Mercury is the closest planet to the Sun, and officially the smallest planet in the Solar System (after the demotion of Pluto to a 'dwarf planet'). Mercury is one of the five planets known of since ancient times – it is named after the Roman messenger god, Hermes in Greek, because he was fleet of foot and the planet moves most quickly though the sky.
- Mass: 0.3302×1024kg
- Equatorial Radius: 2438.7km
- Mean Density: 5427kg/m3
- Length of Day: 4223.4 hours
- Period of Revolution about Sun: 87.97 days
- Acceleration due to Gravity: 3.70m/s2
- Mean Orbital Velocity: 47.87km/s
- Inclination of Axis: 0.01°
- Mean Distance from the Sun: 0.387 AU
Although Mercury has been known about for thousands of years, it has always been difficult to get much information on it. The reason for this is that it is so very close to the Sun - it's never directly overhead, so when observing it you may have to look through as much as eight times the atmospheric thickness as when observing a planet which can be seen overhead (like Mars). The maximum angle between Mercury and the Sun in the sky is 28 degrees.
Mercury's orbit was very carefully studied in the 19th Century. It is very eccentric, varying between 46 million km and 70 million km from the Sun. However, its orbit cannot be fully explained using Newtonian Mechanics, and Mercury's orbit was one of the ways that Einstein's theory of general relativity was proved to be better than Newtonian mechanics - the orbit of Mercury follows the path predicted by the curvature of space-time.
Inside and On the Surface
Our real knowledge of Mercury comes from the three Mariner 10 flybys of 1974 and 1975. On its second flyby the tape recorder stopped working, so it was unable to take as many images as it should have done, but the data from the other two flybys was very valuable.
Mercury probably has a thin silicate mantle, which looks like the moon's crust with lots of meteor impact craters. Under this is an iron core, making Mercury the second densest object in the solar system after Earth. As well as crater marks there is also evidence that Mercury's radius has decreased by about 1km - in other words, at some point Mercury has shrunk. The surface area of Mercury is 15 percent that of the Earth's surface, or around the same area as North America.
Mercury is too hot to have a stable atmosphere like the Earth or Venus; any atmosphere it does have comes from particles from the solar wind. At its nearest approach to the Sun, the dayside temperature is 430°C, but at its furthest from the Sun its temperature drops to 285°C. If you could look at the Sun from Mercury it would appear to be three times the size it is from Earth.
Mercury was thought by Giovanni Schiaparelli in the 1880s to be tidally locked with the Sun, the same way the Moon is with the Earth. This belief persisted until radar observations of the planet determined that actually Mercury is in 3:2 resonance with the Sun, so Mercury has one and a half 'days' during each rotation of the planet.
Although Mercury is the closest planet to the Sun, in 1991 scientists at Caltech suggest that it may well have water ice on its surface. Bright reflections of radio waves bounced off craters on Mercury's North pole are thought to be ice. It would be possible for ice to exist there because the North pole always sees the sun near the horizon, so parts of craters never see the Sun at all, and it could get as cold as -161°C. The water could either have come from the impacts of comets, or from gas released from inside the planet.
Mercury has a very weak magnetic field, which may be caused by molten iron in its core. It has no (known) satellites.
There probably aren't any surprises left about Mercury. Astronomers will get more information about it from NASA's Messenger mission, due for launch in 2004 and to start orbiting Mercury in 2009.
The h2g2 Tour of the Solar System
Take the h2g2 Shuttle for your whistle-stop tour of the Solar System.