Star potential

20th December 1996 at 00:00
They look stunning, but there's more to meteorites than just the wow factor. They can illuminate big chunks of the science syllabus, write John Bridges and Monica Grady.

Look up at the night sky (lying down is a good way to do this!), and sooner or later on a clear night you will see a shooting star. These meteors, produced by tiny particles of dust burning up as they travel through the atmosphere, serve as a reminder that our planet is constantly being bombarded by extraterrestrial material.

Although most people probably associate meteorites with dinosaurs and cavemen, there is a lot more to this subject which teachers can use to illustrate aspects of the national curriculum, in particular part of the science syllabus at key stage 4. A teaching pack prepared at the Natural History Museum and administered by the Particle Physics and Astronomy Research Council is available for loan - at no cost - and contains teachers' notes and lecture slides. Packages containing meteorite samples are also available.

Almost all meteorites are fragments from the asteroid belt between Mars and Jupiter. More than 300 asteroids have perturbed orbits that bring them near to the Earth. Another source of meteoritic material are comets such as Halley's. Their nuclei contain stony material, probably similar in composition to many asteroids, but differ from asteroids in containing abundant carbon-bearing material and ices such as water and methane. The tail of Halley's Comet extends for millions of kilometres and is created from gas and dust, released in jets as evaporation of the ices takes place. Remarkable photographs of Halley's nucleus taken by the Giotto space probe in 1986 show this taking place. Lecture slides in the teachers' package help illustrate the appearance of comets and asteroids and also their relative positions in the solar system.

Studying meteorites can tell us about the age and formation of the solar system because they have not undergone the geological proces-ses that have changed all the other rocks on Earth. The composition of the most abundant stony meteorites (called chondrites) is similar to the composition of the Sun and solar system for non-gaseous elements.

By contrast, iron meteorites offer us a glimpse into the cores of planets. These meteorites were formed by the sinking of dense metal through the silicate mantles of small planetary bodies early in the evolution of the solar system. The age of chondrites - and thus the solar system - is calculated by measuring the abundances of certain radioactive elements and their daughter isotopes. By knowing the rate at which the radioactive element decays, an age can be calculated, and this is how we know that the solar system is 4.6 billion years old. Small amounts of material in chondrites may even pre-date the solar system. Tiny diamonds, isolated from chondrites by dissolving 99 per cent of the rock in acid, contain nitrogen with an isotopic composition different from nitrogen in the solar system. This has led scientists to believe that the diamonds are older than our solar system, relics generated in the stellar winds from red giant stars.

Other stony meteorites (achondrites) are more like terrestrial igneous rocks, consisting of silicate minerals that have crystallised from magma. Some of the achondrites are believed to have been derived from Mars and the Moon rather than asteroids. So how do we know some meteorites come from Mars? Martian meteorites are younger than chondrites, which indicates that they are fragments of a large, geologically active planet. The composition of gases trapped in these meteorites is close to those of the Martian surface analysed by the Viking mission in 1975. Lunar meteorites are similar to some of the pale rocks called anorthosites which were brought back from the Apollo landings.

Over the past 25 years, thousands of meteorites have been found in desert areas where the land is flat, has little vegetation and has not undergone erosion by rivers. If the land surface of such an area remains unchanged for thousands of years then gradually the number of meteorites builds up and they become relatively easy to find because of their dark outer surface. Thousands of meteorites have been found in Antarctica and the Nullabor region of Western Australia. Meteorite samples located in this way (called "finds") now greatly outnumber those meteorites that were seen to fall (called "falls"). Meteorite samples are named after the place where they fall or are found. There is still, however, only one "find" in the British Isles (compared with 23 authenticated "falls") and that was from within an Iron Age archaeological site at Danebury, Hampshire. The last observed fall in Britain was at Glatton in Cambridgeshire in 1991.

The teaching package is most relevant to topics in "The Earth and beyond" section of key stage 4 of the science syllabus. Contact the Particle Physics and Astronomy Research Council at Polaris House, North Star Avenue, Swindon. Tel: 01793 442000 u Dr John Bridges and Dr Monica Grady work on the meteorites and micrometeorites research programme in the Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD u Next week: 16-page TES Science Extra What are meteorites?

Meteorites are fragments of ancient material, natural objects that survive their fall to Earth from space, and are recovered. They are not radioactive and are almost always cold when they land, despite the fireball that can accompany their entry into the Earth's atmosphere. Meteorites are different from meteors, or "shooting stars", which are pieces of dust that burn up in the atmosphere. A meteoroid is a small body travelling through space that may or may not land on earth as a meteorite.

Meteorites were formed at the birth of the solar system - 4.6 billion years ago. Although the Earth was also formed at this time, along with the other planets, none of the original material remains: it has been removed by bombardment and recycled through geological activity. It is only by studying meteorites that we can learn about the processes and materials that shaped the solar system.

How Big Is A Meteorite And How Often Do They Fall?

Meteorite falls are not rare events. They are a lot more common than people realise. They come in all sizes from the very tiny, about one thousandth of a millimetre across, to the very large, bigger than a house. The smallest ones fall all the time. Approximately 40,000 tonnes of extra-terrestrial dust fall on the Earth each year; around four particles per hour per square kilometre of the Earth's surface.

Very big meteorites only fall every thousand years or so. The last really big meteorite fell at Chicxulub in the Gulf of Mexico, 65 million years ago. The environmental changes caused by this impact might have led to the extinction of the dinosaurs.

Several thousand intermediate-sized meteorites, about the size of a bag of sugar fall on the Earth every year, but only half a dozen are seen to fall and brought into museums.

The last meteorite to fall in England landed in 1991 in Glatton near Peterborough. It was a stone meteorite weighing just over 0.5 kilogrammes and fell in the garden of 82-year-old Arthur Pettifor.

Where do Meteorites Come From?

Most meteorites come from the asteroid belt, which lies between the giant Jupiter and Mars (see illustration above).

There are several thousand asteroids - rocky, metallic or carbon-rich bodies which are material remaining after the planets formed. Jupiter's gravitational pull prevented the bodies from joining together to form a single planet.

Asteroids occasionally collide and break up; the fragments then fall to Earth as meteorites. They come in three types - stone, iron or stony-iron. There are also around 20 or so meteorites that have come from the Moon.

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