Stephen P. Broker
Meteorites are small objects of extraterrestrial origin pulled to Earth from near-Earth trajectories, penetrating the atmosphere, and landing on Earth’s surface. Their existence has been known through recorded history, but their Solar System origins have been recognized only for a century. The term “meteoroid” is used for any small object moving through the Solar System and capable of intersecting Earth’s orbit. If a meteoroid enters Earth’s atmosphere, it is termed a meteor, or “shooting star.” If it lands on Earth’s surface, it is then called a meteorite. Meteorites which have an observed entry and which are recovered shortly thereafter are falls. Meteorites discovered by chance some time after their fall to Earth are finds.
Several decades of meteorite study have produced a classification scheme which groups meteorites on the basis of their physical appearance and composition. Current classifications recognize that meteorites exhibit primary (nebular) properties relating to their original locations and processes of formation, and secondary properties caused by events of their later histories. The three broad classes of meteorites are the stones, stony-irons, and irons.
The stone meteorites are extremely diverse in their chemical and physical properties. The two types of stones are chondrites and achondrites. Chondrites are aggregates of different component materials. They contain chondrules, silicate mineral assemblages 0.1 to 1.0 mm large contained in glass or crystalline, bead-shaped structures. The chondrules, which are found in essentially no other rock types, have been partially or totally melted. Chondrites closely mirror the composition of the Sun but have lost the volatile gases (mainly hydrogen and helium) present in the Sun.
Chondrites are divided into nine main classes. The H, L, and LL chondrites, called ordinary chondrites, are the most abundant meteorite type found. The CI, CM, CO, and CV chondrites have high carbon contents and are called carbonaceous chondrites. CI and CM carbonaceous chondrites have hydrated clay-silicate sheets. EH and EL chondrites are enstatite chondrites, high in this mineral.
Achondrites are rare among the meteorites. They are igneous rocks or are derived from igneous rocks. They are highly altered from the original nebular material, due to their cooling histories, melting, metamorphism, and shock. They differ considerably from the Sun in composition. Three of the more common types of achondrites are called eucrites, diogenites, and howardites. Three other achondrites, the shergottites, nakhlites, and Chassigny (known as the SNC meteorites), are believed to have originated on Mars. They have generated great scientific interest in recent months. The achondrites also include unclassified breccias, or aggregated fragments of coarse, angular rocks. The stony-irons are mixtures of stony material and metal. One type, the pallasites, consist of iron-nickel alloy networks with rounded olivine nodules. Irons are pieces of metal, iron-nickel alloys with minor amounts of carbon, sulfur, and phosphorus. They are the meteorites which best fit the popular image of what a meteorite is.
Irons consist of two phases, one called kamacite which is low in nickel, and one called taenite which is high in nickel. The majority of irons, collectively called octahedrites, have an octahedral arrangement of kamacite with interspersed taenite. A characteristic structure within these irons is the Widmanstatten structure, which resembles a crystalline material or is suggestive of frost on a window. “Shock-twins” are another feature of these meteorites. The irons are classified on the basis of their content of the elements gallium, germanium, and nickel. Most common are IAB, IIAB, IIIAB, IVA, and IVB irons.
Other aspects of meteorite structure and composition used in classification are degree of oxidation of iron and isotopic proportions of oxygen 16, 17, & 18. Classification attempts to separate meteorites on the basis of their origins and methods of formation. Secondary properties include metamorphism (here, one considers paleotemperatures and cooling histories), shock, aqueous alteration, and brecciation. Shock effects on meteorites are particularly important because the collisions of parent bodies and impact bodies that produced the shock features also are responsible for sending these objects on an Earth-intersecting trajectory.
Of all the meteorite types, the chondrites are most similar to the Sun in composition, least altered through their histories, considered the best indicators of the early conditions of the Solar System. They formed in the solar nebula. To the extent that there are different types of chondrites, it is a reflection of the different nebular processes at work during formation of the Solar System. The nine chondrite groups and their subgroups may be samples of different regions of the nebula. They give us our closest readings of the age of the Solar System, about 4.56 billion years old.
Approximately one dozen recovered meteorites match nearly identically the chemical makeup of martian rocks studied during the unmanned Mariner satellite visits to Mars. These meteorites (the SNC meteorites referred to above, and dated to 1.3 billion years ago) are recognized as ejecta from asteroidal collisions with Mars. One meteorite of martian origin, called ALH84001 after the Allan Hills locality in Antarctica where the rock was found in 1984, now has been determined to have a “shock age” of 4.0 billion years. A dozen meteorites from our Moon, matching rocks collected on the Moon during the six manned Apollo landings there, have reached Earth in similar fashion, having been knocked off Moon’s surface.
Meteor craters of Earth’s Moon have been observed through recorded time and recognized as structures formed by collision impacts for decades. Meteor craters on Earth were suspected of having extraterrestrial origins just since the latest 19th century. Craters of the other terrestrial planets and those of the larger jovian satellites have become known to us through space exploration of the past several decades. There is an extensive literature on known and suspected meteor craters on Earth. The first meteor craters considered to be of proven extraterrestrial origin were Meteor Crater (also called Barringer Crater) in northern Arizona (studied beginning in 1891 by Albert Foote and subsequently studied exhaustively by Daniel Barringer during the period 1902-1929), the Odessa crater in Texas (1921 discovery), the 13 Henbury craters in central Australia (1931 discovery), the Wabar craters in Saudi Arabia (1932 discovery), the Haviland crater in Kansas (meteoritic fragments first discovered in 1885), the Campo del Cielo craters of Argentina (deduced in 1933), the Kaalijarv craters of Estonia (they have yielded meteorites for centuries), the Boxhole meteor crater of central Australia (1937 discovery), the Wolf Creek crater of western Australia (1937 discovery), the Aouelloul crater of Mauretania, West Africa (ca. 1951 discovery), and the Ungava-Quebec crater (Chubb Crater) of Canada (1943 discovery).
Today, nearly 150 major meteorite craters have been identified on Earth. A dozen or so craters are of considerable size, measuring at least 50 km in diameter and up to 185 km in diameter. Chicxulub crater in the northern Yucatan of Mexico has a diameter of 170 km, but some researchers believe the full dimension of the hidden crater is 300 km in diameter. Our ability to look for and locate impact craters on Earth has developed in part through efforts of the space program. Satellite remote sensing work and the use of gravity anomaly and magnetic surveys continue to disclose new candidates for meteor craters. For example, U.S. Geological Survey research conducted in 1994 disclosed an inner basin and an outer rim of a proposed meteor crater buried 300-500 meters under Chesapeake Bay, caused by an impact event dated at 35.5 million years ago. The existence of a string of three impact craters 12 kilometers wide in Chad was announced by a NASA Jet Propulsion Laboratory scientist and her colleague in March 1996.
Earth’s meteor craters are lost through time by erosion, covered by younger rocks, or hidden in ocean basins. Impacts are well documented by presence of craters, melt rock, shock features, and high-pressure minerals. A main problem is the uncertainty in dating the impacts. A one kilometer diameter meteorite striking Earth would liberate energy equivalent to tens of billions of tons of TNT, millions of times the energy of the Hiroshima atomic bomb.