=121–124}}</ref> as well as [[organic compound]]s. They are composed mainly of [[silicate]]s, [[oxide]]s and [[sulfide]]s, with the minerals [[olivine]] and [[serpentine group|serpentine]] being characteristic. The presence of volatile organic chemicals and water indicates that they have not undergone significant heating (&gt;200&nbsp;°C) since they were formed, and their compositions are considered to be close to that of the [[solar nebula]] from which the [[Solar System]] condensed. Other groups of C&nbsp;chondrites, e.g., CO, CV, and CK&nbsp;chondrites, are relatively poor in volatile compounds, and some of these have experienced significant heating on their parent asteroids.

===CI group===
This group, named after the [[Ivuna meteorite]] (Tanzania), have chemical compositions that are close to that measured in the solar photosphere (aside from gaseous elements, [[Standard solar model#Lithium depletion at the solar surface|and elements such as lithium which are underrepresented in the Sun's photosphere]] by comparison to their abundance in CI&nbsp;chondrites). In this sense, they are chemically the most primitive known meteorites. {{citation needed|date=October 2014}}

CI chondrites typically contain a high proportion of water (up to 22%),<ref name=norton /> and organic matter in the form of [[amino acid]]s<ref name=ehrenfreund>{{cite journal |last=Ehrenfreund |first=Pascale |author2=Daniel P. Glavin |author3=Oliver Botta |author4=George Cooper |author5=Jeffrey L. Bada  |year=2001 |title=Extraterrestrial amino acids in Orgueil and Ivuna: Tracing the parent body of CI type carbonaceous chondrites |journal=Proceedings of the National Academy of Sciences |volume=98 |issue=5 |pages=2138–2141  |doi=10.1073/pnas.051502898 |pmid=11226205 |pmc=30105|bibcode = 2001PNAS...98.2138E |doi-access=free }}</ref> and [[Polycyclic aromatic hydrocarbon|PAH]]s.<ref>{{cite journal |last=Wing |first=Michael R. |author2=Jeffrey L. Bada |year=1992 |title=The origin of the polycyclic aromatic hydrocarbons in meteorites |journal=Origins of Life and Evolution of the Biosphere |volume=21 |issue=5–6 |pages=375–383 |doi=10.1007/BF01808308 |bibcode = 1991OLEB...21..375W |s2cid=11504324 }}</ref> Aqueous alteration promotes a composition of hydrous [[Silicate minerals#Phyllosilicates|phyllosilicate]]s, [[magnetite]], and [[olivine]] crystals occurring in a black matrix, and a possible lack of [[chondrule]]s. It is thought they have not been heated above {{convert|50|°C|°F|abbr=on}}, indicating that they condensed in the cooler outer portion of the solar nebula.

Five CI chondrites have been observed to fall: [[Ivuna meteorite|Ivuna]], [[Orgueil (meteorite)|Orgueil]], [[Alais meteorite|Alais]], [[Tonk meteorite|Tonk]], and [[Revelstoke meteorite|Revelstoke]]. Four others have been found by Japanese field parties in Antarctica. In general, the extreme fragility of CI&nbsp;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===
[[File:NWA 3118 meteorite.jpg|thumb|NWA 3118, CV3]]

This group takes its name from [[Vigarano Mainarda#Trivia|Vigarano]] (Italy). Most of these chondrites belong to the [[Chondrite#Petrologic types|petrologic type]] 3.

CV chondrites observed falls:
*[[Allende meteorite|Allende]]
*[https://www.lpi.usra.edu/meteor/metbull.php?sea=Bali&sfor=names&ants=&nwas=&falls=&valids=&stype=contains&lrec=50&map=ge&browse=&country=All&srt=name&categ=All&mblist=All&rect=&phot=&strewn=&snew=0&pnt=Normal%20table&code=4928 Bali]
*[https://www.lpi.usra.edu/meteor/metbull.php?code=30448 Bukhara]
*[https://www.lpi.usra.edu/meteor/metbull.php?code=11206 Grosnaja]
*[https://www.lpi.usra.edu/meteor/metbull.php?code=12218 Kaba]
*[https://www.lpi.usra.edu/meteor/metbull.php?sea=Mokoia&sfor=names&ants=&nwas=&falls=&valids=&stype=contains&lrec=50&map=ge&browse=&country=All&srt=name&categ=All&mblist=All&rect=&phot=&strewn=&snew=0&pnt=Normal%20table&code=16713 Mokoia]
*[https://www.lpi.usra.edu/meteor/metbull.php?sea=Vigarano&sfor=names&ants=&nwas=&falls=&valids=&stype=contains&lrec=50&map=ge&browse=&country=All&srt=name&categ=All&mblist=All&rect=&phot=&strewn=&snew=0&pnt=Normal%20table&code=24174 Vigarano]

