Mercury is the smallest planet in our Solar System at about 4,880 km (c. 3,010 miles) in diameter. It is the closest planet to the Sun, which it takes just under 88 days to orbit. Originally believed to rotate on its axis once per orbit, showing the same face to the sun at all times, in fact it turns three times on its axis for every two orbits. This is known as a 3/2 spin-orbit resonance. Interestingly, its high orbital precession (43 arcseconds per century) was not understood until Einstein's General Theory of Relativity explained it.
Surface temperatures range from about 80ºK to 700ºK (-193ºC to 427ºC). The wide range is due to mercury having three "days" in every two "years"; that is, it rotates on its axis three times for every two orbits around the sun. The coldest places are at the bottom of craters near the poles, and investigations using radar suggest that there is surface ice there. The very low gravity means that there is no atmosphere to speak of, but traces of hydrogen, helium, oxygen, sodium, calcium and potassium have been observed. Altogether, it is not a very hospitable place!
The surface is heavily cratered, having suffered major bombardment from meteors and comets. The largest crater known is the Caloris Basin, which is 1,550 km (nearly 1,000 miles) across. There are many more that are hundreds of miles across. They differ from the Moon's craters in the way the ejected material falls back to the surface. Mercury's higher gravity limits the extent over which the material spreads compared to the Moon.
Mercury's orbit varies over a period of millions of years from almost circular to highly eccentric. At present it is in a middling stage of eccentricity, though it remains the planet with the most eccentric orbit. Its closest approach to the sun is approximately 46 million km (28,750,000 miles), while at its furthest it is nearly 70 million km (43,500,000 miles) away. This variation in eccentricity is believed to be due to the influence of the other planets, particularly Jupiter. As the maximum eccentricity seems to be increasing, it could reach as far as the orbit of Venus over the next 5 billion years or so. However, as the sun is expected to enter its red giant stage in that period, and both Mercury and Venus are likely to be engulfed, it is unlikely they will get a chance to collide!
Mercury is the second most dense planet after the Earth, and has a very large iron core measuring over 40% of the planet's volume, which is more than half the diameter of Earth's core. There are many theories for this, including the possibility that the planet was once at least twice its current diameter, but was hit by a very large object that removed a substantial amount of surface material leaving the original sized core behind. Being so close to the Sun, and having a highly eccentric orbit, tidal forces are immense, and serve to keep the iron core molten. This creates a dynamo effect that generates a relatively large magnetic field. The mantle around the core is approximately 400 miles thick, with the surface crust between 60 and 180 miles thick.
After several fly-by missions in 2008/9, the Messenger probe went into orbit around Mercury on March 18th 2011. Currently, it is on its second extended mission; Cassini Solstice.
Solar System -
Description of Mercury