Meteorites are rocks that have fallen to Earth from space. Some have lain on Earth for many thousands of years; others arrive all the time. Those observed to traverse Earth's atmosphere, and recovered based on those observations, are called meteorite falls. Those with no record of arrival are meteorite finds when recognized. Meteorites are named for where they fall or are found. Over 25 000 meteorites are known worldwide: 18 000 are from Antarctica, and a few thousand from deserts in Africa and Asia. The best general reference on meteorites is Rocks from Space by O. Richard Norton, 2nd Edition, Mountain Press, 1998.

Often there is confusion about when and where meteorites may fall or be preserved, and about what they are and look like. All are significantly different than Earth rocks. Samples sent to experts for identification, even by other scientists, are usually "meteorwrongs" - terrestrial rocks or minerals, man-made slag, metals, alloys or concrete- that rarely resemble meteorites. Less than 60 identified meteorites are known in Canada. There have been 3 falls since 1994: Tagish Lake, B.C., January 18, 2000; Kitchener, Ontario, July 12, 1998; and St-Robert, Québec, June 14, 1994.

Meteorites probably begin as streams of fragments, meteoroids, that are samples of collisions between larger objects. The calculated orbits of several falls include the Asteroid Belt. Their ultimate origin in space and time may be elsewhere. About 20 meteorites come from the Moon. The petrological and isotopic evidence is compelling that a similar number are from Mars, blasted off the surface by impacts.

Links have been suggested between other types or groups of meteorites, and specific asteroids or asteroid types, or comets, from their reflectance spectra or density, but the provenance of most meteorites is uncertain. Yet they are an unparalleled source of information about our Solar system, from its primitive beginnings to the present. Some even contain extrasolar mineral grains and primordial compounds made in other stars, yielding information about the origin of the Universe. In the absence of extensive space exploration, and to prepare for it, they are scientifically invaluable.

Popular ideas about when and where meteorites fall are connected to the observation of meteors, the brief streaks of light produced when high-speed, interplanetary particles enter Earth's upper atmosphere. Sporadic meteors and meteor showers do not result in meteorites; their fragile cometary debris fragments are reduced to dust high in the atmosphere. Earth collects over 100 tonnes of cosmic dust debris per day. In contrast, stronger, larger, space rocks can survive a fiery passage through the atmosphere and result in meteorites. These first appear as bright fireballs, are slowed to terminal speeds by atmospheric friction, and cease to show a bright trail long before they reach Earth's surface. Fireballs, which may seem very close, are usually quite high in the atmosphere, 50 km or further away from the observer. Even if an extraterrestrial object does reach the surface, it is likely to plunge into the 70% that is water, or be lost among forests, jungles, or mountainous regions. In only uncommonly recorded cases have meteorites landed within a few metres or struck humans (see "Possible Hazards of Meteorite Falls" by C. E. Spratt, JRASC, 85, p. 263, October 1991). Rare meteoroids of masses exceeding 100 tonnes are not slowed appreciably by Earth's atmosphere and produce impact craters. The larger impacts may have dramatically altered the history of life on our planet, but they do not result in meteorites-the kinetic energy is sufficiently high to vaporize the impacting body completely and to deform and melt the target area in a fraction of a second. Crater diameters are typically ten times the diameter of the impacting body. Glassy tektites found scattered over several continents may record such impacts, but they do not have meteorite compositions or characteristics. Information about meteorites, tektites, and impacts is now widely available on the internet.

Meteorites are divided into three groups varying widely both in appearance and properties: stones or stony meteorites (aerolites); stony-irons (siderolites); and irons or iron meteorites (siderites). All usually contain metallic nickel-iron compounds (with traces of other metals) and are mildly to strongly magnetic. Those that have lain on Earth's surface for long periods may be rusted almost beyond recognition. Some require laboratory tests to confirm their identity. Specimens generally have a quite soft, dull black to brown, fusion crust. More prevalent on stones and stony-irons, it may have partially flaked off. Meteorites never contain bubble-like cavities, nor are they ever almost perfectly spherical and smooth. During atmospheric entry, only their surface is affected. Surfaces of irons and stony-irons are dimpled rather than being bulbous. They rust easily so that there may be no bright metal showing. Stony meteorites do not show protuberances; weathered varieties are rusty even on broken surfaces. Fresh stony meteorites may have a whitish rock interior, with bright metal specks. Their crusts are black or smoky grey, varying from quite glassy to dull; telltale lines and bubbled patches orient their flight through the air. More metallic samples may also be oriented.

Stones are the most abundant, resembling some terrestrial rocks, but are denser. Most (called chondrites) consist of spheres of silicate minerals (called chondrules) visible on broken, cut or polished surfaces, and scattered grains of metal. Chondrites are the oldest, most primitive, least altered, meteorites. Rare stony meteorites without chondrules are called achondrites; these are thought to be melt products from chondrites, samples of younger volcanic planetary surfaces. In August 1996, NASA scientists announced discovery of fossil and chemical evidence of bacterial life in an ancient achondrite from Mars, but other scientists disagree. This controversy has resulted in renewed interest in space missions to Mars and in finding water and life on Mars and elsewhere, even in extreme conditions on Earth. Irons and stony-irons are dense, with up to equal amounts of silicates and iron, and are often irregular in shape. They are thought to be core/mantle material of planets formed by melting chondrites.

Rare carbonaceous chondrites are grey to black, some resembling charcoal with silicate inclusions. The dark colour is from iron oxides and sulphides. They contain a few percent carbon in very fine-grained carbonates, carbides, graphite, diamonds, and primitive organic molecules, detectable mainly by isotopic analyses. They may have had biologically significant roles in providing seeds for the origin of life. They probably come from comet nucleii or similar ice-rich asteroids. Their organic molecules originate in interstellar space as coatings on dust. The study of interplanetary or interstellar dust particles (IDP's) has become important to decipher the origins of meteorites, and therefore of our Solar system and everything in it. We are all composed of recycled stardust.

The Geological Survey of Canada (GSC) maintains the National Meteorite Collection of about 2000 specimens, identifies meteorites and supports research on them. It also offers to pay the owner a minimum of $500 for the first specimen of any new Canadian meteorite. Should you find a suspected meteorite, you may forward it to:

The specimen will be examined and reported on free of charge. If it is too large for mailing, a description of its appearance, or a photograph, and its exact location should be sent. The GSC also makes available a free brochure on meteorites, with pictures of meteorites and comparative non-meteorites. Write to the GSC's Publication Office at the above address.

For more information, please contact Richard Herd