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This review discusses the organic content of primitive meteorites and their relevance …

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Carbonaceous chondrites. Murchison, a keystone
- Organic matter in meteorites

It was a Sunday morning, September 28, 1969, in the town of Murchison, Australia. During the time that most residents were attending church, a bright, dramatic fireball appeared, signalling the arrival of hundreds of meteorites. The specimens were scattered across an 8-km2 area within the town, and people gathered them easily from their yards, recovering more than 500 kg of the meteorite material. The Murchison meteorite belonged to the CM carbonaceous chondrite family. Most carbonaceous chondrites are primitive stony meteorites older than 4.5 Gyr that contain up to 10% of their weight in water and more than 3% of their weight in carbon [13]. They are classified by subtle chemical compositional differences on the basis of their similarity to mainly seven, prototypical specimens (Table 1, [34]).

The CI and CM carbonaceous chondrites are the richest in organic matter and water, and for that reason they are very friable and fragile. They must be collected quickly after their fall, before the weather has a chance to destroy them. This is one of the reasons why Murchison has been a crucial stone; hundreds of meteorites were recovered just after they fell, and there was almost no weathering or contamination (Fig. 2). Another unique circumstance coincided with Murchison’s fall in 1969. At that time, laboratories all around the world were set up for the study of the rocks that the Apollo program had brought from the Moon. Murchison was the right meteorite that fell at the right place and at the right time. Most of the work done on meteorite organic chemistry has been carried out on Murchison. Other carbonaceous chondrites whose organic matter has also been studied are Cold Bokkeveld (South Africa, 1838), Orgueil (France, 1864), Murray (USA, 1950) and Allende (Mexico, 1969).

On the morning of January 18, 2000, an asteroid weighing approximately 200 to 250 tons and five to seven meters across impacted the Earth’s atmosphere. The result was that hundreds of meteorites fell in a remote, cold arctic area between British Columbia and Yukon territories, Canada, in a strewn over a field 16 km long and 3 km wide. Aweek later, the first meteorite fragments were found by a nearby resident. The meteorite turned out to be a rare, primitive CI/CM chondrite very rich in organic compounds; it contained 3.6% carbon by weight [6]. A field effort consisting of 234 people, a unique effort in the history of meteoritics, recovered hundreds of specimens that had remained frozen in ice and never touched by human hands. This carbonaceous chondrite, named Tagish Lake after the frozen lake where it fell, has opened a new door on the search of organic compounds in meteorites. The fact that the fragments had been kept in a continuously frozen state minimized the loss of organic molecules and potential contamination.

Carbonaceous chondrites are very special meteorites. They are considered to be among the most primitive solar system materials on the basis of their chemical composition. With the exception of very volatile elements, including hydrogen, helium, carbon, nitrogen and oxygen, carbonaceous chondrites have an elemental composition that corresponds to that of the Sun. Taking into account that the solar system’s elemental composition is equivalent to the chemistry of the Sun, since the latter contains more than 99% of the mass of the whole system, the close correspondence between the elemental composition of the Sun and carbonaceous chondrites indicates that this type of meteorites has not experienced processes leading to extensive chemical fractionation, like those that have altered all the rocks on Earth, for example. For that reason, carbonaceous chondrites provide precious insight allowing us to look back into the past.

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