A rocky body of extra-terrestrial origin that has survived the passage through the atmosphere (see shooting star) and therefore falls to earth . It is thought that a huge meteorite (several kilometres in diameter) was responsible for the disappearance of the dinosaurs 65 million years ago. The mass of meteorites falling to Earth per year is estimated at 10 000 tonnes.
There is a wide variety of meteorites and to sort these out, they have been classified into three main categories.
There are rocky ones, metallic ones and intermediate ones that are called mixed.
According to the interpretation accepted today, these differences in mineralogical composition reflect different conditions in the formation and evolution of bodies in the solar system.
Credits: Stephane Erard and Aurélie Le Bras (obspm.fr).
Rocky meteorites, by far the commonest, are themselves divided into two main groups: chondrites and achondrites.
- Chondrites are primitive meteorites that have remained virtually unchanged since the formation of the solar system 4.566 billion years ago. They owe their name to the presence of chondrules, small spherules from 0.1 to 10 mm in diameter that are not found in terrestrial rocks, and are mainly composed of silicate minerals such as olivine and pyroxene. These are contained in a finely crystallised matrix containing a little iron, sometimes whitish refractory inclusions rich in calcium and aluminium, and in the case of carbonate chondrites, a high proportion of water and carbon.
- Achondrites contain no chondrules. They come from the crust or mantle of a large asteroid and have probably been crystallised from magma. From their texture and their mineralogical and chemical composition, they resemble some terrestrial basalts. Among these, SNC meteorites (Shergottites, Nakhlites, Chassigny) are special and could come from the planet Mars.
In the case of an asteroid at least a few hundreds of kilometres in size, the quantity of radioactive elements initially present would have been enough to heat it strongly, causing partial fusion with the formation of a ferrous core at its centre with a mantle around giving rise to magmatic and volcanic processes.
During violent collisions, some asteroids were pulverised in the shock and this may well be the origin of metallic and mixed meteorites, depending on whether they are composed of an iron-nickel alloy, like the siderites, or a mixture of the latter with other minerals, like the pallasites.
In the latter case, olivine crystals are observed trapped in the metal. It is thought that this may be a piece of the layer at the interface between the core and the mantle of these large asteroids. The same types of rock probably occur at this same interface on earth.