Mercury is the closest planet to the sun, less than half as far from the huge ball of heat as the earth is. You would think that it gets pretty hot on Mercury. But strangely enough, part of the planet is much colder than earth!
Like all planets, Mercury spins on its axis, so that one side is facing the sun and the other side is in darkness. The side facing the sun may have temperatures over 650 degrees Fahrenheit.
But Mercury has no atmosphere to hold in this heat. So when part of Mercury turns away from the sun, it quickly cools off.
On the dark side of Mercury, temperatures may go as low as 270 degrees below zero, colder than anyplace on earth!