Mercury

Mercury (pronounced /ˈmɝkjʊəri/) is the innermost and smallest planet in the solar system, orbiting the Sun once every 88 days. It ranges in brightness from about −2.0 to 5. more...

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5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun (greatest elongation) is only 28.3°. It can only be seen in morning or evening twilight. Comparatively little is known about the planet: the first of two spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only 40%–45% of the planet’s surface. The second, on January 14, 2008, was the MESSENGER spacecraft, which will make two more passes by Mercury, followed by orbital insertion in 2011. It will completely survey and map the planet.

Physically, Mercury is similar in appearance to the Moon as it is heavily cratered. It has no natural satellites and no substantial atmosphere. The planet has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth. It is an exceptionally dense planet, given the core's size. The surface temperatures on Mercury range from about 90 to 700 K (−180 to 430 °C), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

Recorded observations of Mercury date back to the time of the Sumerians, in the third millennium BC. The Romans named the planet after the Roman god Mercurius, equated to the Greek Hermes and the Babylonian Nabu. The astronomical symbol for Mercury is a stylized version of the god’s head and winged hat atop his caduceus, an ancient astrological symbol. The Greeks of Hesiod's time called it Στίλβων Stilbon (“the gleaming”) and Hermaon. Before the 5th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, the other only at sunset. In India, the planet was named Budha (बुध), after the son of Chandra (the Moon). The Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet as the water star (水星), based on the Five Elements. The Hebrews named it Kokhav Hamah (כוכב חמה), “the star of the hot one” (“the hot one” being the Sun). Mercury is smaller (though still having a greater mass) than the largest of the natural satellites in our solar system, Ganymede and Titan.

Internal structure

Mercury is one of the four terrestrial planets that is a rocky body like the Earth. It is the smallest of the four, with a diameter of 4879 km (3,030 mi) at its equator. Mercury consists of approximately 70% metallic and 30% silicate material. The density of the planet is the second highest in the solar system at 5.43 g/cm³ (water is 1.00 g/cm²), only slightly less than Earth’s density. 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 g/cm³ versus Earth’s 4.4 g/cm³.

Read more at Wikipedia.org