Identification
How do we recognise a meteorite?
fusion crust
Credit: (fluxa) - CC BY 2.0
Fusion crust
Almost all meteorites have a fusion crust. This crust results from the melting of minerals due to the extreme temperature reached by friction, when the rock enters the atmosphere. Thus, 80% of the meteorite is abraded and only the remaining 20% reaches the ground. The molten envelope crystallizes instantaneously and creates this glassy black crust, typical of meteorites. A meteorite that has remained in the desert for thousands of years can, through alteration, lose its fusion crust. Meteorites also have irregular angles, witnessing the loss of material during their atmospheric travel.
Presence of metal
A large majority of meteorites contain metal, making these objects magnetic (except achondrites - 7% of meteorites). Chondrites contain iron-nickel metal grains, in proportions which vary according to groups. Metallic and mixed meteorites contain large amounts of metal because they sampled the core or mantle-core boundary of differentiated asteroids. Thus, the weight of a heavier rock compared to terrestrial rock for a similar size may be suggestive of the presence of metal. This is why meteorites that do not contain metal, i.e., achondrites from the crust of differentiated asteroids or planets, such as lunar or Martian meteorites, are more difficult to find and to identify.
Mbale meteorite - Credit: LOMO56 - CC BY-SA 2.0
Allende/MNHN/A.Stephant
Composants
Chondrites, undifferentiated meteorites, contain characteristic components attesting to their formation at the origins of the Solar System. Thus, the presence of chondrules, round-shaped globules of molten silicates, inserted in a fine-grained matrix, can be observed when cutting the meteorite.
Size
Although there are meteorites of various sizes, ranging from few millimeters to few meters, the majority of meteorites found are small in size. Indeed, if the meteoroid that crosses the atmosphere is large, then there is a good chance that the rock will explode in the air and fall in several pieces, which can be spread over several kilometres.
NWA 10074/ASU/J. Davidson
Figures
How many are there in the world today? To what frequency fall on Earth?
Location
Where are they found? Where are the craters vestiges of their arrival on Earth?
Origin
Where do they come from? How long do they wander around in interplanetary space?
