Mercury

The Greeks and Romans called this small planet Hermes and Mercury respectively, for the messenger of their gods because, as observed from Earth, it moves faster than any other planet. This is because it is the closest planet to the Sun and has a year of just 88 Earth days, although a Mercurian day lasts 59 Earth days.

We didn't know a great deal about Mercury until 1974, when the Mariner 10 probe analysed Mercury. Mariner 10 is the only probe that has visited Mercury (It flew past Mercury 3 times during 1974-1975). Mercury is too near the Sun for it to be mapped by the Hubble Space Telescope, and Mariner 10 only managed to map 45% of the surface). The photographs it took portray Mercury as a barren, rocky landscape, it's only distinguishing features being the large amount of craters marking Mercury's surface.

Mercury has a diameter of 4878 kilometers and a mass of 3.30E23 kg. It's main distinguishing feature is the Caloris Basin, a lowland area that is 1350 kilometers across, and is thought to have been caused by a very large and ancient meteorite.

Mercury is only 52 million kilometers away from the Sun (0.38 AU), and it's temperature in the face of the Sun is 326 degrees C. At night, however, the surface chills to -150 degrees C. This no doubt plays an important role in eroding and changing the planets changing surface features.

Mercury has a highly eccentric orbit, at the perihelion (the point in its orbit where a planet is closest to the Sun) is is only 46 million kilometers away from the Sun, and at the aphelion (the point in its orbit where a planet is furthest away from the Sun), it is 70 million kilometers).

For a long time it was thought that Mercury's day was the same length as its year so as to keep the same face to the Sun (just like the Moon does to the Earth), but was proven to be false in 1965. It is now known that Mercury rotates 3 times every 2 years.

Becuse of Mercury's rotation and erratic orbit, a person on Mercury's surface would observe some strange effects. At some longitudes the Sun would seem to rise and then gradually increase in size. The Sun would then seem to stop, breifly reverse course, stop again before resuming its path toward the horizon and decreasing in apparent size.

Mercury, like the Moon, has no plate tectonics, although Mercury is much denser (5.43 g/cm cubed).

Mercury's core is made of iron, with a diameter of 3600 km to 3800 km. The silicate outer shell is only 500 to 600 km thick. At least some of the core is molten.

Radar observations of Mercury's north pole (a region not mapped by Mariner 10) show evidence of water ice in the protected shadows of some craters. Mercury also has a small magnetic feild (about 1% of Earth's)