===CM group===
{{Main|CM chondrite}}
The group takes its name from [[Mighei chondrite|Mighei]] (Ukraine), but the most famous member is the extensively studied [[Murchison (meteorite)|Murchison]] meteorite. Many falls of this type have been observed and CM&nbsp;chondrites are known to contain a rich mix of complex organic compounds such as amino-acids and purine/pyrimidine nucleobases.<ref name="meteoritefrcarb"/><ref>{{Cite APOD
| date = 28 April 2012
| title = Sutter's Mill Meteorite
| access-date = 2012-05-06}}</ref><ref name="Pearce & Pudritz (2015)">{{cite journal|last1=Pearce|first1=Ben K. D.|last2=Pudritz|first2=Ralph E.|title=Seeding the Pregenetic Earth: Meteoritic Abundances of Nucleobases and Potential Reaction Pathways|journal=Astrophysical Journal|date=2015|volume=807|issue=1|page=85|doi=10.1088/0004-637X/807/1/85|arxiv = 1505.01465 |bibcode = 2015ApJ...807...85P |s2cid=93561811}}</ref>
CM chondrite famous falls:
*[[Murchison meteorite|Murchison]]
*[[Sutter's Mill meteorite|Sutter's Mill]]
*Aguas Zarcas<ref>{{Cite web|title=Meteoritical Bulletin: Entry for Aguas Zarcas|url=https://www.lpi.usra.edu/meteor/metbull.php?sea=Aguas+Zarcas&sfor=names&ants=&nwas=&falls=&valids=&stype=&lrec=50&map=ge&browse=&country=All&srt=&categ=All&mblist=All&rect=&phot=&strewn=&snew=0&pnt=Normal%20table&code=69696|access-date=2020-08-21|website=www.lpi.usra.edu}}</ref><ref>https://www.science.org/content/article/unusual-meteorite-more-valuable-gold-may-hold-building-blocks-life </ref>
*[[Jbilet Winselwan meteorite|Jbilet Winselwan]]
*[[Winchcombe meteorite]]

===CR group===
The group takes its name from [[Cento#Trivia|Renazzo]] (Italy). The best [[parent body]] candidate is [[2 Pallas]].<ref name="meteoritefrcarb" />

CR chondrites observed falls:
*[[Al Rais meteorite|Al Rais]]
*[[Kaidun meteorite|Kaidun]]
*[[Renazzo meteorite|Renazzo]]

Other famous CR chondrites:
*[https://www.lpi.usra.edu/meteor/metbull.php?sea=&sfor=names&ants=&nwas=&falls=&valids=&stype=contains&lrec=50&map=ge&browse=&country=Libya&srt=name&categ=All&mblist=All&rect=&phot=&strewn=&snew=0&pnt=Normal%20table&code=6121 Dar al Gani 574]
*[[El Djouf 001]]
*[[Northwest Africa 801]]

===CH group===
"H" stands for "high metal" because CH chondrites may contain up to as much as 40% of metal.<ref name=norton2>{{cite book |title=The Cambridge Encyclopedia of Meteorites |last=Norton |first=O. Richard |year=2002 |publisher=Cambridge University Press |location=Cambridge |isbn=978-0-521-62143-4 |page=139}}</ref> 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.<ref name="meteoritefrcarb">[http://www.meteorite.fr/en/classification/carbonaceous.htm "Carbonaceous chondrite" Meteorite.fr: All About Meteorites: Classification] {{webarchive|url=https://web.archive.org/web/20091012121808/http://www.meteorite.fr/en/classification/carbonaceous.htm |date=2009-10-12 }}</ref>

