Carbonaceous chondrites, also called C chondrites, are a type of meteorite. They include at least 8 known groups and many ungrouped meteorites. They contain some of the oldest meteorites found. C chondrites make up about 4.6% of all meteorites that fall to Earth.

Some famous carbonaceous chondrites are: Allende, Murchison, Orgueil, Ivuna, Murray, Tagish Lake, Sutter's Mill, and Winchcombe.

General description

C chondrites have a relatively high amount of carbon (up to 3%), which exists as graphite, carbonates, and organic compounds, such as amino acids. They also contain water and minerals that have been changed by the presence of water.

Carbonaceous chondrites were not exposed to high temperatures, so they remained largely unchanged by heat. Some carbonaceous chondrites, like the Allende meteorite, include calcium-aluminum-rich inclusions (CAIs). These are materials that formed early in the solar system’s history, condensed from the solar nebula, and are the oldest minerals found in the Solar System.

Some carbonaceous chondrites, such as the CM chondrite Murchison, include presolar minerals, like moissanite (a natural form of silicon carbide) and very small diamonds. These minerals likely formed outside the Solar System, possibly during the explosion of a nearby supernova or near a pulsating red giant (called an AGB star) before joining the material cloud that formed the Solar System. Explosions from such stars can create pressure waves that help form new clouds, stars, and planetary systems.

Another carbonaceous chondrite, the Flensburg meteorite (2019), shows evidence of the earliest known liquid water in the young Solar System.

Composition and classification

Carbonaceous chondrites are grouped based on unique compositions that reflect the type of parent body from which they came. These C chondrite groups are named using a standard two-letter CX designation, where C stands for "carbonaceous" (other chondrite types do not begin with this letter) and X is often the first letter of the name of a well-known meteorite—usually the first one discovered in the group. These meteorites are often named after the location where they fell, which does not provide information about the group's physical characteristics. The CH group, where H stands for "high metal," is the only exception. Details about each group's name are provided below.

Several carbonaceous chondrite groups, such as the CM and CI groups, contain high amounts of water (3% to 22%) and organic compounds. These meteorites are mainly made of silicates, oxides, and sulphides, with minerals like olivine and serpentine being common. The presence of water and volatile chemicals suggests these meteorites have not been heated above 200°C since their formation. Their compositions are considered similar to the solar nebula, the cloud of gas and dust from which the Solar System formed. Other C chondrite groups, such as CO, CV, and CK chondrites, have fewer volatile compounds and some may have experienced significant heating on their parent asteroids.

This group is named after the Ivuna meteorite (Tanzania) and has a chemical composition close to that of the solar photosphere, except for gaseous elements and lithium, which are less common in the Sun's photosphere compared to CI chondrites. In this way, they are the most chemically primitive known meteorites.

CI chondrites usually contain a high amount of water (up to 22%) and organic matter, including amino acids and PAHs. Water-related changes create a composition of hydrous phyllosilicates, magnetite, and olivine crystals in a black matrix, with few or no chondrules. These meteorites are thought to have remained below 50°C (122°F), suggesting they formed in the cooler outer regions of the solar nebula.

Five CI chondrites have been observed falling: Ivuna, Orgueil, Alais, Tonk, and Revelstoke. Four others were found by Japanese teams in Antarctica. CI chondrites are extremely fragile, making them vulnerable to Earth's weathering and causing them to break down quickly after falling.

This group is named after Vigarano (Italy). Most of these chondrites belong to petrologic type 3.

CV chondrites observed falls:
• Allende
• Bali
• Bukhara
• Grosnaja
• Kaba
• Mokoia
• Vigarano

This group is named after Mighei (Ukraine), but the most famous member is the Murchison meteorite, which has been extensively studied. CM chondrites contain a rich mix of complex organic compounds, such as amino acids and purine/pyrimidine nucleobases.

CM chondrite famous falls:
• Murchison
• Sutter's Mill
• Aguas Zarcas
• Jbilet Winselwan
• Winchcombe

This group is named after Renazzo (Italy). The best parent body candidate is 2 Pallas.

CR chondrites observed falls:
• Al Rais
• Kaidun
• Renazzo

Other famous CR chondrites:
• Dar al Gani 574
• El Djouf 001
• Northwest Africa 801

"H" stands for "high metal" because CH chondrites may contain up to 40% metal. This makes them one of the most metal-rich chondrite groups, second only to CB chondrites and some ungrouped chondrites like 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 to petrologic types 2 or 3.

This group is named after Bencubbin (Australia). These chondrites contain over 50% nickel-iron metal but are not classified as mesosiderites because their mineralogical and chemical properties are similar to CR chondrites.

This group is named after Karoonda (Australia). These chondrites are closely related to the CO and CV groups.

This group is named after Ornans (France). The chondrule size averages about 0.15 mm. All specimens are of petrologic type 3.

