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Meteorites - Messengers From the Past

From The Natural History Museum of London, Mineralogy Project 4.

Meteorites were formed at the birth of the solar system, about 4,560 million years ago. We have no material on Earth as old as this. Only by studying meteorites can we learn about the processes that shaped our solar system and our planet. Meteorites are pieces of ancient material that survive their fall to Earth from space. A meteorite is named after a place nearest to where it landed. Meteorites are often covered with a thin, black crust - the fusion crust - formed where the surface of the meteorite has melted through frictional heating as it came through the atmosphere. Only the outermost layer of the meteorite melts: the inside remains cold.

What are meteorites made of?

Most meteorites are stony and made of the same minerals as terrestrial rocks - minerals which contain silicon, oxygen, magnesium, iron and calcium. Stony meteorites can be sub-divided into those which formed from melts (igneous rocks, like basalts) on their parent bodies, and those which remained unchanged since formation (or aggregation) of the parents. The latter are known as 'chondrites', after the small rounded droplets of once-molten material (chondrules) which they contain.

Many meteorites are almost totally iron metal, and were formed by melting on the parent bodies on which the meteorites originated. Iron meteorites are the closest physical analogy we have to the material which forms the Earth's core. Iron meteorites are, as their name implies, dominantly made of iron metal, and most contain 5-15 per cent weight of nickel. Their characteristic "Widmanstatten pattern" is formed by the intergrowth of two forms of iron metal, each containing different concentrations of nickel. The slice also shows dark areas, which are patches of iron sulphides and graphite.


Widmanstatten pattern in an iron-nickel meteorite

A further sub-division of meteorites is the stony-irons: a mix, as the name suggests, of stone and metal. These very rare meteorites are particularly beautiful. Their appearance is caused by the intergrowth of silicate minerals with iron metal. On the right is a polished section of the Brenham pallasite, showing the orange-brown olivine grains (an iron, magnesium silicate) set in iron-nickel metal. Pallasites formed by melting and represent material from the core-mantle boundary of the parent body.

Where do meteorites come from?

Most meteorites are pieces broken from asteroids, and come from the asteroid belt between Mars and Jupiter. Over a dozen meteorites have come from the Moon - they have been compared with samples brought back by the Apollo and Luna missions.

There are also twelve meteorites that have come from Mars - their compositions have been compared to the martian soil and atmosphere analysed by the Viking space probes in 1976. On the right is a stone meteorite from a shower which fell over the Egyptian town of El Nakhla al Baharia in 1911. This meteorite crystallised 1,300 million years ago - that is, it formed on a body that was still geologically active at this time. Trapped gases in related meteorites imply that came from Mars.

The most primitive meteorites - the carbonaceous chondrites - are rich in water, sulphur and organic compounds. It is meteorites like these which brought volatile materials to the newly-formed Earth, and helped establish our planet's atmosphere. Without carbonaceous chondrites, there would probably be no life on Earth.

The figure on the right is one of the stones of the Orgueil meteorite, which fell in France in 1864. The specimen is approximately ten centimetres wide. This is one of only five Type One carbonaceous chondrites - the most volatile-rich of all meteorites. It is thought they might be the remains of burnt-out comets.

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