Mercury

Mercury (pronounced /ˈmɝkjʊəri/) is the innermost and smallest planet in the solar system, orbiting the Sun once every 88 days. Mercury is bright when viewed from Earth, ranging from −2. more...

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0 to 5.5 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 and evening twilight. Comparatively little is known about it; the first of two spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only about 45% of the planet’s surface. The second was the MESSENGER spacecraft, which mapped another 30% of the planet during its flyby of January 14, 2008. MESSENGER will make two more passes by Mercury, followed by orbital insertion in 2011, and will survey and map the entire planet.

Physically, Mercury is similar in appearance to the Moon. It is heavily cratered, has no natural satellites and no substantial atmosphere. It 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 due to the large size of its core. 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 Sumerians in the third millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes. The English name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated with the Greek Hermes. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.

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 at its equator. 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. The density of the planet is the second highest in the solar system at 5.43 g/cm³ (water is 1.00 g/cm³), 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.

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. 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.

Read more at Wikipedia.org