Mercury

Mercury is the first planet in the solar system, only about 60 million km away from the sun. It takes Mercury about 88 days to go round the sun once and one Mercury day is about 58 Earth days long. Mercury is a small planet - not much larger than our moon. Temperatures go up to 500 °C when it faces the sun and -150° C on its dark side.

The surface of Mercury is very much like that of the moon. Scientists believe that over 3 billion years ago, when the solar system was forming, the planet was constantly hit by asteroids and rocks . For its size, Mercury is a heavy planet. Gravity is about one third of the Earth's. The core of Mercury is probably still liquid .

Some scientists believe that Mercury is really the moon of another planet that flew off course and began its own orbit as a planet. The orbit of Mercury is egg-shaped - which means it is closer to the sun at some times of the year. The sky is pitch black and all of a sudden the sun begins to rise. Because it is so close to Mercury it looks enormous, a huge blazing ball that is twice as big as it is on Earth.

Mercury
Image : National Aeronautics and Space Administration/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington,
Public domain, via Wikimedia Commons