Some Facts About Meteorites
What are meteorites and where do they come from?
A meteorite is a solid body from outer space that has fallen to the Earth's surface. A "fall" is a meteorite that was observed to fall and then collected. A "find" is a meteorite that was not observed to fall but that was recognized by diagnostic features. Most of the "shooting stars" that are observed in the night sky are actually pieces of dust rather than large objects. However, the biggest objects that fall to Earth may result in large craters, such as Barringer Meteor Crater, on the planet's surface.
Barringer Meteor Crater near Winslow, Arizona. The crater is believed to have formed from the impact of a large (30-50 m diameter) iron meteorite 50,000 years ago. The crater is about 1.2 km in diameter. Photo by D. Roddy and K. Zeller, USGS.
Most meteorites are believed to originate from the asteroid belt between Mars and Jupiter. They are the remains of a planet that never formed and are considered to represent the building blocks of the terrestrial planets, including Earth. A handful of meteorites appear to come from the Moon and Mars. Meteorites escape their parent bodies through collisions with other objects in the solar system or they are pulled from their orbits by the Sun's large gravitational field.
How do we know they're from space?
Meteorites that come from the asteroid belt are about the same age as the solar system, approximately 4.5 billion years old. No Earth rocks are this old because they have been processed by plate tectonics and erosion. Meteorites from Mars and the Moon are distinguished from Earth rocks and other meteorites by their chemical and isotopic compositions, mineralogy, and age. Lunar meteorites are also distinguished by their resemblance to the lunar rocks returned by the Apollo astronauts. One day scientists hope to return samples from Mars for a direct comparison as well!
What are the different types of meteorites?
Stony meteorites: These meteorites, the most common type, contain 75-90% silicate minerals (like olivine), 10-25% nickel-iron metal alloy, and iron sulfide. Stony meteorites are the most common type of meteorite to fall, making up about 94% of observed falls. Of the two subgroups, chondrites are the most abundant, making up approximately 86% of all observed meteorite falls.
Chondrites - Chondrites, the most abundant type of stony meteorite, are very primitive in terms of chemistry. They also contain many of the first objects to have formed in the solar system, such as calcium-aluminum-rich inclusions and chondrules (from whence they get their name). Most chondrites also contain tiny flecks of nickel-iron metal.
Mezö-Madaras, Romania, ordinary chondrite. This close-up of a cut and polished face of Mezö-Madaras measures ~ 7 cm from left to right. Chondrites are named for the nearly spherical, silicate-rich objects they contain called chondrules, which were among the first objects to have formed in our solar system. As is evident in this picture, Mezö-Madaras has abundant large chondrules. Photo by D. Ball, ASU.
Allende, Mexico, carbonaceous chondrite. This chondrite also contains chondrules (note the round cavity left by the removal of a large chondrule). In addition, Allende, like many carbonaceous chondrites, contains calcium-aluminum-rich inclusions (CAIs). Unlike chondrules, which are round and composed mostly of silicate minerals like olivine and pyroxene, CAIs are predominantly white to light gray in color, irregularly shaped, and rich in refractory (high- temperature) minerals like melilite and spinel. They are believed to pre-date chondrules by at least 2 million years. This specimen is ~ 11 cm from left to right. Photo by D. Ball, ASU.
Achondrites - These meteorites underwent melting or other types of processing on their asteroid or planetary parent body (lunar and martian meteorites included). Because achondrites closely resemble Earth rocks to the untrained eye, they are rarely found. For this reason, most of the achondrites in our collections were seen to fall and then collected.
Sioux County, NE, eucrite. This achondrite specimen, a fall, has been broken open to reveal the stark distinction between its black, shiny, and smooth fusion crust and its light-colored interior. This specimen is ~ 7 cm from left to right. Photo by J. Kurtzmen.
Johnstown, CO, diogenite. Diogenite achondrites are primarily composed of orthopyroxene, a silicate mineral. Johnstown is brecciated and contains large orthopyroxene grains in a groundmass of crushed and broken orthopyroxene. This specimen is ~ 13 cm long. Photo by D. Ball, ASU.
Stony-iron meteorites: These meteorites contain ~ 50% silicates and 50% nickel-iron metal.
Pallasites formed where an asteroid's silicate mantle and metal core mixed.
Albin, WY, pallasite. Pallasites are composed of about half metal and half olivine, a greenish silicate mineral. This slice, which is ~ 18 cm long, has been illuminated from behind to distinguish its olivine from the surrounding metal. Photo by D. Ball, ASU.
Mesosiderites, the other type of stony-iron, likely formed from the collision of a metal-rich asteroid with a silicate-rich asteroid.
Hainholz, Germany, mesosiderite. This cut and polished face of the Hainholz meteorite, which is ~ 9 cm from left to right, demonstrates the complex mixture of half metal and half pyroxene, a silicate mineral, typical of mesosiderites. Photo by D. Ball, ASU.
