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Planets and moons


Colour photo of MercuryColour photo of Mercury Mercury is the smallest planet in the Solar System, and the nearest planet to the Sun. Mercury can be seen from Earth only with difficulty: it is always low in the dawn or twilight sky close to the Sun. For many years, much of what we knew about Mercury was gained by space probe Mariner 10, which flew by the planet in 1974. On the third of its three flybys, it came within 327 kilometres (203 miles) of Mercury, but was able to map only between 40 and 45% of the planet's surface. A survey of the planet using radar signals beamed from Earth in 1991 revealed further details of Mercury’s landscape. Since 2011, NASA’s MESSENGER space probe has been orbiting Mercury and sending back more detailed images than ever before.

Close-up view of one of Mercury's cratersClose-up view of one of Mercury's craters
Mercury's surface, showing craters and wrinklesMercury's surface, showing craters and wrinkles


On first appearance Mercury looks quite similar to our Moon—bare and rocky and covered with craters and lava plains. Originally molten, Mercury’s surface shrank as it cooled after the bombardment that produced all the craters began to ease about 3000 million years ago. As with a dried fruit, the shrinkage resulted in “wrinkles”. On Mercury’s surface, these wrinkles appear as high cliffs. With no atmosphere to cause winds or water to erode the rocks, Mercury’s landscape has remained the same ever since.


Mercury is smaller than two moons: Jupiter's Ganymede and Saturn's Titan.

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