What is a CM chondrite?
A CM chondrite is a carbonaceous chondrite altered by water, rich in clay minerals and organic compounds, such as Murchison and Winchcombe.
CM chondrites are one of the most studied groups of carbonaceous chondrites. They are named after the Mighei meteorite, which fell in Ukraine in 1889.
What they look like. CM chondrites are dark grey to black, soft and often crumbly. Inside, small chondrules and white CAIs sit in a dark matrix. Many are breccias, made of fragments altered to different degrees.
What they are made of. Their matrix is dominated by clay minerals, especially serpentine group minerals, formed when liquid water altered the original rock inside their parent asteroid. They also contain carbonates, sulfides such as tochilinite and pentlandite, magnetite, and about 2 per cent carbon, much of it in organic compounds. They hold several per cent water bound in minerals.
Organic chemistry. CMs contain amino acids, nucleobases, sugars and many other organic molecules. Murchison, which fell in Australia in 1969, is the best-known example.
Alteration scale. Scientists rank CMs by how much water changed them, from about CM2.0, heavily altered, to around CM2.9, least altered. The Paris meteorite is among the least altered.
Examples. Murchison, Mighei, Cold Bokkeveld, Murray, Sutter's Mill and Winchcombe, which fell in England in 2021.
Parent bodies. CMs are probably linked to dark C-type asteroids. They resemble but differ from samples from Ryugu and Bennu, which are more like CI chondrites.
CMs are valuable for understanding water and organic chemistry in the early Solar System.