A hot old world

Mercury is the planet in the solar system nearest to the Sun, with an average orbital radius of 58 million kilometres. It is also the smallest planet of the inner solar system, with a diameter of only 4880 km, making it about the size of our Moon. Mercury travels fast, taking just 88 days to orbit the Sun, but it takes a slow 58.65 days to rotate once on its axis. Of all the terrestrial planets, Mercury's orbit is the most elliptical. Its elliptical orbit and slow rotation give it large variations in surface temperatures. During the day temperatures can reach a blistering 430°C, while at night they can drop to a freezing -180°C. No other planet experiences such a wide range of temperatures.

Mercury is thought to have formed at the same time as the other planets in the solar system, about 4.5 billion years ago. Because it is so close to the Sun, Mercury must have been very hot and in a molten state before it cooled to become a solid planet. As Mercury cooled, it also began to contract. The surface of Mercury has been churned up by many meteorite impacts.

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