The planet Mercury is the closest planet to the Sun in our Solar System. It was named after "Mercury" the speedy Roman god. It was given this name because it appears to move faster than any other planet. Mercury has quite a lot in common with our moon, although it is larger. (Two other moons are actually larger than Mercury, they are Titan and Ganymede). The only planet smaller than Mercury is Pluto.

The surface of Mercury is very similar to that of the moon. It is covered in craters, jagged cliffs, and dusty hills. From Mercury's surface the sun would take up nearly half of the black sky (the sky on Mercury is black, not blue, this is because the planet has almost no atmosphere to generate the illusion of a blue sky). From the nighttime side, both Earth and Venus would be visible about half of the time.

It is difficult to view Mercury from Earth. It can only be seen during the daytime (which is not exactly a great time to attempt to view planets), or for a brief period before the sunrise and after the sunset (this isn't much better, as it appears just over the horizon, and the light is going through a lot more atmosphere than if you were viewing it head on). Most of our information about Mercury comes from the Mariner 10 spacecraft, which flew past Mercury three times in the mid 1970s.

Early astronomers thought that Mercury was tidally locked, and only showed one face towards the Sun (like our own moon). By the early 1960s that theory had been refuted, because researchers determined (via radio waves), that Mercury's dark side was too hot for it to possibly be tidally locked. By 1965 they had determined a rotational period of 59 days, later the Mariner 10 data showed an exact rotational period of 58.646 days. So it is not tidally locked, but its rotation an orbit are related. It has exactly 1.5 days per Mercurian year (in other words it spins around 3 times for every two times it rotates the sun). This peculiar trait means that a "day" on Mercury actually lasts 176 Earth days (even though it only takes 58 days to rotate). There is much evidence to support the theory that Mercury once had a much faster rotational period (as quick as 8 hours), but that the Sun has been slowing it down for millions of years. If this theory is true, then Mercury will eventually become tidally locked.

The Mariner 10 space probe flew past Mercury three times in 1974 and 1975. During these flights we learned quite a bit about the planet, and managed to photograph nearly half of its surface. The biggest surprise was the discovery of a magnetic field. Everyone had thought that Mercury was far too small to support the required core of molten iron to generate a magnetic field via the dynamo effect. This field is a weak one, only 1 percent of the strength of the Earth's magnetic field. A few researchers say that the field actually comes from magnetized iron rocks scattering the planet, and not a molten core at all. One thing we do know for certain is that Mercury is made up mostly of metal. The planet is just as dense as Earth is, and it appears to be made up of at least sixty percent metal (iron mostly).

The surface of Mercury is that of a dead world. It is mostly covered with great plains, left over from prehistoric lava flows. These plains are broken up by great cliffs that run for hundreds of kilometers. These cliffs were apparently created far in the past when Mercury contracted in size (because of a reduction in temperature). Finally there are the craters, they are everywhere. Mercury is as littered with craters as our own moon. Mariner 10 photographed craters from as small as its minimum resolution (100 meters), to as large as 1300 kilometers (the Caloris Basin). These craters are of all different ages, from billions of years old, right up to fresh craters that are only a few thousand years old (virtual youngsters).

Recent data has suggested that Mercury may have a bit of water at its North Pole. Radar data indicates that the interior of several craters (which are in polar areas that are never in direct sunlight), may have frozen water just below the surface. Although further research is needed before this can be considered a fact. But luckily further research is on the way, in the form of the Messenger space probe, which was originally scheduled to visit Mercury on March 23, 2004. The Messenger launch was unfortunately delayed, and it did not end up being launched until August 3, 2004, and the first Mercury flyby is not scheduled until 2008, with actual orbit being scheduled for 2011. The reason it is going to take so long is because NASA set things up so gravity does almost all the work.

Mercury, by the numbers
  • Mass 3.303e+23 Kilograms
  • Mass (Earth = 1) 5.5271e-02
  • Equatorial radius 2,439.7 Kilometers
  • Equatorial radius (Earth = 1) 3.8252e-01
  • Mean density (gm/cm^3) 5.42
  • Mean distance from the Sun (km) 57,910,000
  • Rotational period 58.6462 Days
  • Orbital period 87.969 Days
  • Mean orbital velocity 47.88 km/sec
  • Orbital eccentricity 0.2056
  • Tilt of axis 0
  • Orbital inclination (degrees) 7.004 Degrees
  • Equatorial surface gravity (m/sec^2) 2.78
  • Equatorial escape velocity (km/sec) 4.25
  • Visual geometric albedo 0.10
  • Magnitude -1.9 Vo
  • Mean surface temperature 179°C
  • Maximum surface temperature 427°C
  • Minimum surface temperature -173°C
  • Atmospheric composition Helium 42%, Sodium 42%, Oxygen 15%, Other 1%