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Mercury (Planet): Surface
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The plains of Mercury have two distinct ages: the younger plains are less heavily cratered and probably formed when lava flows buried earlier terrain. One unusual feature of the planet's surface is the numerous compression folds which crisscross the plains. It is thought that as the planet's interior cooled, it contracted and its surface began to deform. The folds can be seen on top of other features, such as craters and smoother plains, indicating that they are more recent. Mercury's surface is ... flexed by significant tidal bulges raised by the Sun—the Sun's tides on Mercury are about 17% stronger than the Moon's on Earth.
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Because Mercury orbits so near the Sun, it can be difficult to observe from Earth. The planet is only a few degrees above the horizon for short periods in the early evening or just before dawn, often visible only during twilight and seen through hazy air. Like the Moon and Venus, it goes through phases and varies noticeably in brightness. Optical telescopes on Earth have revealed little detail of its surface and space telescopes such as the Hubble Space Telescope cannot be safely pointed at an object so close to the Sun. Radar... can observe Mercury in the sky during the daytime. Radar studies in the 1960s discovered its 3:2 rotation and orbital period relationship—scientists had previously assumed that Mercury always kept the same face to the Sun the way the Moon does with the Earth.
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The orbit of Mercury is the most eccentric of the major planets, with the planet's distance from the Sun ranging from 46,000,000 to 70,000,000kilometers. It takes 88 days to complete the orbit. The diagram on the left illustrates the effects of the eccentricity, showing Mercury’s orbit with a circular orbit with the same semi-major axis. The higher velocity of the planet when it is near perihelion is clear from the greater distance it covers in each 5-day interval. The size of the spheres, inversely proportional to their distance from the Sun, illustrates the varying heliocentric distance. This varying distance to the Sun, combined with a unique 2:3 resonance of the planet's rotation around its axis, result in complex variations of the surface temperature.
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Surface temperatures on Mercury vary more than those of any other major body in the solar system, with a maximum range of about 650°C (1170°F/ 650°K) between the hottest and coldest extremes. The side facing the Sun gets very hot—up to 450°C (840°F/725°K)—while the side facing away quickly cools to frigid temperatures, -183°C (-297.4°F/90°K). Because its axis is vertical, Mercury does not have seasons. The floors of craters at the north and south poles receive very little sunlight and always remain extremely cold—about -200°C (-328°F/70°K)—while its equatorial region experiences extreme changes, reaching 450°C (840°F/725°K) at perihelion when facing the Sun—hot enough to melt zinc. (The surface of Venus is even hotter because of the greenhouse effect caused by its dense atmosphere, reaching 462°C (864°F/736°K), hot enough to melt lead). The same spot on Mercury faces the Sun at perihelion every second orbit.
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A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004, from the Cape Canaveral Air Force Station aboard a Boeing Delta 2 rocket. The MESSENGER spacecraft will make several close approaches to planets to place it onto the correct trajectory to reach an orbit around Mercury. It made a close approach to the Earth in February 2005, and to Venus in October 2006. Another Venusian encounter will follow in 2007, followed by three close approaches to Mercury in 2008 and 2009, after which it will enter orbit around the planet in March 2011.
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Mercury's cratered surface appears very similar to the Moon. Mercury's most distinctive surface feature (of what has been photographed) is Caloris Basin, a impact crater ~1350km in diameter. The planet is marked with scarps, which apparently formed billions of years ago as Mercury's core cooled and shrank causing the crust to wrinkle. The majority of Mercury's surface is covered with plains of two distinct ages; the younger plains are less heavily cratered and probably formed when lava flows buried earlier terrain. In addition, Mercury has "significant" tidal bulges.
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