Mercury is the planet closest to the Sun. Its distance from the Sun is approximately 58 million km (36 million miles).  Its diameter is 4875 km (3030 miles), and its volume and mass is about 6% of that of the Earth.
Mercury takes 88 days to revolve around the Sun and takes just under 59 days to rotate once.
Mercury has a very thin atmosphere containing sodium and potassium.  The force of gravity on Mercury is about one-third of that on the Earth.
The Mariner 10 spacecraft passed Mercury in 1974 and again in 1975.  It sent back pictures of a moon-like, cratered surface and reported temperatures on the surface to be about 430 degrees centigrade on the sunlit side and  -180 degrees centigrade on the dark side.

In 1991 powerful radio telescopes on Earth revealed signs of vast sheets of ice in Mercury's polar regions.  Investigation by the Hubble Space Telescope is not possible as the planet is too close to the Sun.
Like the surface of the moon, much of the surface of Mercury is covered by large craters.  The largest of these is the Caloris Basin, which is almost 1300 km in diameter  It was probably caused by a very large impact early in the history of the solar system.  Mercury also has regions of smooth terrain, some of which may be the result of volcanic activity.
Mercury is often visible with binoculars or the naked eye, but it is always very near to the Sun and is therefore difficult to see in the twilight sky.

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