This image was created from a mosaic of photographs of Mercury taken by the Mariner 10 spacecraft on 29 March 1974.
Source: NASA, USGS, Mark Robinson.
Average Distance from the Sun:
57.9 million km (0.387 AU)
Size (Equatorial Diameter):
4 880 km (0.383 x that of Earth)
3.59 x 1023 kg (0.06 x that of Earth)
Length of Day (Solar Rotation Period):
176 Earth days
Length of Year (Sidereal Orbital Period):
87.97 Earth days
360 ºC (day side)
-180 ºC (night side)
2.78 m/s2 (0.28 x that of Earth)
Mercury is the closest planet to the Sun. It is one of the four terrestrial or rocky planets of the inner Solar System. From Earth it can at times be seen with the naked eye close to the Sun, around the morning or evening twilight. It was one of the five planets known to the ancients but was popularly believed to be two separate objects. The ancient Greeks called it Apollo when it appeared in the morning and Hermes when it appeared in the evening.
Mercury is in many ways similar to the Moon. It has a grey, rocky surface which is covered with a thick layer of dust. It is heavily cratered and has many valleys and cliffs. Some of the cliffs reach 3 km in height, and are hundreds of kilometres in length.
Mercury has almost no atmosphere. This is why the temperature difference between the day side and the night side is so large. Without an atmosphere the planet cannot trap the heat that it receives from the Sun. On the day side of the planet temperatures reach approximately 400ºC, but can drop to -180ºC on the night side.
It spins once on its Axis every 58.65 Earth days. This is 2/3 of Mercury's year, hence Mercury's day (Sunrise to Sunrise) is 176 Earth days or two Mercury years long.
Mercury was a Roman god, presiding over trade. He was associated with the ancient Greek Hermes, the son of Zeus and Zeus's messenger, the guide of shades to Hades, protector of travellers, bringer of luck and patron deity of thieves and merchants. He was depicted in art as a young man wearing a wide brimmed winged hat, winged sandals and carrying a herald's staff (caduceus) crowned with two snakes.