Mercury is the smallest of all the planets.

It is also the closest planet to the sun, with an average distance of about 36 million miles. Despite its small size, Mercury has a very dense core, which is thought to be composed mostly of iron. Its surface is heavily cratered and has a thin atmosphere made up of gases like oxygen, sodium, hydrogen, and helium. Mercury's surface temperature can vary greatly, ranging from extremely hot during the day to very cold at night.

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