CM chondrites
Water-rich carbonaceous chondrites full of organic molecules, Murchison and Winchcombe among them.
- Classification
- Stony meteorites › Chondrites › Carbonaceous chondrites › CM chondrites
- Share of witnessed falls
- about 1.5%
- In our catalogue
- 461 meteorites, 15 seen to fall
- Heaviest
- Murchison, 100 kg
- Earliest recorded fall
- Cold Bokkeveld, 1838
- Most recent fall
- Maribo, 2009
- Type specimen
- Mighei, fell in Ukraine in 1889
CM chondrites, named after Mighei in Ukraine, are the most commonly seen carbonaceous chondrites to fall. They are mostly petrologic type 2: water altered them heavily, turning much of their material into serpentine and other clays, but small chondrules and CAIs survive.
They are rich in organic compounds. Murchison, which fell in Australia in 1969, contains more than 70 amino acids and thousands of other organic molecules, and dust grains older than the Sun. Winchcombe, which fell in England in 2021, was collected within days and is one of the least contaminated CMs ever studied.
Mineralogy and chemistry
CM chondrites contain small chondrules (around 0.3 mm), CAIs and fragments of olivine set in a large amount of fine matrix, roughly 60% to 70% of the rock. Water has altered much of this matrix into serpentine clays and distinctive intergrowths of tochilinite and cronstedtite, minerals that hold iron, sulphur and water. Carbonates, magnetite and sulphides are common. They contain about 2% carbon and up to about 10–13% water by weight.
Alteration varies from stone to stone, so CMs are graded on a scale from CM2.9 (least altered) to CM2.0 or CM1 (completely altered). Many are breccias of differently altered fragments. Their oxygen isotopes lie well below the terrestrial line, with altered stones shifted towards heavier values as the water exchanged with the rock.
Organic chemistry
CMs are the richest meteorites for soluble organic molecules. Murchison contains more than 90 different amino acids, many of them not used by life on Earth, plus nucleobases, sugar-related compounds and tens of thousands of other carbon molecules. Some amino acids show a slight excess of the left-handed form, the kind used by life, which has prompted debate about how life came to prefer it. Murchison also holds presolar grains of silicon carbide that are older than the Sun.
Parent body and formation
The CM parent body accreted rock and ice a few million years after the first solids. Gentle heating melted the ice, and the water reacted with the rock for a few million years at temperatures of roughly 0 to 120 °C. Some CMs were later heated again, perhaps by impacts, driving out some water. Dark C-type asteroids are likely sources. The CO chondrites share CM-like oxygen isotopes and refractory inclusions, and the two may be dry and wet versions of related material.
Abundance and famous examples
CMs are the most common carbonaceous chondrites among falls, about 1.5% of all witnessed falls. Murchison fell in Victoria, Australia, on 28 September 1969, and more than 100 kg was collected. Winchcombe fell in Gloucestershire on 28 February 2021; its fireball was filmed by camera networks, so its orbit is known, and the first pieces were gathered within about 12 hours, making it one of the most pristine CMs studied. Sutter's Mill fell in California in 2012 and Aguas Zarcas in Costa Rica in 2019. Cold Bokkeveld (South Africa, 1838) and Murray (Kentucky, 1950) are classic falls.
How to recognise one
A CM is dark grey to black inside, with tiny pale chondrules and white specks of CAIs in a dark matrix, and only a thin dull crust. It is light, fragile and weakly magnetic. Freshly broken stones have been described as smelling of solvent or tar. Keep any suspected fall dry and untouched by bare hands.
Famous examples
- Murchison CM2, Australia, 1969 · 100 kg
- Winchcombe Carbonaceous Chondrite, United Kingdom, 2021 · 602 g
- Mighei CM2, Ukraine, 1889 · 8 kg
- Cold Bokkeveld CM2, South Africa, 1838 · 5.2 kg
- Murray CM2, United States, 1950 · 12.6 kg
- Sutter's Mill C, United States, 2012 · 993 g
- Aguas Zarcas Unclassified, Costa Rica, 2019 · 27 kg
Related groups
- CO chondrites Small-chondrule carbonaceous chondrites, close relatives of the CMs.
- CI chondrites The chemically most primitive rocks known: their make-up matches the Sun's.
- Carbonaceous chondrites Rare, often dark chondrites rich in carbon, water-bearing minerals and organic compounds, including the building blocks of life.
Photos shared under Creative Commons licences (Wikimedia Commons).