Text: Jan Olov Nyström
Meteorites are extraterrestrial
material that fall to Earth. The large majority of them are stones (94 %), the remainder
are irons composed of iron-nickel metal and a small number are mixtures of stone and iron
(the stony-iron meteorites). The meteorites originate from asteroids which through
collision with each other were fragmented and thrown into Earth-crossing orbits. Comets
may also be a source for meteorites, and some very rare types of meteorites are derived
from the planet Mars or from the Moon.
Meteorites have characteristic properties which make them easy to identify by a specialist. All meteorites unaffected by weathering have a thin and well-defined black fusion crust formed by frictional heating in the atmosphere. Shooting stars or meteors are very small meteorites consumed by melting and vaporization in the atmosphere.
Iron meteorites have high densities and are strongly magnetic. Etching of a polished surface with a mixture of nitric acid in alcohol reveals a structure that is shown only by meteoritic iron and which is difficult to produce artificially.
|Iron meteorite from Chinautla, Guatemala, found 1902. Polished and etched
surface, with the characteristic structure.
Photo: Dan Holtstam
meteorites: chondrites and achondrites
Stony meteorites contain smaller amounts of metallic iron and are less magnetic than the iron meteorites. They are mainly composed of olivine and a pyroxene mineral virtually absent in terrestrial rocks. The minerals form millimeter-sized, nearly spherical particles of different internal structure called chondrules. Most stony meteorites contain chondrules and therefore are called chondrites.
Chondrites have not been melted since their formation and are composed of almost the same proportion of elements as the Earth. They are believed to represent the materials the Earth formed from. The most primitive of all meteorites are the carbonaceous chondrites. Small amounts of organic compounds which formed in space without involvement of life are present in these meteorites.
The achondrites are stony meteorites without chondrules. They are divided into several groups that differ strongly from each other. The Mars meteorites make up three of the groups.
The photo, taken in a polarization microscope,
Photo: Erik Jonsson
Meteorites are uncommon. As an average, five new meteorites from falls are recorded globally each year. The list of Swedish meteorites is short: nine falls and a handful of finds have been recovered during the last 130 years.
The collections of the Department of Mineralogy at the Swedish Museum of Natural History contain 1008 specimens from 307 different meteorites. Their name, type and country of origin are listed in a Meteorite catalogue that can be searched.
The most well-known Swedish stony meteorite, called Hessle after the place where it was seen to fall, exploded 1869 in the atmosphere and gave rise to a meteorite shower; hundreds of individual stones were recovered. The iron Muonionalusta also consists of many pieces. They represent fragments of a large meteorite that fell in northernmost Sweden more than 800.000 years ago. Glacial erosion during the Ice Age appears to have obliterated the impact crater. However, there are many rusty fragments of the iron meteorite in the moraine still waiting to be discovered.
Very large meteorites strike the Earth with cosmic velocity (more than 11 km/sec) and are vaporized in a cataclysm that may have global consequences. A well-known example is an impact in Mexico 65 million years ago that is considered to have caused the extinction of the dinosaurs. A Swedish example of a major impact is the 365 million years old circular Siljan structure. The lakes Siljan, Orsasjön, Skattungen and Oresjön mark the location of the structure.
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