Mercury is the closest planet to the Sun and is the second smallest in the Solar System. The Sun on Mercury would look 2 and a half times bigger than it does on Earth, but the sky always remains black because there is virtually no atmosphere. It takes Mercury 88 Earth days to orbit the Sun, and one day on Mercury- sunrise to sunset- is 176 Earth days long. Most of what we know about Mercury has come from the Mariner 10 spacecraft, which made three passes of the planet in 1973 and 1974. The most significant discovery made my Mariner 10 is that Mercury has a weak magnetic field, which is about 1% that of the Earth's.
The Mariner 10 photos of Mercury show a terrain similar to that of our Moon. The surface is pocketed with craters, basins and hills, and dusty plains. The planet is geologically inactive except for the occasional impact by meteorites. Volcanic activity ended around 3.9 billion years ago. One of the most prominant features on Mercury is the Caloris Basin. It is 1,300 kilometers in diameter, and was probably created from an impact from an object 100 kilometers wide. The impact blasted material 600 to 800 kilometers around the planet, and created mountains up to three miles high.
Mercury is 57,910,000 kilometers from the Sun, and is very dense. The mean surface temperature is 179 degrees Celsius, but it can get as hot as 427 degrees Celsius or as cold as -173 degrees Celsius. Mercury has a rotational period of 58.6462 days and orbits the Sun at 47.88 kilometers per second. Even though Mercury shines at a bright magnitude of -1.9 in our sky, it is hard to observe from Earth because of its close posistion to the Sun.
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