What is a carbonaceous chondrite?
A carbonaceous chondrite is a primitive stony meteorite rich in carbon compounds and often in water-bearing minerals. These meteorites are some of the least altered samples of the material that built the planets.
Carbonaceous chondrites are usually dark grey to black, sometimes crumbly, and look rather unremarkable. Inside, though, they hold some of the richest scientific treasures any rock can offer. They contain carbon, up to a few per cent by weight, much of it as complex organic molecules, and many contain clay-like minerals that formed when liquid water flowed through their parent asteroids.
Scientists split them into groups labelled with C plus a second letter, often named after a typical member. CI chondrites, such as Orgueil, which fell in France in 1864, have a composition closer to the Sun's than any other rock known, apart from the lightest gases. CM chondrites include Murchison, which fell in Australia in 1969, and Winchcombe, which fell in England in 2021. CV chondrites, such as Allende, which fell in Mexico in 1969, are famous for their large chondrules and pale calcium-aluminium-rich inclusions, the oldest solids made in the Solar System. Tagish Lake, which fell onto a frozen lake in Canada in 2000, is an unusual member of the wider family.
These meteorites matter because they preserve material from the very beginning. They contain amino acids and other organic compounds, water locked into minerals and grains of stardust older than the Sun. Many researchers think asteroids of this kind helped supply early Earth with water and carbon.
Carbonaceous chondrites make up only a few per cent of observed falls, partly because they are fragile and often break apart in the atmosphere. They also weather quickly on the ground. Fresh falls, collected rapidly and kept clean, are especially valuable to scientists, which is why recoveries like Winchcombe attract so much attention. The samples returned from asteroids Ryugu and Bennu turned out to be related material.