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( Mercury (planet))
Mercury (pronounced /'m??kj??ri/&_160;(help·info)) is the innermost and smallest planet in the solar system,[7] orbiting the Sun once every 88 days. Mercury is bright when viewed from Earth, ranging from -2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun is only 28.3°. It can only be seen in morning or evening twilight. Comparatively little is known about it; the first of two spacecraft to visit Mercury was Mariner&_160;10, which mapped only about 45% of the planet’s surface from 1974 to 1975. The second is the MESSENGER spacecraft, which mapped another 30% during its flyby of January 14, 2008. MESSENGER will make one more pass by Mercury in 2009, followed by orbital insertion in 2011, and will then survey and map the entire planet. Mercury is similar in appearance to the Moon it is heavily cratered, has no natural satellites and no substantial atmosphere. However, unlike the moon, it has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth.[8] It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about 90 to 700 K (-183&_160;°C to 427&_160;°C, -297&_160;°F to 801&_160;°F),[9] with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest. Mercury is one of four terrestrial planets in the solar system, and is a rocky body like the Earth. It is the smallest planet in the solar system, with an equatorial radius of 2439.7&_160;km.[3] Mercury is even smaller—albeit more massive—than the largest natural satellites in the solar system, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material.[12] Mercury's density is the second highest in the Solar System at 5.427&_160;g/cm³, only slightly less than Earth’s density of 5.515&_160;g/cm³.[3] If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3&_160;g/cm³ versus Earth’s 4.4&_160;g/cm³.[13] Mercury’s density can be used to infer details of its inner structure. While the Earth’s high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not nearly as strongly compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.[14] Geologists estimate that Mercury’s core occupies about 42% of its volume; for Earth this proportion is 17%. Recent research strongly suggests Mercury has a molten core.[15][16]
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