Iron meteorites: Composed of almost entirely nickel-iron metal, these meteorites come from the cores of large differentiated asteroids. Therefore, they are considered analogous to the Earth's core...Iron meteorites make up only about 5% of observed falls. However, they are overrepresented in our collections, in part because they are more easily recognized than the other types of meteorites.
Rancho Gomelia, Mexico, octahedrite. This iron meteorite has been cut, polished, and etched with acid on one face to reveal an interlocking crystal structure of variable composition nickel-iron alloys. This pattern, called the "Widmanstätten" pattern, is unique to the subgroup of iron meteorites called octahedrites. The etched face is ~ 14 cm from bottom to top. Photo by D. Ball, ASU.
What do most meteorites look like?
Size - Meteorites vary in size from a few centimeters across to several feet in diameter.
Shape - Meteorites are rarely round in shape. Typically, they are irregular in shape with rounded edges.
Plainview, TX, ordinary chondrite. The exterior of this complete specimen of Plainview is typical for stony meteorite finds. It is blocky in shape with rounded edges and corners, has a smooth surface with a few regmaglypts, and is black to brown in color. The specimen measures ~ 19 cm from left to right. Photo by D. Ball, ASU.
Weight - In general, meteorites are heavier than Earth rocks of the same size due to their increased abundance of nickel-iron metal. Naturally occurring terrestrial rocks and materials are typically metal- and nickel-poor compared to meteorites.
Canyon Diablo, AZ, octahedrite. This iron meteorite is part of the object that formed Barringer Meteor Crater, pictured above. Its exterior appearance is typical for iron meteorite finds. Because it is mostly nickel-iron metal, it is very heavy for its size. It measures ~ 15 cm tall and weighs ~ 8.1 kg (~ 18 lb). Photo by D. Ball, ASU.
Color - The surface of a freshly fallen meteorite will appear black and shiny due to the presence of fusion crust (see Sioux County and Johnstown above). Fusion crust is the result of frictional heating and ablation of the outer surface layer of a meteoroid during its transit through the atmosphere. However, meteorites will weather to a rusty brown color over time (see Plainview, above), which may cause the fusion crust to disappear completely.
Surface Features - Most meteorites have very smooth surfaces, with no holes. However, some meteorites will exhibit thin flow lines (see Camel Donga, below) or "thumbprint"-shaped features called regmaglypts (see Plainview, above, and Henbury, below). Flow lines are basically cooled streams of once molten fusion crust. Regmaglypts are most likely due to the selective melting and ablation of the various compositional components of a meteoroid during its transit though the atmosphere.
Camel Donga, Australia, eucrite. This complete specimen exhibits a shiny, black fusion crust with flow lines. This specimen is ~ 19 cm from left to right. Photo by D. Ball, ASU.
Henbury, Australia, iron meteorite. The Henbury iron meteorite exhibits many regmaglypts. This specimen is ~ 26 cm long. Photo by D. Ball, ASU.
Interior features - Most stony meteorites, especially ordinary chondrites which are the most common type of meteorite to fall, will exhibit tiny metallic flecks on a broken, cut, or polished surface (see Warden, below). In addition, most stony meteorites will exhibit small round chondrules (see Plainview, below). As their name implies, iron meteorites will be made almost entirely of metal, while stony-iron meteorites will contain approximately half metal.
Warden, Australia, ordinary chondrite. Chondrites, particularly ordinary chondrites, commonly contain flecks of metal, as can be seen in this polished slice of Warden. The slice is ~ 16 cm across. Photo by D. Ball, ASU.
Plainview, TX, ordinary chondrite. Visible in this cut and polished specimen of Plainview are numerous small, round chondrules. This specimen also contains flecks of metal; however they are difficult to see in the lighting used to take this photo. This specimen is ~ 10 cm from top to bottom. Photo by D. Ball, ASU.
Magnetism - A magnet will be attracted to most meteorites, even stony meteorites, due to the iron-nickel metal they contain.
Where can I find meteorites?
Meteorites fall all over the planet, but they are best preserved and most easily found in deserts, whether they are hot (like Arizona) or cold (like Antarctica) deserts. The dry climate of a desert slows rusting of the metal within many meteorites and the lack of vegetation in deserts makes meteorites easier to find. Each star on the map below represents a location in Arizona where a meteorite has been found! Meteorites are typically named for the nearest town or geographic feature.
What can we learn from meteorites?
Star evolution - Some meteorites actually contain chemical species and grains of dust that were produced by stars before the formation of our solar system. Study of the behavior of these chemical species and the composition of the dust can increase our understanding of stars and their role in the universe.
Solar system evolution - We can discern the changing chemical, temperature, and pressure conditions within the early solar system from the study of different meteorite components.
Age of solar system and components - By measuring the abundances of particular isotopes in meteorites, we can estimate the age of the solar system and the relative order in which different meteorite components formed.
Geologic history of the Earth and Moon - Large meteorite impacts have helped shape the face of our planet and the Moon. Many scientists believe that a very large impact was even responsible for the formation of the Moon.
History of life - Meteorites may have delivered to Earth the chemicals necessary for life!
