Mineral Sciences - Division of Meteorites
Meteorites are stony or metallic bodies that fall to the Earth from space. All meteorites show peculiarities not observed in known rocks of Earth, and it is therefore possible to recognize them even if they have not been seen to fall (see TESTING FOR SUSPECTED METEORITES).
Most meteorites come from asteroids, a rare few come from larger bodies such as the Moon and Mars, and many of the smallest meteorites, "micrometeorites", are dust from comets.
Many meteorites preserve chemical and physical properties that were established 4.5 billion years ago, during the earliest history of the solar system, and thus provide some of the best clues to the nature of the events that occurred in that remote time.
There are three major classes of meteorites; stony meteorites, iron meteorites, and stony-iron meteorites.
A small meteorite may fall without any sound or light effects. The fall of a larger meteorite can be accompanied by startling light and sound effects. A fiery mass suddenly appears in the sky travelling swiftly in an arc and leaving a luminous trail behind. It may then disintegrate with a loud explosion and its fragments fall to the ground. Most meteorites break into pieces during the luminous flight and produce multiple individual fragments.
Meteorites are heated by friction with the air as they pass through Earth's upper atmosphere. Their outer surface is melted and continually removed by airflow. Although the meteorite in flight appears incandescent, the heat generated in this process is mainly lost to the surrounding atmosphere, so little heat actually penetrates the cold meteorite. Sonic booms are frequently produced at this stage of flight. The energy resulting from the high velocity of entry into the atmosphere is dissipated within a few seconds while the meteorite is still at high altitude, and the body then falls freely and comparatively slowly to the ground. This long plunge through the cold atmosphere cools the meteorite considerably. Meteorites do not ignite grass or fall in flames.
Stony Meteorites: Chondrites and Achondrites
Stony meteorites are by far the most common. More than 95% of meteorites observed to fall to Earth are stony. They can be divided into chondrites and achondrites. Both types are composed mostly of silicate minerals, but the great majority also contain metallic iron in small-scattered grains.
Chondrites are named for their most prominent feature - millimeter-sized spherical bodies called chondrules. These chondrules (from the Greek for small sphere) formed 4.5 billion years ago in the Solar Nebula - the cloud of gas and dust from which the Sun, planets, asteroids, and comets formed. Chondrules are not found in terrestrial rocks. These chondrules, along with small mineral grains, accreted to form asteroids during the birth of the Solar System. Chondrites are, by far, the most abundant type of stony meteorite.
Less common, comprising only a few percent of all meteorites, are achondrites. These are also stony meteorites composed primarily of silicates, but these meteorites have experienced familiar geologic processes of melting and differentiation - although these happened long ago. Most achondrites formed on asteroids during the birth of the Solar System, but a small number formed on Mars and the Moon.
Stony-iron meteorites, contain about equal proportions of metal and silicate material, and are rare (less than 2% of all known meteorites). Stony-iron meteorites form in places where metal and silicate are mixed. One type of stony-iron are pallasites - rocks composed of a network of iron-nickel metal surrounding a greenish, silicate mineral called olivine. Pallasites probably form when the olivine-rich mantle of an asteroid mixes with the metallic core. Mesosiderites are mixtures of iron-nickel metal and basalt and probably formed by the collision of two asteroids.
Iron meteorites are really composed of iron and nickel and are extremely dense. They are pieces of the cores of asteroids. Early in Solar System history, asteroids melted and the dense iron-nickel metal sank to the center to form a core - much like the Earth has a core. Iron meteorites are the samples of the cores of ancient worlds. While they are rare among meteorites seen to fall to Earth (only a few percent), they are among the most common type of meteorites in our collections, because they can be recognized long after their fall, are very different from Earth rocks, and are resistant to weathering. One of the most distinguishing features of meteorites is the presence of the Widmanstatten pattern - the distinctive series of bands in geometric patterns. This pattern is created by the intergrowth of two different iron-nickel minerals formed during very slow cooling (a few degrees every million years) in the core of the asteroid. The presence of nickel is a universal feature of iron meteorites.
If you think you have a meteorite, the following simple tests will tell if your sample is a candidate for further examination.
1. Does the sample have a dark-colored (typically black) thin exterior coating that shows evidence of melting and is clearly different from the interior (typically light colored)? It is important that you compare the outer surface and interior and this may require removing a small piece of the rock by breaking or sawing with a diamond-impregnated saw blade.
2. Is the sample round?
3. Is the sample very spongy (contain numerous holes)?
4. Is the sample unusually heavy?
5. Does it differ from the rocks typically found in that area?
6. Does the sample attract a magnet?
These simple tests can be helpful in determining if a rock might be a meteorite. Meteorites have exterior surfaces that have been melted during passage through the atmosphere (the fusion crust); they only very rarely contain holes; they are usually solid objects with irregular, but not spherical, shapes; they will be obviously different than the local rocks; they are unusually heavy; and they will attract a magnet.