===CB group===
[[File:Gujba meteorite, bencubbinite (14785860604).jpg|thumb|Gujba meteorite, a  bencubbinite found in Nigeria. Polished slice, 4.6&nbsp;×&nbsp;3.8&nbsp;cm. Note the nickel-iron chondrules, which have been age-dated to 4.5627 billion years.]] 
The group takes its name from the most representative member: [https://www.lpi.usra.edu/meteor/metbull.php?sea=Bencubbin&sfor=names&ants=&nwas=&falls=&valids=&stype=contains&lrec=50&map=ge&browse=&country=All&srt=name&categ=All&mblist=All&rect=&phot=&strewn=&snew=0&pnt=Normal%20table&code=5014 Bencubbin] (Australia). Although these chondrites contain over 50% nickel-iron metal, they are not classified as [[mesosiderite]]s because their mineralogical and chemical properties are strongly associated with CR chondrites.<ref name="meteoritefrcarb" />

===CK group===
This group takes its name from [[Karoonda meteorite|Karoonda]] (Australia). These chondrites are closely related to the CO and CV groups.<ref name="meteoritefrcarb" />

===CO group===
The group takes its name from [[Ornans meteorite|Ornans]] (France). The chondrule size is only about 0.15&nbsp;mm on average. They are all of petrologic type&nbsp;3.

Famous CO chondrite falls:
*[[Ornans meteorite|Ornans]]
*[https://www.lpi.usra.edu/meteor/metbull.php?code=12229 Kainsaz]
*[https://www.lpi.usra.edu/meteor/metbull.php?code=24215 Warrenton]
*[[Moss meteorite|Moss]]

Famous finds:
*[[Dar al Gani 749]]

===CL group===
Officially recognized in 2022<ref>{{cite journal |last1=Gattacceca |first1=Jérôme |last2=McCubbin F. M. |last3=Grossman J. |last4=Bouvier A. |last5=Chabot N. L. |last6=D'Orazio M. |last7=Goodrich C. |last8=Greshake A. |last9=Gross J. |last10=Komatsu M. |last11=Miao B. |last12=Schrader D. |title=The Meteoritical Bulletin, No. 110 |journal=Meteoritics and Planetary Science |date=2022 |volume=57 |issue=11 |pages=21022105|doi=10.1111/maps.13918 |bibcode=2022M&PS...57.2102G |s2cid=253089085 |url=https://hal-insu.archives-ouvertes.fr/insu-03863070/file/Meteorit%20Planetary%20Scien%20-%202022%20-%20Gattacceca%20-%20The%20Meteoritical%20Bulletin%20No%20110.pdf }}</ref> after minimum specimens (five) described.<ref>{{cite journal |last1=Metzler |first1=K. |last2=Hezel, D. C. |last3=Barosch, J. |last4=Wölfer, E. |last5=Schneider, J. M. |last6=Hellmann, J. L. |last7=Berndt, J. |display-authors=etal |title=The Loongana (<span class="ltr">CL</span>) Group of Carbonaceous Chondrites |journal=Geochimica et Cosmochimica Acta |date=2021 |volume=304 |pages=1–31|doi=10.1016/j.gca.2021.04.007 |bibcode=2021GeCoA.304....1M |s2cid=234847404 |url=http://oro.open.ac.uk/76111/1/1-s2.0-S0016703721002222-main.pdf }}</ref> CL chondrites, named after type specimen(<span class="ltr">s</span>) Loongana, are chondrite-rich, metal-rich, and volatile-poor.

===C ungrouped===
The most famous members:
*[[Tagish Lake (meteorite)|Tagish Lake]]
*Tarda

==Organic matter==
[[File:Murchison crop.jpg|thumb|200px|Murchison meteorite]]
Most of the [[organic compound|organic]] [[carbon]] in CI and CM carbonaceous chondrites is an insoluble complex material.  That is similar to the description for [[kerogen]].  A kerogen-like material is also in the [[ALH84001]] [[Martian meteorite]] (an [[achondrite]]).

The CM meteorite [[Murchison meteorite|Murchison]] has over 96 extraterrestrial [[amino acid]]s and other compounds including [[carboxylic acid]]s, hydroxy carboxylic acids, sulphonic and phosphonic acids, aliphatic, aromatic and polar [[hydrocarbon]]s, [[fullerene]]s, [[heterocycle]]s, [[carbonyl]] compounds, [[Alcohol (chemistry)|alcohol]]s, [[amine]]s and [[amide]]s.

