Meteorite - astronomy.

Meteorite - astronomy. Meteorite, meteor that reaches the surface of Earth or of another planet before it is entirely consumed. Meteorites found on Earth are classified into types, depending on their composition: irons, those composed chiefly of iron, a small percentage of nickel, and traces of other metals such as cobalt; stones, stony meteors consisting of silicates; and stony irons, containing varying proportions of both iron and stone. Although most meteorites are now believed to be fragments of either asteroids or comets, recent geochemical studies have shown that a few Antarctic stones came from the Moon and Mars, from which they presumably were ejected by the explosive impact of asteroids. Asteroids themselves are fragments of planetesimals, formed some 4.6 billion years ago, while Earth was forming. Irons are thought to represent the cores of planetesimals, and stones (other than the aforementioned Antarctic ones) the crust. Meteorites generally have a pitted surface and fused, charred crust. A meteorite that landed in Texas in 1998 was found to have water trapped in its rock crystals. The discovery helped scientists theorize about whether water exists in other parts of the solar system. Large meteorites strike Earth with tremendous impact, creating huge craters. The largest known meteorite, estimated to weigh about 60 metric tons, is situated at Hoba West near Grootfontein, Namibia. The next largest, weighing more than 31 metric tons, is the Ahnighito (the Tent); it was discovered, along with two smaller meteorites, in 1894 near Perlernerit (Cape York), Greenland, by American explorer Robert Edwin Peary. Composed chiefly of iron, the three masses had long been used by the Inuit as a source of metal for the manufacture of knives and other weapons. Peary brought the Ahnighito to the United States, and it is now on display at the American Museum of Natural History in New York City. The three largest known impact structures are located in Vredefort, South Africa; Sudbury, Canada (north of Lake Huron); and off the coast of the Yucatán Peninsula of Mexico. The original craters from these impacts have eroded away, but the remaining structures indicate that they were all about 300 km (about 190 mi) in diameter. The meteorites that formed craters as large as the ones in Vredefort, Sudbury, and the Yucatán must have had a devastating effect on the nearby environment, and they also probably affected global weather patterns. The force of collision would have spewed molten rock far around the impact site. Dust and poisonous gases that were produced by the crash when it vaporized minerals in the ground would have darkened the sky over a huge area for months or even years. Many scientists believe that the event that caused the crater in the Yucatán Peninsula may have created global climate changes that led to the extinction of the last of the dinosaurs. If a meteorite that size were to land in a populated area today, the results would be devastating. The amount of property damage and number of lives lost in the immediate area would be great. Dust and gas circulating in the atmosphere could cut off sunlight for months, killing crops and reducing the food supply for the entire world. Fortunately, astronomers calculate the average frequency of major collisions at only about one collision every 300,000 years. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.