By Richard Herd, Geological Survey of Canada,
Reprinted from the 'Observer's Handbook' of the Royal Astronomical Society of Canada
Meteorites are rocks that have fallen to Earth from space. Those that are observed to fall and are recovered shortly thereafter are called meteorite falls. For the majority however, there is no record of their fiery passage through the atmosphere. Those that are found at some later time are called meteorite finds. Both falls and finds are important for scientific research. Identified meteorites are quite rare. Fifty have been recovered in Canada. The worldwide total is a few thousand, apart from large numbers recovered in recent years from Antarctica.
Following a spectacular fireball and sonic boom on the evening of June 14, 1994, pieces of an H5 stony meteorite (chondrite) were recovered and identified near the town of St-Robert-de-Sorel, northeast of Montreal. The Meteorite and Impacts Advisory Committee (MIAC) of the Canadian Space Agency played a central role in investigating this event. The Geological Survey of Canada (GSC) paid $10 000 for the first (2.3 kg) piece of the meteorite, which was subjected to laboratory gamma-ray analysis within 68 hours of the fall, and it is now part of the National Meteorite Collection of Canada in Ottawa. GSC also owns the largest (6.5 kg) fragment and half of a smaller individual. Other pieces from this meteorite shower, the first known in eastern Canada, have been acquired by museums and universities in Quebec and Ontario. Information on the fall is available from the GSC (address below). An article by Terence Dickinson entitled "It Came From Outer Space" appears in the Canadian Geographic, May/June 1995,115,#3, p. 30, and features the St-Robert event and other aspects of meteorite and crater research. The dynamical aspects of the fall are discussed by P. E. Brown et al., "The Fall of the St-Robert Meteorite", in Meteoritics & Planetary Science,31,p. 502, 1996.Often there is confusion about when and where meteorites may fall or be preserved, and about what they are and look like. Samples sent to experts for identification, even by other scientists, are usually "meteorwrongs" — terrestrial rocks or man-made debris such as slag, and do not resemble meteorites.
Often there is confusion about when and where meteorites may fall or be preserved, and about what they are and look like. Samples sent to experts for identification, even by other scientists, are usually "meteorwrongs" -- terrestrial rocks or man-made debris such as slag, and do not resemble meteorites.
Popular ideas about when and where meteorites fall are connected to the observation of meteors (or "shooting stars" or "falling stars" as they are frequently called). The term "meteor" applies to the brief streak of light produced when a high-speed, interplanetary particle enters Earth's upper atmosphere. Although the submission of material for identification sometimes coincides with the publicity surrounding annual meteor showers, these showers do not result in meteorites. Meteor showers are associated with cometary debris which is generally too fragile to survive passage through Earth's atmosphere -- the fragments are reduced to dust high in the atmosphere. In contrast, stronger rocky/metallic fragments the size of a fist and larger can survive passage through the atmosphere and result in meteorites. These first appear as bright fireballs. The vast majority are relatively small, are slowed to terminal speeds by atmospheric friction, and cease to show a bright trail long before they reach Earth's surface. As far as we know, most meteorites are asteroidal fragments, that, like Earth, are part of the Solar System. A few meteorites have apparently come from the Moon, and a few are believed to have come from Mars, having been blasted off of these bodies by impacts.
Rare meteors of masses exceeding 100 tonnes are not slowed appreciably by Earth's atmosphere and produce craters upon impact (100 tonnes is also an estimate of the total mass of meteoritic material entering the atmosphere each day). 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 about 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.
Bright meteors 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 a dozen or so recorded cases have meteorites landed within a few metres of a human being (see "Possible Hazards of Meteorite Falls" by C.E. Spratt, Journal of the RASC,85,p.263, October 1991). Many meteorites have been recovered from open areas such as deserts, farmland, and the Antarctic ice cap. Some have lain on Earth for many thousands of years.
Meteorites are divided into three groups, which vary widely both in appearance and properties:
All usually contain metallic iron compounds (involving nickel and 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.
The identification of any object as a possible meteorite depends largely on: (1) knowledge of the features of previously recovered meteorites whose fall was observed and whose chemistry, mineralogy, textures and age have been studied; (2) recognition of features consistent with the specimen having passed through the atmosphere and having undergone frictional heating and fragmentation; and 3) comparison with samples of Earth rocks, from which meteorites generally differ in several respects.
Despite many resemblances to natural rocks, minerals, slags and metals, meteorites usually possess a number of distinctive characteristics. However, some may not exhibit these characteristics and may require laboratory tests to confirm their identity. Specimens generally have a fusion crust, which is dull black to brown in colour, and quite soft. The crust is more prevalent on stones and stony-irons and may have partially flaked off.
Stones are the most abundant and resemble some terrestrial rocks, but are denser. They contain scattered grains of metallic iron, that are visible on broken or polished surfaces, in a silicate matrix. Most (called chondrites) also commonly contain spheres of silicate minerals called chondrules. These are generally visible to the naked eye in broken or cut specimens. Rare stony meteorites without chondrules are called achondrites.
Irons and Stony-irons are dense (very heavy for their size), with up to equal amounts of silicate minerals, and are often irregular in shape.
Meteorites are never porous, (i.e. do not have 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 (shallow depressions) rather than being bulbous. Also they rust easily so that there may be no bright metal showing. Stony meteorites do not show protuberances, and weathered varieties are rusty even on broken surfaces. Fresh stony meteorites may have a whitish rock interior, with bright whitish metal specks; their surfaces are black or smoky grey, varying in lustre from quite glassy to dull, and perhaps with telltale lines from their flight through the air.
The best way to determine if an object is a possible meteorite is to compare it with the appearance of known meteorites. The Geological Survey of Canada (601 Booth St., Ottawa, ON, K1A 0E8) maintains a National Meteorite Collection of over 1700 specimens, and supports research on them. there are also a number of other collections across the country. Should you find a suspected meteorite, please contact the nearest member of MIAC/CCMI. Meteorites of Canadian origin are subject to the provisions of the Cultural Property Import and Export Act and may not be exported from Canada without a permit.