Carbonaceous chondrites

Carbonaceous chondrites are rare, often dark and crumbly, and scientifically priceless. They carry water, organic molecules and the oldest solids in the Solar System.

What makes them special

Carbonaceous chondrites make up only about 4 per cent of observed falls. The name comes from the carbon some of them contain, often a few per cent by weight, much of it as complex organic matter. Not all carbonaceous chondrites are rich in carbon, though; the name is partly historical.

What unites them is a combination of chemistry and oxygen isotopes. Compared with ordinary chondrites, they tend to contain more of the elements that condense at high temperature, such as calcium and aluminium, relative to silicon, and their oxygen isotopes plot in a distinctive region.

Many carbonaceous chondrites contain water bound in clay minerals, and several groups contain abundant CAIs. Some of them are among the most primitive rocks known, preserving dust that predates the planets.

The groups

Carbonaceous chondrites are divided into groups, each named with C followed by a letter from a typical member.

CI chondrites, named after Ivuna in Tanzania, are type 1, heavily altered by water, and have no visible chondrules. Their chemistry matches the Sun's outer layers most closely. Orgueil, which fell in France in 1864, is the best-known CI.

CM chondrites, after Mighei in Ukraine, are mostly type 2 and rich in organics. Murchison and Winchcombe are CMs.

CV chondrites, after Vigarano in Italy, have large chondrules and big CAIs. Allende is a CV3.

CO chondrites, after Ornans in France, have tiny chondrules.

CR chondrites, after Renazzo in Italy, contain abundant metal and preserved organic matter. CK chondrites, after Karoonda in Australia, are oxidised and often heated. CH and CB chondrites are unusually rich in metal.

Organic molecules and water

The Murchison meteorite, which fell in Victoria, Australia, in 1969, became famous because scientists found dozens of amino acids in it, the building blocks of proteins. Crucially, many of them are rare or absent in Earth's living things, and they contain isotopic signatures showing they formed in space, not from contamination.

Since then, studies of carbonaceous chondrites have revealed thousands of different organic compounds, including sugars and the nucleobases used in DNA and RNA. None of this means life existed in these asteroids. It shows that complex chemistry happened naturally in the early Solar System, and that such material could have been delivered to the young Earth.

The hydrogen isotopes of water in many carbonaceous chondrites resemble those of Earth's oceans, which is one reason they are considered a possible source of some of Earth's water.

Handling a fragile treasure

Many carbonaceous chondrites, especially CI and CM types, are fragile and react with Earth's air. Their clay minerals can absorb moisture, and sulphates can grow on surfaces within weeks or months. Organic compounds can be contaminated by fingerprints, plastics and dust.

That is why the recovery of a fresh fall like Winchcombe is handled so carefully. Pieces are collected with clean tools, wrapped in aluminium foil or sealed bags, and moved quickly to a dry, clean store. Some samples are kept under dry nitrogen.

The same lessons apply to asteroid samples. The material returned from the asteroids Ryugu and Bennu was handled in clean, nitrogen-filled cabinets. It resembles CI chondrites but is even more pristine than CI meteorites in museums, many of which have altered since their fall.

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