Carbonaceous chondrites, or C chondrites, are a type of chondritic meteorite. They include at least 8 known groups and many ungrouped meteorites. These meteorites are some of the oldest known. C chondrites make up about 4.6% of all meteorites that fall to Earth.
Some well-known examples include Allende, Murchison, Orgueil, Ivuna, Murray, Tagish Lake, Sutter's Mill, and Winchcombe.
General description
C chondrites have a significant amount of carbon, up to 3%, which exists as graphite, carbonates, and organic compounds, including amino acids. They also contain water and minerals that were changed by water.
Carbonaceous chondrites were not heated to high temperatures, so they remained mostly unchanged by heat. Some carbonaceous chondrites, like the Allende meteorite, contain clumps rich in calcium and aluminum. These clumps formed early in the solar system from the cloud of gas and dust that created the planets and the sun.
Some primitive carbonaceous chondrites, such as the CM chondrite Murchison, include minerals that existed before the solar system formed. These include moissanite (a type of natural silicon carbide) and tiny diamonds that are just a few billionths of a meter in size. These minerals likely formed during the explosion of a nearby star or near a type of aging red giant star before joining the cloud of material that became the solar system. When stars explode, they create pressure waves that can cause gas and dust clouds to form, leading to the creation of new stars and planets.
Another carbonaceous chondrite, the Flensburg meteorite (2019), shows the earliest known sign of liquid water in the young solar system.
Composition and classification
Carbonaceous chondrites are divided into groups based on their unique compositions, which are believed to reflect the parent bodies from which they came. These groups are labeled with a two-letter code, "CX," where "C" stands for "carbonaceous" (other chondrite types do not start with this letter), and "X" is usually the first letter of the name of a well-known meteorite in the group, often the first one discovered. These meteorites are frequently named after the location where they fell, which does not provide information about their physical properties. The exception is the "CH" group, where "H" stands for "high metal." More details about group names are provided below.
Some carbonaceous chondrite groups, such as CM and CI, contain large 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 organic chemicals suggests these meteorites were not heated above 200°C since their formation. Their compositions are thought to closely match that of the solar nebula, the cloud of gas and dust from which the Solar System formed. Other groups, such as CO, CV, and CK, have fewer volatile compounds and some may have experienced significant heating on their parent asteroids.
The CI group is named after the Ivuna meteorite in Tanzania. Its chemical composition closely matches measurements of the solar photosphere, except for gaseous elements and elements like lithium, which are less common in the Sun’s atmosphere than in CI chondrites. This makes CI chondrites the most chemically primitive known meteorites.
CI chondrites often contain up to 22% water and organic matter, including amino acids and PAHs. Aqueous alteration has shaped their composition, resulting in hydrous phyllosilicates, magnetite, and olivine crystals in a black matrix. They may lack chondrules and are believed to have remained below 50°C since forming, indicating they condensed 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 in Antarctica by Japanese teams. Due to their fragility, CI chondrites are easily damaged by Earth’s weather and rarely survive long after falling.
The CV group is named after Mighei in Ukraine. The Murchison meteorite is a famous member of this group and contains a wide variety of organic compounds, including amino acids and nucleobase molecules.
The CM group is named after Renazzo in Italy. The best parent body candidate for this group is the asteroid 2 Pallas.
The CR group is named after Vigarano in Italy. Most CR chondrites belong to petrologic type 3.
The CH group is named after the first discovered meteorite, ALH 85085. These chondrites can contain up to 40% metal, making them among the most metal-rich chondrites, second only to CB chondrites and some ungrouped types. They are chemically similar to CR and CB groups and are classified as petrologic types 2 or 3.
The CB group is named after Bencubbin in Australia. These chondrites contain over 50% nickel-iron metal but are not classified as mesosiderites because their properties are closely linked to CR chondrites.
The CO group is named after Karoonda in Australia. These chondrites are related to CO and CV groups.
The CL group is named after Loongana, the type specimen. Recognized in 2022, CL chondrites are rich in chondrites, metal, and low in volatiles.
The CO group is named after Ornans in France. These chondrites have chondrules averaging about 0.15 mm in size and are all classified as petrologic type 3.
Famous CO chondrite falls include:
Organic matter
Most of the organic carbon in CI and CM carbonaceous chondrites is a material that does not dissolve easily and is complex in structure. This is similar to a substance called kerogen. A material like kerogen is also found in the ALH84001 Martian meteorite, which is not a chondrite.
The CM meteorite Murchison contains over 96 types of amino acids and other compounds, including carboxylic acids, hydroxy carboxylic acids, sulphonic acids, 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 early in its history and how life might have developed. 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-proteinogenic amino acids, such as α-aminoisobutyric acid and isovaline, which are rare on Earth. Over time, scientists have identified 96 amino acids in Murchison, including 12 of the 20 common biological amino acids. Many of these amino acids are not commonly found on Earth, reducing the chance that they came from terrestrial sources.
Amino acids can have two mirror-image structures, called enantiomers, which are referred to as left-handed (L) and right-handed (D) based on the structure of glyceraldehyde. Living organisms use L-amino acids, but there is no known 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 D- and L-enantiomers. This means the ratio of enantiomers in a sample can help determine if amino acids formed through biological or non-biological processes. In the first study of Murchison, all chiral amino acids were found in equal amounts, suggesting a non-biological origin. This matches the Strecker synthesis, a process that creates equal mixtures of enantiomers.
Ehrenfreund et al. (2001) found that amino acids in CI chondrites Ivuna and Orgueil were present in much lower amounts than in CM chondrites, and their composition was different. These samples had 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, amino acids in several carbonaceous chondrites have been found to have significant L-enantiomeric excesses. For example, the Murchison and Murray meteorites show L-excesses of 3–15% in some non-protein α-dialkyl amino acids. These amino acids are not found in biological systems and have higher levels of heavy isotopes in carbon and deuterium compared to Earth-based samples, indicating an extraterrestrial origin. Studies of L-isovaline excesses up to 20.5% in various carbonaceous chondrite groups suggest that increased hydrothermal alteration in the meteorite may lead to higher 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 showing an extraterrestrial origin. In Tagish Lake, some proteinogenic amino acids, such as glutamic acid, serine, and threonine, show L-excesses of 50–99%, while alanine is present in equal amounts of both enantiomers.
It has been proposed that the L-excesses observed in extraterrestrial amino acids result from differences in how the enantiomers crystallize. Experiments have shown that circularly polarized ultraviolet light can create L-excesses in amino acids under conditions similar to those on asteroids. This process is thought to be the main source of chiral symmetry breaking in space. The fact that only L-enantiomers are observed in extraterrestrial samples suggests that this non-biological process may explain the preference for L-amino acids in life on Earth.
NASA has suggested a "Ladder of Life Detection" threshold of more than 20% enantiomeric excess in amino acids to identify potential biosignatures from space. However, recent studies have shown that abiotic processes can produce even larger enantiomeric excesses. To determine if a sample contains biological amino acids, Glavin et al. (2020) emphasize three criteria: chiral asymmetry, a light carbon isotope composition, and a simplified distribution of structural isomers. If an extraterrestrial sample shows chiral asymmetry, prefers certain structural isomers, and has lower levels of carbon, nitrogen, and deuterium compared to surrounding inorganic material, it may indicate a biological origin. With upcoming missions to return samples from carbonaceous asteroids (e.g., OSIRIS-REx) and Mars, analyzing uncontaminated samples will be key to identifying possible biosignatures in the Solar System.