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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. Much of what we know about Mercury was gained by space probe Mariner 10, which flew by the planet in 1974. From a distance of just under 20,000 kilometres (12,000 miles), it could distinguish features as small as 100 metres (60 miles) across. 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 spacecraft 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 wrinkles.Mercury'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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