=== 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 [[Abiogenesis|development of life]]. Shortly after its fall and recovery in Australia in 1969, the [[Murchison meteorite|Murchison]] meteorite was found to host five protein amino acids ([[glycine]], [[alanine]], [[valine]], [[proline]], and [[glutamic acid]]) in addition to 12 non-[[Proteinogenic amino acid|proteinogenic]] amino acids including [[2-Aminoisobutyric acid|α-aminoisobutyric]] acid and [[isovaline]], which are rare on Earth.<ref name=":0">{{Cite journal |last1=Kvenvolden |first1=Keith |last2=Lawless |first2=James |last3=Pering |first3=Katherine |last4=Peterson |first4=Etta |last5=Flores |first5=Jose |last6=Ponnamperuma |first6=Cyril |last7=Kaplan |first7=I. R. |last8=Moore |first8=Carleton |date=1970 |title=Evidence for Extraterrestrial Amino-acids and Hydrocarbons in the Murchison Meteorite |url=https://www.nature.com/articles/228923a0 |journal=Nature |language=en |volume=228 |issue=5275 |pages=923–926 |doi=10.1038/228923a0 |pmid=5482102 |bibcode=1970Natur.228..923K |s2cid=4147981 |issn=1476-4687}}</ref> 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.<ref>{{Citation |last1=Glavin |first1=Daniel P. |title=Chapter 3 - The Origin and Evolution of Organic Matter in Carbonaceous Chondrites and Links to Their Parent Bodies |date=2018-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780128133255000033 |work=Primitive Meteorites and Asteroids |pages=205–271 |editor-last=Abreu |editor-first=Neyda |access-date=2023-05-01 |publisher=Elsevier |language=en |doi=10.1016/b978-0-12-813325-5.00003-3 |isbn=978-0-12-813325-5 |last2=Alexander |first2=Conel M. O'D. |last3=Aponte |first3=José C. |last4=Dworkin |first4=Jason P. |last5=Elsila |first5=Jamie E. |last6=Yabuta |first6=Hikaru|hdl=2060/20180004493 |hdl-access=free }}</ref> 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.<ref>{{Citation |last1=Cronin |first1=John R. |title=Organic Matter in Meteorites: Molecular and Isotopic Analyses of the Murchison Meteorite |date=1993 |url=https://doi.org/10.1007/978-94-011-1936-8_9 |journal=The Chemistry of Life's Origins |pages=209–258 |editor-last=Greenberg |editor-first=J. M. |access-date=2023-05-01 |place=Dordrecht |publisher=Springer Netherlands |language=en |doi=10.1007/978-94-011-1936-8_9 |isbn=978-94-011-1936-8 |last2=Chang |first2=Sherwood |volume=416 |bibcode=1993ASIC..416..209C |editor2-last=Mendoza-Gómez |editor2-first=C. X. |editor3-last=Pirronello |editor3-first=V.}}</ref>

