A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids. When it enters the atmosphere, impact pressure causes the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting/falling star. The term bolide refers to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface.
More generally, a meteorite on the surface of any celestial body is a natural object that has come from elsewhere in space. Meteorites have been found on the Moon and Mars.
Meteorites that are recovered after being observed as they transited the atmosphere or impacted the Earth are called falls. All other meteorites are known as finds. As of mid-2006, there are approximately 1,050 witnessed falls having specimens in the world's collections. In contrast, there are over 31,000 well-documented meteorite finds.
Meteorites have traditionally been divided into three broad categories: stony meteorites are rocks, mainly composed of silicate minerals; iron meteorites are largely composed of metallic iron-nickel; and, stony-iron meteorites contain large amounts of both metallic and rocky material. Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy
Most meteoroids disintegrate when entering Earth's atmosphere. However, an estimated 500 meteorites ranging in size from marbles to basketballs or larger do reach the surface each year; only 5 or 6 of these are typically recovered and made known to scientists. Few meteorites are large enough to create large impact craters. Instead, they typically arrive at the surface at their terminal velocity and, at most, create a small pit. Even so, falling meteorites have reportedly caused damage to property, livestock and people.
Campo del Cielo iron meteorite with natural holeVery large meteoroids may strike the ground with a significant fraction of their cosmic velocity, leaving behind a hypervelocity impact crater. The kind of crater will depend on the size, composition, degree of fragmentation, and incoming angle of the impactor. The force of such collisions has the potential to cause widespread destruction. The most frequent hypervelocity cratering events on the Earth are caused by iron meteoroids, which are most easily able to transit the atmosphere intact. Examples of craters caused by iron meteoroids include Barringer Meteor Crater, Odessa Meteor Crater, Wabar craters, and Wolfe Creek crater; iron meteorites are found in association with all of these craters. In contrast, even relatively large stony or icy bodies like small comets or asteroids, up to millions of tons, are disrupted in the atmosphere, and do not make impact craters. Although such disruption events are uncommon, they can cause a considerable concussion to occur; the famed Tunguska event probably resulted from such an incident. Very large stony objects, hundreds of meters in diameter or more, weighing tens-of-millions of tons or more, can reach the surface and cause large craters, but are very rare. Such events are generally so energetic that the impactor is completely destroyed, leaving no meteorites. (The very first example of a stony meteorite found in association with a large impact crater, the Morokweng crater in South Africa, was reported in May 2006.
Several phenomena are well-documented during witnessed meteorite falls too small to produce hypervelocity craters.The fireball that occurs as the meteoroid passes through the atmosphere can appear to be very bright, rivaling the sun in intensity, although most are far dimmer and may not even be noticed during daytime. Various colors have been reported, including yellow, green and red. Flashes and bursts of light can occur as the object breaks up. Explosions, detonations, and rumblings are often heard during meteorite falls, which can be caused by sonic booms as well as shock waves resulting from major fragmentation events. These sounds can be heard over wide areas, up to many thousands of square km. Whistling and hissing sounds are also sometimes heard, but are poorly understood. Following passage of the fireball, it is not unusual for a dust trail to linger in the atmosphere for some time.
As meteoroids are heated during atmospheric entry, their surfaces melt and experience ablation. They can be sculpted into various shapes during this process, sometimes resulting in deep "thumb-print" like indentations on their surfaces called regmaglypts. If the meteoroid maintains a fixed orientation for some time, without tumbling, it may develop a conical "nose cone" or "heat shield" shape. As it decelerates, eventually the molten surface layer solidifies into a thin fusion crust, which on most meteorites is black (on some achondrites, the fusion crust may be very light colored). On stony meteorites, the heat-affected zone is at most a few mm deep; in iron meteorites, which are more thermally conductive, the structure of the metal may be affected by heat up to 1 cm below the surface. Meteorites are sometimes reported to be warm to the touch when they land, but they are never hot. Reports, however, vary greatly, with some meteorites being reported as "burning hot to the touch" upon landing, and others forming a frost upon their surface.
Meteoroids that experience disruption in the atmosphere may fall as meteorite showers, which can range from only a few up to thousands of separate individuals. The area over which a meteorite shower falls is known as its strewn field. Strewn fields are commonly elliptical in shape, with the major axis parallel to the direction of flight. In most cases, the largest meteorites in a shower are found farthest down-range in the strewn field.
meteorite types :
Most meteorites are stony meteorites, classed as chondrites and achondrites. Only 6% of meteorides are iron meteorites or a blend of rock and metal, the stony-iron meteorites. Modern classification of meteorites is complex, the review paper of Krot et al. (2007) summarizes modern meteorite taxonomy.
About 86% of the meteorites that fall on Earth are chondrites, which are named for the small, round particles they contain. These particles, or chondrules, are composed mostly of silicate minerals that appear to have been melted while they were free-floating objects in space. Certain types of chondrites also contain small amounts of organic matter, including amino acids, and presolar grains. Chondrites are typically about 4.55 billion years old and are thought to represent material from the asteroid belt that never formed into large bodies. Like comets, chondritic asteroids are some of the oldest and most primitive materials in the solar system. Chondrites are often considered to be "the building blocks of the planets".
About 8% of the meteorites that fall on Earth are achondrites (meaning they do not contain chondrules), some of which are similar to terrestrial mafic igneous rocks. Most achondrites are also ancient rocks, and are thought to represent crustal material of asteroids. One large family of achondrites (the HED meteorites) may have originated on the asteroid 4 Vesta. Others derive from different asteroids. Two small groups of achondrites are special, as they are younger and do not appear to come from the asteroid belt. One of these groups comes from the Moon, and includes rocks similar to those brought back to Earth by Apollo and Luna programs. The other group is almost certainly from Mars and are the only materials from other planets ever recovered by man.
About 5% of meteorites that fall are iron meteorites with intergrowths of iron-nickel alloys, such as kamacite and taenite. Most iron meteorites are thought to come from the core of a number of asteroids that were once molten. As on Earth, the denser metal separated from silicate material and sank toward the center of the asteroid, forming a core. After the asteroid solidified, it broke up in a collision with another asteroid. Due to the low abundance of irons in collection areas such as Antarctica, where most of the meteoric material that has fallen can be recovered, it is possible that the actual percentage of iron-meteorite falls is lower than 5%.
Stony-iron meteorites constitute the remaining 1%. They are a mixture of iron-nickel metal and silicate minerals. One type, called pallasites, is thought to have originated in the boundary zone above the core regions where iron meteorites originated. The other major type of stony-iron meteorites is the mesosiderites.
Tektites (from Greek tektos, molten) are not themselves meteorites, but are rather natural glass objects up to a few centimeters in size which were formed—according to most scientists—by the impacts of large meteorites on Earth's surface. A few researchers have favored Tektites originating from the Moon as volcanic ejecta, but this theory has lost much of its support over the last few decades.