Mercury is the first and smallest of the planets in the solar system. Although astronomers have known of this planet since it was first identified in the ancient Middle East, only a pair of space probes have been sent to this planet. From what we do know about it, it largely resembles our own Moon: small, grey, and pockmarked with impact craters.
- Orbit and Day -
Mercury orbits the Sun at less than half the distance that Earth does. As a result, its year is far shorter: just 88 days long. At the same time, it also has one of the longest days: Mercury rotates about twice in an Earth-year, once every 175 days. This creates an unusual situation in that the Mercury year is twice as long as the Mercury day.
The reason for this bizarre situation is Mercury's close proximity to the Sun. Large enough gravitational forces in close orbits lead to a process called tidal locking. Fully locked objects rotate at exactly the same speed as they orbit due to these tidal forces: our own Moon, for example, is fully locked, which is why the same face of the Moon always faces toward Earth. Mercury is not tidally locked, but it does rotate extremely slowly.
Mercury and Venus are the only planets in the solar system which have no moons. Mercury is so close to the Sun that any large objects which did get captured in its orbit would eventually be pulled away by the Sun.
- Rocky Planet -
Like the other inner planets of the solar system (Venus, Earth, and Mars), Mercury is a rocky planet, also known as a terrestrial planet, made up mostly of surface silicates and a molten iron core (again, much like the Earth). However, it is by far the smallest: with a radius of just 1500 miles, it is actually smaller than Jupiter's moon Ganymede, and Saturn's moon Titan.
What makes Mercury different than the other planets, however, is its incredibly scarred surface. Although Mercury does have an atmosphere, it is a very thin one, with the top layers being continually stripped away by solar radiation. Without an atmosphere to burn up meteors, its surface has been bombarded for billions of years, so that it looks much like our own Moon. Most bizarre are a series of massive ridges, or folds in the surface. Why its surface should have been torn in this way is unclear: perhaps there was a giant impact in its past, or perhaps it is the result of the enormous tidal forces exerted by the nearby Sun.
- Atmosphere and Climate -
Mercury has extreme weather: the side facing away from the Sun plunges hundreds of degrees below zero, while the side facing the Sun is hundreds of degrees above freezing. The extreme heat is the result of Mercury's proximity to the Sun: it receives as much as ten times the energy from sunlight that the Earth does. Despite this, because the planet is not tilted like Earth, there are some deep craters in the polar regions which never receive sunlight. As a result, there is actually ice on Mercury, in the places where sunlight never reaches.
The planet's atmosphere is very thin, again due to the proximity of the Sun. A number of compounds are present, like hydrogen, helium, oxygen, and even trace amounts of water vapour, but the atmosphere is permanently unstable due to the solar radiation. At any given time, some gases are entering the atmosphere from meteor impacts and chemical reactions in the ground, while others are escaping out of the atmosphere, bleeding away into free space.
- Space Probes to Mercury -
Reaching Mercury is not easy: spacecraft first have to accelerate toward the Sun, and then accelerate away from it to avoid falling into the Sun's massive gravity well. (By contrast, reaching planets in the outer solar system is easy: they can simply coast outwards, gradually slowed by the Sun's gravity.) As a result, only two spacecraft have been sent to Mercury so far, with a third now being prepared for a mission.
The first space probe, Mariner 10, flew by Mercury twice, in 1974 and 1975, and took the first pictures of the surface. The second, Messenger, is now en route to Mercury, and will arrive in 2011. This will be our first chance to study the planet in detail. Unfortunately, because of the pull of the Sun's gravity, Messenger's lifespan will probably be short, as little as one or two years. By the time it fails, a third space probe, BepiColombo, will be on its way to the planet to continue the mission. No further probes are planned after that.
- Seeing Mercury -
Mercury can be visible from the Earth, but often it is too close to the Sun to be visible in the sunlight. It does become visible in certain locations during the pre-dawn and twilight hours. Interestingly, Mercury cannot be studied from orbit by satellites like the Hubble Space Telescope, only by surface telescopes. This is because pointing orbital telescopes so close to the Sun would result in critical damage to their sensitive systems.