...lanets are those that revolve around the sun in orbits larger in diameter than the Earths orbit. The planets may also be classified into to groups according to their gross physical characteristics. The terrestrial, or Earth-like, planets are close to the sun and are composed primarily of rock and metal. They include Mercury, Venus, Earth, and Mars. The terrestrial planets are also called the inner planets. The Jovian, or Jupiter-like, planets are very large compared to the terrestrial planets and are much farther from the sun. They are also called the outer planets. They include Jupiter, Saturn, Uranus, and Neptune. These planets are composed mostly of hydrogen and helium in gaseous and liquid form. Pluto, the outermost planet, is usually considered neither a terrestrial nor a Jovian planet. It is composed of ice and rock and is much smaller than the other planets. Formation and Evolution Although the origin of the solar system is uncertain, most scientists believe that it began to develop about 4 12 billion years ago from a large cloud of gas and dust. The cloud began to contract. As the material ithin the cloud became compressed, it gre hot. Most of this mass as dran toard the center of the cloud, eventually forming the sun. The remaining material, less than 1 percent of the original, formed a spinning disk, called the solar nebula, around the center. The planets and satellites evolved from the nebula as it cooled. See also Solar System, Past and Future of the Solar System. Close to the center, the material in the disk condensed into small particles of rock and metal that collided and stuck together, gradually groing into larger bodies called planetesimals. As they traveled around the center, the largest planetesimals sept up smaller material in their paths, a process knon as accretion. Eventually these accreting bodies evolved into the terrestrial planets. The numerous impact craters still evident on the oldest surfaces of some planets are believed to have been created during this phase, hen the nascent planets collided ith other bodies. Farther from the center of the disk, cooler temperatures alloed not only rock and metal but also ice and gas to develop. These materials formed small eddies in the spinning disk that evolved into the Jovian planets. Each young planet had its on, relatively cool nebula from hich its satellites formed. As the planets and satellites accreted, their interiors gre hot and melted. In a process knon as differentiation, heavier materials sank to the centers, generating more heat in the process and gradually forming cores. In the case of the terrestrial planets, mantles of rock formed around metal-rich cores and ere covered by thin surface crusts. Lighter elements escaped from the interiors and formed atmospheres and, on Earth, oceans. In addition to the heat generated by accretion and differentiation, the planets and satellites had a third source of internal heat the decay of certain radioactive elements ithin the bodies see Radioactivity. Since their formation, many of the physical characteristics of the planets have been determined by the manner in hich the bodies generated and lost their internal heat. For example, the release of internal heat accounts for the volcanic and tectonic activity that shapes the crusts of the terrestrial planets. See also Continent Geology Plate Tectonics Volcano. These bodies have solid surfaces that have preserved a record of their geologic histories. In smaller bodies such as Earths moon, Mercury, Mars, and the satellites of the outer planets, the internal heat escapes to the surface relatively quickly. As a result, the surface initially undergoes rapid, violent changes. Then, hen most of the bodys internal heat has dissipated, the surface features stabilize and remain largely undisturbed as the body ages. Larger bodies, like the Earth and Venus, lose their heat more sloly. In fact, they are still subject to the forces of volcanism and tectonism. The landforms of the terrestrial bodies that lack atmospheres have been shaped primarily by these volcanic and tectonic activities, combined ith cratering caused by impacts that occurred during the solar systems formation. The same is true for those terrestrial bodies that have atmospheres, but their landforms have been modified by the action of ind and, in some cases, ater. The evolution of the Jovian planets cannot be reconstructed by analyzing their surface features--they have no solid surfaces. These planets are so large that much of their internal heat is still being released. Mercury Mercury, the planet nearest the sun, is difficult to observe from the Earth because it rises and sets ithin to hours of the sun. Consequently, little as knon about the planet until the Mariner 10 spacecraft made several flybys in 1974 and 1975. Mercurys surface has several different types of terrain. Planetary scientists can estimate the age of a surface by the number of impact craters on it in general, the older the surface, the more craters it has. Some regions on Mercury are heavily cratered, suggesting that they are very old surfaces that ere probably formed about 4 billion years ago. Beteen these regions are areas of gently rolling plains that may have been smoothed by volcanic lava flos or by accumulated deposits of fine material ejected from impacts. These plains are also old enough to have accumulated a large number of impact craters. Elsehere on the planet are smooth, flat plains ith fe craters. These plains are probably younger and volcanic in origin. Sometime beteen the formation of the intercrater plains and the formation of the smooth plains, the hole planet may have shrunk as it cooled, causing the crust to buckle and form the long, steep cliffs called scarps. The largest impact basin on Mercury, Caloris, is about 800 miles 1,300 kilometers across and is surrounded by mountains that rise to heights of about 1.2 miles 2 kilometers. It as probably created from the impact of a large planetesimal hen Mercury as forming. On the opposite side of the planet from Caloris is an area of hilly, lineated terrain that probably resulted from seismic aves caused by the same impact. Like other airless, solid bodies in the solar system, the entire surface of Mercury is covered ith a layer of rubble called regolith. Regolith is composed of material, ranging from dust to boulders, that as scattered hen impact craters ere formed. This debris as in turn broken up and redistributed by subsequent impacts. Mercury is very dense and has a magnetic field that is about 1 percent as strong as Earths. This suggests the existence of a core composed of iron and nickel and constituting about 40 percent of the planets volume. The surface gravity is about one third as strong as Earths. A thin atmosphere of hydrogen, helium, potassium, and sulfur surrounds the planet. Radar images taken of Mercury in 1991 sho hat are considered to be large ice ...
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