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الورقة 4

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الورقة الثالثة

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الورقة الثانية

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Meteorites are space rocks that fall to Earth's surface. Meteorites are the last stage in the existence of these type of space rocks. Before they were meteorites, the rocks were meteors. Before they were meteors, they were meteoroids. Meteoroids are lumps of rock or metal that orbit the sun. Meteoroids become meteors when they crash into Earth's atmosphere and the gases surrounding them briefly light up as "shooting stars." While most meteors burn up and disintegrate in the atmosphere, many of these space rocks reach Earth's surface in the form of meteorites. Dust-sized particles called micrometeorites make up 99 percent of the approximately 50 tons of space debris that falls on the Earth's surface every day. Some meteorites, however, are as large as boulders. The largest meteorite found on Earth is the Hoba meteorite discovered in Namibia in 1920. The Hoba meteorite weighs roughly 54,000 kilograms (119,000 pounds). The Hoba meteorite is so big, and so heavy, it has never been moved from where it was found! Most meteorites look very much like rocks found on Earth, except meteorites usually have a dark, burned exterior. This exterior is formed as friction from the atmosphere melts the meteorite as it crashes toward Earth. Known as thermal ablation, this process can also give meteorites a roughened, smooth, or thumbprint surface. Thermal ablation creates these different textures due to different chemicals present in the meteorite. Meteorites crash through the atmospheres of all planets and moons in our solar system. Some planets and moons don't have enough atmosphere to break apart meteors, resulting in large meteorites. These larger meteorites create deep, round impact craters that can be found all over our Moon, Mercury, and Mars. In 2005, the first meteorite found on another planet was discovered by Opportunity, one of NASA's Mars rover spacecraft. In 2014, Opportunity's sister spacecraft, Curiosity, discovered a meteorite that was 2 meters (7 feet) wide, making it the largest yet discovered on Mars. Types of Meteorites More than 60,000 meteorites have been found on Earth. Scientists have divided these meteorites into three main types: stony, iron, and stony-iron. Each of these types has many sub-groups. minerals that contain silicates-material made of silicon and oxygen. They also contain some metal-nickel and iron. There are two major types of stony meteorites: chondrites and achondrites. Chondrites themselves are classified into two major groups: ordinary and carbonaceous. Ordinary chondrites are the most common type of stony meteorite, accounting for 86 percent of all meteorites that have fallen to Earth. They are named for the hardened droplets of lava, called chondrules, embedded in them. Chondrites formed from the dust and small particles that came together to form asteroids in the early solar system, more than 4.5 billion years ago. Because they were formed at the same time as the solar system, chondrites are integral to the study of the solar system's origin, age, and composition. Ordinary chondrites can be classified into three main groups. The groups indicate the meteorite's quantity of iron. The H chondrite group has a high amount of iron. The L chondrite group has a ow amount of iron. The LL group has a ow amount of iron and a low amount of metal in general. Carbonaceous chondrites are much more rare than ordinary chondrites. Astronomers think carbonaceous chondrites formed far away from the sun as the early solar system developed. As their name implies, carbonaceous chondrites contain the element carbon, usually in the form of organic compounds such as amino acids. Carbonaceous chondrites also often contain water or material that was shaped by the presence of water. Like ordinary chondrites, carbonaceous chondrites can be more minutely classified based on their mineral composition. All groups of carbonaceous chondrites are marked with a two- or three-letter code starting with C. Carbonaceous chondrites are often named after the first specimen of that type recovered. The CI group, for instance, is named after the Ivuna meteorite, which crashed into Tanzania in 1938. CI meteorites have a high amount of carbon, as well as clays. Carbonaceous chondrites can also be named after the place where the first specimen of the type was found. The CV group is named after a meteorite that crashed near the city of Vigarano, Italy, in 1910. The most famous CV meteorite is probably the Allende meteorite, which fell to Earth near Pueblo de Allende, Chihuahua, Mexico, in 1969. The Allende meteorite has thousands of tiny chondrules made of the mineral olivine. The Allende meteorite also has grains of a special kind of carbon-diamonds. These diamonds are actually older than the solar system, and astronomers think they were produced as blast material from a nearby, ancient supernova. Achondrites do not contain the lava droplets (chondrules) present in chondrites. They are very rare, making up about 3 percent of all known meteorites. Most achondrites form from the brittle outer layers of asteroids, which are similar to Earth's Stony MeteoritesStony meteorites are made up of crust. There are many classifications of achondrites. The "primitive achondrite" group, for instance, has a very similar mineral composition to chondrites. Lunar meteorites are achondrites that crashed to Earth from the Moon, while Martian achondrites crashed to Earth from our neighbor planet, Mars. Very few meteorites, only about 0.2 percent, come from Mars and the Moon. These achondrites are the results of Mars and the Moon's own meteorite impacts. Large meteorites hit the surface of Mars and the Moon, blasting off bits of rock. These rock bits rarely make their way to our atmosphere as meteors and even more rarely hit the Earth's surface. cores of asteroids and account for about 5 percent of meteorites on Earth. Iron meteorites are the most massive meteorites ever discovered. Their heavy mineral composition (iron and nickel) often allows them to survive the harsh plummet through Earth's atmosphere without breaking into smaller pieces. The largest meteorite ever found, Namibia's Hoba meteorite, is an iron meteorite. pallasites, contains yellow-green olivine crystals encased in shiny metal. Astronomers think many pallasites are relics of an asteroid's core-mantle boundary. Their chemical composition is similar to many iron meteorites, leading astronomers to think maybe they came from different parts of the same asteroid that broke up when it crashed into Earth's atmosphere. Meteorite Impact Craters Meteorites crash through the Earth's atmosphere with tremendous force. The largest meteorites leave enormous holes in the ground called impact craters. The best-preserved impact crater in the world is the Barringer Meteorite Crater, near Winslow, Arizona. There, more than 50,000 years ago, a meteorite weighing about 270,000 metric tons (300,000 tons) slammed into the Earth with the force of 2.5 million tons of TNT. The impact blasted a hole 1 kilometer (.6 miles) wide and about 230 meters (750 feet) deep. The fragments left of the space rock show that it was an iron meteorite. More than a hundred impact craters have been identified on Earth. Perhaps the most famous is the Chicxulub Crater, in Yucatan, Mexico. The Chicxulub Crater can be identified on land, beneath dozens of meters of sediment, although about half of the feature is submerged in the Gulf of Mexico. It is one of the larges impact craters ever discovered on Earth. Despite its size, the Chicxulub Craber is famous for another reason. Many scientists think the large meteorite that created the Chicxulub Crater-measuring roughly 10 kilometers (6 miles) wide-triggered the extinction of the dinosaurs and other animal and plant life 65 million years ago. Iron MeteoritesIron meteorites are mostly made of iron and nickel. They come from the Stony-Iron Meteorites Stony-iron meteorites have nearly equal amounts of silicate minerals (chemicals that contain the elements silicon and oxygen) and metals (iron and nickel). One group of stony-iron meteorites, the