Amino acids may be structurally [[Chirality|chiral]], meaning that they have two possible non-superimposable mirror image structures, termed [[Enantiomer|enantiomers]]. Conventionally, these are referred to as left-handed (<span class="ltr">L</span>) and right-handed (<span class="ltr">D</span>) 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.<ref>{{Cite journal |last1=Milton |first1=R. C. deL. |last2=Milton |first2=S. C. F. |last3=Kent |first3=S. B. H. |date=1992-06-05 |title=Total Chemical Synthesis of a D-Enzyme: The Enantiomers of HIV-1 Protease Show Reciprocal Chiral Substrate Specificity |url=https://www.science.org/doi/10.1126/science.1604320 |journal=Science |language=en |volume=256 |issue=5062 |pages=1445–1448 |doi=10.1126/science.1604320 |pmid=1604320 |issn=0036-8075}}</ref> In contrast with terrestrial biology, early laboratory studies, including the famous [[Miller–Urey experiment|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.<ref>{{Cite journal |last=Miller |first=Stanley L. |date=1953-05-15 |title=A Production of Amino Acids Under Possible Primitive Earth Conditions |url=https://www.science.org/doi/10.1126/science.117.3046.528 |journal=Science |language=en |volume=117 |issue=3046 |pages=528–529 |doi=10.1126/science.117.3046.528 |pmid=13056598 |bibcode=1953Sci...117..528M |issn=0036-8075}}</ref> 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.<ref name=":0" /> 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 amino acid synthesis|Strecker synthesis]] which produces racemic mixtures of enantiomers.<ref>{{Cite journal |last1=Wolman |first1=Yecheskel |last2=Haverland |first2=William J. |last3=Miller |first3=Stanley L. |date=1972 |title=Nonprotein Amino Acids from Spark Discharges and Their Comparison with the Murchison Meteorite Amino Acids |journal=Proceedings of the National Academy of Sciences |language=en |volume=69 |issue=4 |pages=809–811 |doi=10.1073/pnas.69.4.809 |issn=0027-8424 |pmc=426569 |pmid=16591973 |bibcode=1972PNAS...69..809W |doi-access=free }}</ref>
[[File:Strecker-Synthese Übersicht V3.svg|thumb|400x400px|The Strecker synthesis of alpha amino acids from carbonyl compounds in the presence of ammonia and cyanide.]]
Ehrenfreund et al. (2001)<ref name="ehrenfreund" /> 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-[[Gamma-Aminobutyric acid|ABA]], and [[beta-Aminobutyric acid|β-ABA]] but low in [[2-Aminoisobutyric acid|α-aminoisobutyric acid (AIB)]] and [[isovaline]]. This implies that they had formed by a different synthetic pathway, and on a different parent body from the CM chondrites.

==== 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.<ref>{{Cite journal |last1=Cronin |first1=John R. |last2=Pizzarello |first2=Sandra |date=1997-02-14 |title=Enantiomeric Excesses in Meteoritic Amino Acids |url=https://www.science.org/doi/10.1126/science.275.5302.951 |journal=Science |language=en |volume=275 |issue=5302 |pages=951–955 |doi=10.1126/science.275.5302.951 |pmid=9020072 |bibcode=1997Sci...275..951C |s2cid=10979716 |issn=0036-8075}}</ref> Their extraterrestrial origin is indicated by their absence in biological systems and significant heavy [[isotope]] enrichments in <sup>13</sup>C and deuterium compared to terrestrial values.<ref>{{Cite journal |last1=Elsila |first1=Jamie E. |last2=Callahan |first2=Michael P. |last3=Glavin |first3=Daniel P. |last4=Dworkin |first4=Jason P. |last5=Brückner |first5=Hans |date=2011 |title=Distribution and Stable Isotopic Composition of Amino Acids from Fungal Peptaibiotics: Assessing the Potential for Meteoritic Contamination |url=http://www.liebertpub.com/doi/10.1089/ast.2010.0505 |journal=Astrobiology |language=en |volume=11 |issue=2 |pages=123–133 |doi=10.1089/ast.2010.0505 |pmid=21417942 |bibcode=2011AsBio..11..123E |issn=1531-1074}}</ref> Further characterization of L-isovaline excesses up to 20.5% in a range of carbonaceous chondrite groups have supported a hypothesis that increasing [[hydrothermal alteration]] of the host meteorite correlates with increasing observed L-enantiomeric excess.<ref>{{Cite journal |last1=Glavin |first1=Daniel P. |last2=Callahan |first2=Michael P. |last3=Dworkin |first3=Jason P. |last4=Elsila |first4=Jamie E. |date=2010 |title=The effects of parent body processes on amino acids in carbonaceous chondrites: Amino acids in carbonaceous chondrites |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2010.01132.x |journal=Meteoritics & Planetary Science |language=en |volume=45 |issue=12 |pages=1948–1972 |doi=10.1111/j.1945-5100.2010.01132.x|hdl=2060/20100032396 |s2cid=62883414 |hdl-access=free }}</ref> Large L-excesses for α-H amino acids have also been reported, but these are more problematic due to the potential for terrestrial contamination.<ref>{{Cite journal |last1=Glavin |first1=Daniel P. |last2=Elsila |first2=Jamie E. |last3=McLain |first3=Hannah L. |last4=Aponte |first4=José C. |last5=Parker |first5=Eric T. |last6=Dworkin |first6=Jason P. |last7=Hill |first7=Dolores H. |last8=Connolly |first8=Harold C. |last9=Lauretta |first9=Dante S. |date=2021 |title=Extraterrestrial amino acids and L‐enantiomeric excesses in the CM 2 carbonaceous chondrites Aguas Zarcas and Murchison |url=https://onlinelibrary.wiley.com/doi/10.1111/maps.13451 |journal=Meteoritics & Planetary Science |language=en |volume=56 |issue=1 |pages=148–173 |doi=10.1111/maps.13451 |bibcode=2021M&PS...56..148G |hdl=10150/638053 |s2cid=212671033 |issn=1086-9379|hdl-access=free }}</ref> The ungrouped C2 chondrite [[Tagish Lake (meteorite)|Tagish Lake]] has L-[[aspartic acid]] excesses up to ~60%, with carbon isotope measurements indicating an extraterrestrial origin due to significant enrichments in <sup>13</sup>C.<ref name=":1">{{Cite journal |last1=Glavin |first1=Daniel P. |last2=Elsila |first2=Jamie E. |last3=Burton |first3=Aaron S. |last4=Callahan |first4=Michael P. |last5=Dworkin |first5=Jason P. |last6=Hilts |first6=Robert W. |last7=Herd |first7=Christopher D. K. |date=2012 |title=Unusual nonterrestrial l-proteinogenic amino acid excesses in the Tagish Lake meteorite: l-amino acid excesses in the Tagish Lake meteorite |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2012.01400.x |journal=Meteoritics & Planetary Science |language=en |volume=47 |issue=8 |pages=1347–1364 |doi=10.1111/j.1945-5100.2012.01400.x|s2cid=52227545 }}</ref> 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.<ref name=":1" />

