Why do CI chondrites match the composition of the Sun?
CI chondrites formed from material that never melted or lost most of its elements, so apart from gases, their chemistry closely matches the Sun's atmosphere.
The Sun contains more than 99.8 per cent of the Solar System's mass, and its composition reflects the original cloud of gas and dust. Astronomers can measure the Sun's atmospheric composition by studying its spectrum.
When scientists compare these measurements with meteorites, they find a remarkable match for one group: the CI chondrites, such as Orgueil, which fell in France in 1864, and Ivuna, which fell in Tanzania in 1938. For most elements apart from the very volatile ones, such as hydrogen, helium, carbon, nitrogen, oxygen and the noble gases, their abundances relative to silicon agree with the Sun's within about 10 per cent.
Why? CI chondrites formed from fine dust that gathered in a cold region of the disc and was never melted. Although water altered their minerals on the parent asteroid, it did not move elements far, so the bulk chemistry stayed close to the original mix. Other meteorites lost or gained some elements through heating, melting or the sorting of different components.
Because of this, CI chondrites are used as a reference for the composition of the Solar System. Chemists plot other rocks' compositions as ratios to CI values, revealing how far they have been changed. Samples returned from the asteroids Ryugu and Bennu closely resemble CI chondrites and have confirmed and refined these values. CI chondrites are extremely fragile and rare, so every gram is precious for research.