The planet Mercury is the closest world to the Sun. Mercury is a small and ancient world. It is only about one third the size of the Earth and about one twentieth the mass. It has no real atmosphere, just few wisps of gas, and is a dry rocky place. It has no moon. Its surface is covered with big holes, or craters, where objects like asteroids or comets have hit it, making big explosions and scattering rock and dust for large distances.
It takes Mercury only 88 days to orbit all the way around the Sun because it is only about one third of the distance from the Sun that the Earth is, and it spins slowly – only once every 59 days instead of 24 hours like the Earth.
Think about it
Because Mercury spins so slowly day and night times are very strange. If you stood on the surface of the planet then daytime would seem to last for about 88 Earth days, or more than two thousand hours! Then night time would last for another 88 Earth days. Since Mercury is so close to the Sun and has no atmosphere then it gets incredibly hot during the long day, and then incredibly cold during the long night. At the hottest it can be 800 degrees and at the coldest a chilly -330 degrees.
Illustration of why a day or night lasts for 88 Earth days on Mercury – because it rotates three times for every two times it goes around the Sun.
The only places on Mercury that may never have sunlight on them are deep craters at the North and South poles. Inside these craters it can be permanently dark, and always cold. Astronomers have found evidence that there is water ice inside some of these craters at the poles. This ice must be ancient, since it is hard to imagine water ice forming on a planet so close to the Sun.
Mercury is composed of rock and metals. It has a huge core, or center, of about 2,200 miles in diameter. The core seems to have more iron in it than any other big planet in the Solar System.
Illustration of the inside of Mercury. There is a thin rock crust, only 60 to 200 miles thick. Below that is a rocky layer called the mantle about 400 miles thick, and then below that is a huge core that contains a lot of iron.
We think that Mercury may be like this because four billion years ago it collided with another big object. In that collision the outside part of the planet may have been blasted away – like the soft outsides of a snowball. All that was left was the denser insides of the planet.
Visiting Mercury is hard because it is closer in to the Sun than any other planet. This means that any object, like a spacecraft, must be able to deal with more intense light and heat. Imagine the sunburn you’d get if the Sun was ten times brighter than it usually is when you go to the beach! It also means that spacecraft have to be able to maneuver in the stronger gravitational field of the Sun. Mercury moves around the Sun with almost twice the speed that Earth moves. In 1974 and 1975 the robotic spacecraft Mariner 10 flew past Mercury three times, taking pictures and measurements. It found that Mercury has a magnetic field. Thirty years later the next mission to Mercury is called Messenger and it is another robotic spacecraft that will orbit Mercury and make a detailed map of the planet’s surface. Until this finally happens, no one will have actually seen what the whole surface of Mercury looks like. Keep up to date with this mission online and you could be among the first people in history to see what parts of another planet actually look like!