Some rocks are often confused with meteorites. Industrial slags, commonly used in railroad beds, typically contain numerous holes (vesicles) and the exterior surface, while showing evidence of melting, does not differ from the interior. Iron sulfide and iron oxide concretions are sometimes rounded and can be easily broken. In contrast, it is virtually impossible to break an iron meteorite - they must be cut.
Given recent security concerns, the Division of Meteorites of the Smithsonian Institution is not currently accepting suspected meteorites for examination or testing.
"L" indicates this book may only be found in a library
Beatty, J. Kelly, Chaikin, Andrew, (1990), The New Solar System, Sky Publishing & Cambridge University Press, 326 pp.
L Dodd, R.T. (1981) Meteorites: A petrologic-chemical synthesis, Cambridge University Press, 368 pp.
Dodd, R.T. (1986) Thunderstones and Shooting Stars: The Meaning of Meteorites, Harvard University Press, 196 pp.
Heide, F. and Wlotzka, F. (1995) Meteorites, Messengers from Space, Springer-Verlag, New York, 231 pp.
L Hutchison, R. (1983) The Search for Our Beginning: An Enquiry Based on Meteorite Research Into the Origin of Our Planet and of Life, British Museum (Natural History) and Oxford University Press, 164 pp.
Hutchison, R. and Graham, A. (1993) Meteorites: The Key to Our Existence, Sterling Publishing Co., Inc., New York, 60 pp.
L Kerridge, J.F. and Matthews, M.S. (1988) Meteorites and the early Solar System, The University of Arizona Press, 1269 pp.
L Mason, B. (1962) Meteorites, John Wiley and Sons, Inc., New York, 274 pp.
McSween, H.Y. (1987) Meteorites and their Parent Bodies, Cambridge University Press, 233 pp.
Norton, O.R. (1998) Rocks from Space, Second Edition, Mountain Press Publishing Co., Missoula, Montana, 447 pp.
L Wasson, J.T. (1985) Meteorites: Their Record of Early Solar-System History, W.H. Freeman and Company, 267 pp.
Voyage Through the Universe: Comets Asteroids and Meteorites, (1992), Time-Life Books, Alexandria, VA 144 pp.
BOOKS ON IMPACT CRATERS (General)
L Grieve, R.A.F. et al. (1988) Astronaut's Guide to Terrestrial Impact Craters, LPI Technical Report No. 88-03, 89 pp.
Mark, Kathleen (1987) Meteorite Craters, The Univ. of Arizona Press, 288pp.
Powell, J.W. (1998) Night Comes to the Cretaceous, Dinosaur Extinction and the Transformation of Modern Geology. W.H. Freeman and Company, New York, 250 pp.
BOOKS ON COMETS AND ASTEROIDS
L Binzel, R.P., Gehrels, T., and Mathews, M.S., editors (1989) Asteroids II, The University of Arizona Press, 1258 pp. (technical)
L Delsemme, A.H., editor (1977) Comets, Asteroids, Meteorites: interrelations, evolution and origins, The University of Toledo, 587 pp. (technical)
L Gehrels, T., editor (1979) Asteroids, The University of Arizona Press, 1181 pp. (technical)
L Glass, B.P. (1982) Introduction to Planetary Geology, Cambridge University Press, pp. 325-351 (Chapter 10: Asteroids and Comets).
L Whipple, Fred L. (1985) The Mystery of Comets, Smithsonian Institution Press, Washington, D.C., 276 pp.
L Wilkening, L.L., editor (1982) Comets, The University of Tucson Press, 766 pp. (technical)
HISTORY OF METEORITICS
L Burke, John G. (1986) Cosmic Debris: Meteorites in History, University of California Press, Berkeley, 445 pp.
L Clarke, R.S.Jr., editor (1993) The Port Orford, Oregon, Meteorite Mystery. Smithsonian Contributions to the Earth Sciences, No. 31, Smithsonian Institution Press, Washington, D.C., 43 pp.
L Hoyt, William Graves (1987) Coon Mountain Controversies: Meteor Crater and the Development of Impact Theory, The University of Arizona Press, Tucson, 442 pp.
L Willey, Richard R. (1987) The Tucson Meteorites: Their History from Frontier Arizona to the Smithsonian, Smithsonian Institution Press, Washington, D.C., 47 pp. Reprinted by University of Arizona Press, 1997.
Carlise, Madelyn Wood, (1992) Let’s Investigate Magical, Mysterious Meteorites, Barrons, 32 pp.
Lauber, Patricia (1989) Voyagers from Space: Meteors and Meteorites, Harper and Row, 74 pp.
Fodor, R.V. (1976) Meteorites: Stones from the Sky, Dodd, Mead & Company, New York, 47 pp.
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