Fast Fact

Ablation Blackening Thermal ablation, the process that burns off the surface layer of a meteorite and causes it to appear blackened, is the same process that blackens the outside of returning spacecraft, such as tiles on the space shuttle.

Fast Fact

Mars and the Moon As of July 2014, there were 133 Martian meteorites and 183 lunar meteorites found on Earth-not a lot. Scientists are able to tell the rocks came from Mars and the Moon because their composition matches chemical analysis of rocks conducted during NASA's robotic explorations of Mars and the "moon rocks" recovered during the Apollo lunar missions.

Fast Fact

Meteorite or Meteorwrong?
How can you tell if that rock you found fell from the sky? First of all, meteorites get burned when they enter Earths atmosphere, so they are usually black and crusty on the outside. Also, meteoriteseven stony meteoritescontain iron, so a magnet will stick to them.

Fast Fact

Natural Hazards Most meteorites fall to Earth harmlessly. Sometimes, however, they can cause great damage. The extinction of most life on Earth 65 million years ago is a good example of that. A less catastrophic impact hit a driveway in Peekskill, New York, in 1992. Although no one was harmed, the meteorite slammed through the trunk of a parked Chevrolet Malibu, barely missing the gas tank, before creating a small impact crater beneath the car. The Chevy is nicknamed the "Peekskill Meteorite Car."

Fast Fact

Rocky Cookie
The best place to hunt for meteorites is in Antarctica. Because most of Antarctica is covered in ice and snow, rocky meteorites stand out like chocolate chips in a cookie.

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Meteorites are space rocks that fall to Earth's surface. Meteorites are the last stage in the existence of these type of space rocks. Before they were meteorites, the rocks were meteors. Before they were meteors, they were meteoroids. Meteoroids are lumps of rock or metal that orbit the sun.