Famous CO chondrite falls:
• Ornans
• Kainsaz
• Warrenton
• Moss
• Dar al Gani 749

CL chondrites were officially recognized in 2022 after at least five specimens were described. Named after the type specimen Loongana, these chondrites are rich in chondrites, metal, and low in volatile compounds.

Most famous members:
• Tagish Lake
• Tarda

Organic matter

Most of the organic carbon in CI and CM carbonaceous chondrites is a hard-to-dissolve, complex material. This is similar to a substance called kerogen, which is found in Earth’s rocks. A material like kerogen is also present in the ALH84001 Martian meteorite, which is a type of achondrite.

The CM meteorite Murchison contains more than 96 types of amino acids and other compounds, including carboxylic acids, hydroxy carboxylic acids, sulphonic and phosphonic acids, aliphatic and aromatic hydrocarbons, fullerenes, heterocycles, carbonyl compounds, alcohols, amines, and amides.

Amino acids in carbonaceous chondrites are important for understanding how organic compounds may have reached Earth and contributed to the development of life. Shortly after falling in Australia in 1969, the Murchison meteorite was found to contain five protein amino acids (glycine, alanine, valine, proline, and glutamic acid) and 12 non-protein amino acids, such as α-aminoisobutyric acid and isovaline, which are rare on Earth. Since then, scientists have identified 96 amino acids in Murchison, including 12 of the 20 common biological amino acids. Many other amino acids have been detected but are not yet fully characterized. While some amino acids in Earth’s soil could cause contamination, most of the amino acids found in Murchison are rare or absent on Earth.

Amino acids can have mirror-image structures, called enantiomers. These are often labeled as left-handed (L) or right-handed (D), similar to how glyceraldehyde is described. Living organisms use L-amino acids, but there is no clear reason why one form is preferred over the other in biological systems. In contrast, laboratory experiments, such as the Miller-Urey Experiment, have shown that amino acids can form under non-living conditions with equal amounts of L- and D-enantiomers. This means that the ratio of L- to D-enantiomers in a sample can help determine whether the amino acids formed from living or non-living processes. Early studies of Murchison found that all chiral amino acids were present in equal amounts (racemic mixtures), suggesting an abiotic origin. This matches with the Strecker synthesis, a process that forms racemic mixtures of amino acids in CM chondrites.

Ehrenfreund et al. (2001) found that amino acids in CI chondrites like Ivuna and Orgueil were present in much lower amounts compared to CM chondrites (~30%). These amino acids had different compositions, with high levels of β-alanine, glycine, γ-ABA, and β-ABA, but low levels of α-aminoisobutyric acid (AIB) and isovaline. This suggests they formed through a different process and came from a different parent body than CM chondrites.

More recently, scientists have identified significant L-enantiomeric excesses (more L-amino acids than D) in several carbonaceous chondrites. For example, Murchison and Murray meteorites show L-excesses of 3–15% in some non-protein amino acids. These amino acids are not found in biological systems, and their carbon and deuterium isotope levels are much higher than those on Earth, indicating an extraterrestrial origin. Studies also found L-isovaline excesses up to 20.5% in various carbonaceous chondrites, supporting the idea that increased hydrothermal activity in the meteorite’s parent body may lead to greater L-excesses. Large L-excesses in α-H amino acids have also been reported, but these are harder to interpret due to possible contamination from Earth. The ungrouped C2 chondrite Tagish Lake has L-aspartic acid excesses up to ~60%, with carbon isotope measurements confirming an extraterrestrial origin. In Tagish Lake, some protein amino acids like glutamic acid, serine, and threonine show large L-excesses, while others, like alanine, are racemic.

Scientists have suggested that the L-excesses in extraterrestrial amino acids may result from differences in how the enantiomers crystallize. Experiments using circularly polarized ultraviolet light, similar to conditions on asteroids, have shown that this light can create L-excesses in amino acids. This is thought to be the main way chiral symmetry (favoring one enantiomer) occurs in space. Notably, only L-enantiomers are observed in extraterrestrial amino acids, suggesting that the process responsible for these excesses may explain why life on Earth uses L-amino acids.

NASA has proposed a "Ladder of Life Detection" threshold of more than 20% enantiomeric excess in amino acids to identify potential biosignatures. However, recent studies have shown that abiotic processes can also produce even larger enantiomeric excesses. To determine if chiral asymmetry in an extraterrestrial sample is of biological origin, scientists like Glavin et al. (2020) suggest three criteria: chiral asymmetry, light carbon isotope composition, and a simplified distribution of structural isomers. If an extraterrestrial sample shows chiral asymmetry, has a preference for certain structural forms, and has lower carbon, nitrogen, and deuterium levels compared to surrounding inorganic material, it could strongly suggest a biological origin. With upcoming sample return missions from carbonaceous asteroids (e.g., OSIRIS-REx) and Mars, analyzing uncontaminated samples will provide the best chance to find biosignatures in the Solar System.