Meteorites

Meteorites

Meteorites are small extraterrestrial bodies that reach the Earth's surface. They are small asteroids, approximately boulder-sized or less. While still in space these bodies are called meteoroids. When they enter the Earth's atmosphere, but before reaching the surface, they are called meteors.

Seventy nine percent of meteorites are chondrites. Chondrites are balls of mafic minerals with small grain size indicative of rapid cooling. In most chondrites small spherules, called chondrules, can be found. Chondrites are typically about 4.6 billion years old and are thought to represent material from the asteroid belt. It is unknown how they formed. Carbonaceous chondrites, thought to be unaltered solar nebula material, constitute about five percent of meteorites and contain small amounts of organic materials, including amino acids. Also, presolar grains are identified in carbonaceous chondrites. About six percent of meteorites are iron meteorites with intergrowths of iron-nickel alloys, such as kamacite. Unlike chondrites, the crystals are large and appear to represent slow crystallization. Iron meteorites are thought to be the core material of one or more planets that subsequently broke up. Stony iron meteorites constitute the remaining two percent. They are a mixture of iron-nickel and silicate minerals. They are thought to have originated in the boundary zone above the core regions where iron meteorites originated.

When a meteorite enters the Earth's atmosphere, air drag and friction cause the body to heat up and emit light, thus forming a fireball or shooting star. Throughout the ages, meteorites were highly thought of as sacred objects by different cultures and ancient

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civilizations. The extravagant fall of a meteorite, accompanied by light and sound phenomena, such as falling stars and sonic booms, has always kindled the human imagination, evoking fear and awe in everyone who witnesses such an event. For obvious reasons, the remnants of these incidents, the actual meteorites, were often kept as sacred stones or objects of power. They were worshiped, and used in their respective religious ceremonies.

Most meteors disintegrate when entering the Earth's atmosphere, making impact events on the surface uncommon. On Average, about 500 baseball-sized rocks reach the surface each year. Large meteorites may strike the ground with considerable force, leaving behind an impact crater, a circular, bowl-shaped depression on the earth's surface. The kind of crater will depend on the size, composition, degree of fragmentation, and incoming angle of the meteor. The force of collision may cause widespread destruction. Occasional damage to property, livestock, and even people has been recorded in historic times. Mexico's Chicxulub crater is believed to be the site of a meteorite impact so immense that the resulting environmental changes caused the extinction of the dinosaurs 65 million years ago. In 1908, in the Tunguska Basin in Siberia, a meteor that was probably a stony asteroid about 100 ft (30 m) in diameter completely disintegrated before hitting the ground, so no crater was formed; however, all the trees were flattened and wildlife killed in an area 30 miles (50 kilometers) in diameter, more than half the size of Rhode Island. A meteorite estimated to weigh 60 tons rests where it was discovered, near Grootfontein, Namibia. Among the exhibits at the Hayden Planetarium in New York City are three large meteorites brought from Greenland