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Modern classification schemes for the composition of asteroids and meteorites divide them into either chondrites or nonchondrites. The nonchondrites are divided into primitive and differentiated types. The differentiated nonchondrites have three groups, including the achondrites, stony-irons, and irons, based on the chemistry and texture of the meteorites and reflecting their origin. Chondrites have compositions similar to the sun, and represent the average composition of the solar system, thought to be close to the original composition of the solar nebula. Many chondrites contain chondrules, which are small, originally liquid melt drops of the original material that condensed to form the solar system 4.6 billion years ago. some chondrites contain calcium-aluminum inclusions, which may represent presolar system material. one class of chondrites, carbonaceous condrites, contain complex organic molecules. The variation in chondritic meteorites is thought to represent formation at different depth in a large, 100-120-mile (165-200-km) wide asteroid destroyed in a catastrophic collision early in the history of the solar system, dispersing the fragments across the solar system.

The nonchondrites are divided into primitive and differentiated types. The differentiated nonchon-drites have three groups, including the achondrites, stony-irons, and irons. Achondrites are rocky silicate igneous rocks, whereas the irons consist of mixtures of iron and nickle. stony-irons represent a transitional group. These meteorites are also thought to have formed in several parent bodies destroyed by collisions in what is now the asteroid belt, but these bodies were initially large enough (120-240 miles [200-400 km]) across that they were able to differentiate into crust, mantle, and core. The irons are from the core of these bodies, the achondrites from the mantle and crust, and the stony-irons from the transition zone. Some unusual achondrites have been shown to have been ejected from the Moon and Mars during impacts, eventually landing on Earth.

Most meteorites are thought to come from the asteroid belt, where 1-2 million asteroids with diameters greater than 0.6 miles (1 km) are orbiting the Sun between Mars and Jupiter. They may get pushed into Earth-crossing orbits after being deflected by collisions in the asteroid belt or by gravitational perturbations during complex orbital dynamics. Spectral measurements of some of the asteroids show that their compositions correlate with the meteorites sampled on Earth, and a crude gradation of compositions in the asteroid belt is thought to represent both the original distribution of different parent bodies that broke up during collisions and the initial compositional trends across the solar nebula. Asteroids closer to the Sun are rockier, with more silicates and metals, whereas those farther out have more ices of nitrogen, methane, and water.

Several different groups of asteroids have unstable orbits that cross the paths of the planets in the inner solar system. These objects represent grave dangers to life on Earth, as any impacts with large objects are likely to be catastrophic. These Earth and Mars orbit-crossing asteroids are classified according to their increasing distance from the Sun into Aten-, Apollo-, and Amor-class asteroids. Some of these asteroids are being tracked, to monitor the risk to life on Earth, since collisions of asteroids of this size are known to cause mass extinction events, such as the Cretaceous-Tertiary extinction that killed the dinosaurs. Major impacts occur on Earth about every 300,000 years.

The outer solar system also hosts belts of asteroids, and the number and mass of these objects pales in comparison with the amount of material in the inner solar system. There are many names for asteroids and other bodies orbiting in specific regions, but the bodies of most significance include the Trans-Neptunian objects that orbit beyond the orbit of Neptune at 30 A.U. and into the Kuiper belt, that extends to about 49 A.U. Beyond this there is a relatively empty gap before the beginning of the Oort Cloud at 60 A.U. Most objects in the Kuiper belt and the Oort Cloud consist of mixtures of rock and ice, and are the source region for comets. There are thought to be thousands of Kuiper belt objects with diameters greater than 600 miles (1,000 km), 70,000 asteroids or comets with diameters greater than 60 miles (1,000 km), and half a million objects with diameters greater than 30 miles (50 km).

The oort Cloud represents the outer reaches of the solar system, and may actually extend into the Oort Cloud of the nearby star system, Proxima Cen-tauri. There are thought to be trillions of comets in the Oort Cloud over 0.8 miles (1.3 km) in diameter, totally several Earth masses. The Oort Cloud is the source of long-period comets, with orbits longer than 200 years. Comets typically have a rocky core and emit jets of ices consisting of methane, water, and ammonia, and other ice compounds. Many comets are coated by a dark surface consisting of complex organic molecules, and these may be the source for much of the carbon and volatile elements on the Earth that presently make up much of the atmosphere and the oceans. Some scientists speculate that comets may be responsible for bringing the complex organic molecules to Earth that served as the building blocks for life.

See also astronomy; astrophysics; comet; meteor, meteorite; origin and evolution of the Earth and solar system; solar system.


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asthenosphere The asthenosphere is the layer of the Earth's mantle between the lithosphere and the mesosphere. Its depth in the Earth ranges from about 155 miles (250 km) to zero miles below the midocean ridges, and 31 to 62 miles (50-100 km) below different parts of the continents and oceans. Some old continental cratons have deep roots that extend deeper into the asthenosphere. The asthenosphere is characterized by small amounts (1-10 percent) of partial melt that greatly reduces the strength of the layer and is thought to accommodate much of the movement of the plates and vertical isostatic motions. The name derives from the Greek for "weak sphere." S-wave seismic velocities clearly demarcate the asthe-nosphere and show a dramatic drop through the asthenosphere because of the partial melt present in this zone. Because of this the asthenosphere is also known as the low-velocity zone, and it shows the greatest attenuation, or weakening, of seismic waves anywhere in the Earth.

The asthenosphere is composed of the rock type peridotite, consisting primarily of the mineral olivine, with smaller amounts of the minerals orthopyroxene, clinopyroxene, and other accessory minerals including spinels such as chromite. The term peridotite is a general term for many narrowly defined ultramafic rock compositions including harzburgite, lherzolite, websterite, wehrlite, dunite, and pyroxenite. Peri-dotites are not common in the continental crust but are common in the lower cumulate section of ophio-lites, in the mantle, and in continental layered intrusions and ultramafic dikes. Peridotites have unstable compositions under shallow crustal metamorphic conditions, and in the presence of shallow-surface hydrating weathering conditions, they commonly become altered to serpentinites through the addition of water to the mineral structures.

The asthenosphere is flowing in response to heat loss in the deep Earth, and geologists are currently debating the relative coupling between the flowing asthenosphere and the overlying lithosphere. In some models the convection in the asthenosphere exerts a considerable mantle drag force on the base of the lithosphere, and significantly influences plate motions. In other models the lithosphere and asthenosphere are thought to be largely uncoupled, with the driving forces for plate tectonics being more related to the balance between the gravitational ridge push force, slab pull force, slab drag force, transform resistance force, and subduction resistance force. There is also a current debate on the relationship between upper-mantle (asthenosphere) convection and convection in the mesosphere. Some models propose double or several layers of convection, whereas other models purport that the entire mantle is convecting as a single layer.

See also convection and the Earth's mantle; energy in the earth system; mantle; plate tectonics.

astronomy Astronomy is the study of celestial objects and phenomena that originate outside the Earth's atmosphere. The name is derived from the Greek words astron for star and nomos for law and includes the study of stars, planets, galaxies, comets, interstellar medium, the large-scale structure of the universe, and the natural laws that describe these features. Astronomy is also concerned with the chemistry and meteorology of stellar objects, the physics of motion, and the evolution of the universe through time.

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