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.<ref name=":2">{{Cite journal |last1=Glavin |first1=Daniel P. |last2=Burton |first2=Aaron S. |last3=Elsila |first3=Jamie E. |last4=Aponte |first4=José C. |last5=Dworkin |first5=Jason P. |date=2020-06-10 |title=The Search for Chiral Asymmetry as a Potential Biosignature in our Solar System |url=https://pubs.acs.org/doi/10.1021/acs.chemrev.9b00474 |journal=Chemical Reviews |language=en |volume=120 |issue=11 |pages=4660–4689 |doi=10.1021/acs.chemrev.9b00474 |pmid=31743015 |s2cid=208185504 |issn=0009-2665|doi-access=free }}</ref> [[Circular polarization|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).<ref>{{Cite journal |last1=Garcia |first1=Adrien D. |last2=Meinert |first2=Cornelia |last3=Sugahara |first3=Haruna |last4=Jones |first4=Nykola C. |last5=Hoffmann |first5=Søren V. |last6=Meierhenrich |first6=Uwe J. |date=2019-03-16 |title=The Astrophysical Formation of Asymmetric Molecules and the Emergence of a Chiral Bias |journal=Life |volume=9 |issue=1 |pages=29 |doi=10.3390/life9010029 |issn=2075-1729 |pmc=6463258 |pmid=30884807 |bibcode=2019Life....9...29G |doi-access=free }}</ref> 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)<ref name=":2" /> emphasize three criteria that must be met: chiral asymmetry, light <sup>13</sup>C isotopic composition, and simplified distribution of [[Structural isomer|structural isomers]]. If a distribution of amino acids in an extraterrestrial sample is found to be chirally asymmetric, display structural isomeric preference, and carry <sup>13</sup>C, <sup>15</sup>N, 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 mission|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==
* [[Glossary of meteoritics]]
* [[List of meteorite minerals]]

==References==
{{Reflist}}
*[http://www.daviddarling.info/encyclopedia/C/carbchon.html Carbonaceous chondrites at ''The Encyclopedia of Astrobiology, Astronomy, and Spaceflight'']
*{{cite book |title= Origins of earth and life|last=Gilmour |first=I. |author2=Wright, I. |author3=Wright, J.  |year=1997 |publisher=The Open University |location=Bletchley |isbn=978-0-7492-8182-3 }}

==External links==
{{commons category|Carbonaceous chondrites}}
*[http://www.meteorites.com.au/collection/CREchondrites.html Carbonaceous Chondrite Images] from Meteorites Australia - Meteorites.com.au

{{Meteorites}}

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