Of the five planets visible to the naked eye on the night sky, Mercury is the most difficult to find. Its close orbit to the sun makes it elusive.
Mercury's size is almost one third of the Earth's and it is the second smallest planet in our known solar system. In fact, it is even smaller then Jupiter's moon Ganymede and Saturn's moon Titan.
Mercury's eccentric orbit takes it as close as 46 million km from the sun at perihelion (when it is closest to the sun) to about 70 million km at aphelion (when it is furthest away) and it orbits the sun in merely 88 days. Its vicinity to the sun has left Mercury with a very thin atmosphere and a surface temperature of about 400 degrees Kelvin.
The surface of Mercury is very similar to our moon's: It is severely scarred with craters it has received in its lifetime. Some craters at the north/south poles cast permanent shadows where the temperature could get as low as -200 degrees Celsius.
The only probe that has visited Mercury to this date is Mariner 10 (1974-1975). During its flybys, it took over 2,700 pictures, covering 45% of Mercury's surface.
Astronomers long believed that Mercury had a tidally locked orbit, which means that it always shows the same "face" towards us, like our moon always shows the same face towards us. Mercury has a 3:2 spin-orbit resonance, which means that for every 3 times it spins (one spin is one Mercurian day) around its axe, it completes 2 orbits around the sun. The reason was that when the planet was best placed for observation it was always at the same state in this resonance.
However, doppler radar observations in 1965 showed that this was wrong.
Mercury could have had a much faster rotation period in the past, perhaps it only took eight hours to make a spin around its axis. This fast spinning could have been slowed down by the sun's tidal forces, making a spin slower and slower.
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Mercury has a surprisingly large iron core, which occupies about 75% of the radius. Mercury is thought to consist of about 70% of metals and the rest to be silicates. Outside the core is a 600 km thick crust. The density is 5.44 g/cm^3, which is slightly less than Earth's 5.52g/cm^3. But why is the mercurian core so large compared to the other terrestrial planets?
One theory suggests that the planet was much larger in the past (2.25 times its current mass), and was struck by an object 1/6th of that mass. The collission was so powerful that it stripped off much matter from Mercury's outer crust and mantle, leaving the iron core and some of the crust behind.
Another theory states that Mercury formed very early when the sun had yet not stabilized and there were much gas in that region. The temperature could have been as high as 2 000 - 3 500 degrees, vaporizing the surface.
A third theory says that the intensity of the sun light has been eroding the surface for billions of years.
Mariner 10 discovered that Mercury has a magnetic field that is 1% as strong as Earth's. As the core of iron is believed to be solid astronomers think that there might be another liquid substance that is generating it, possibly sulphur.
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As mentioned earlier, Mariner 10 is the only spacecract that has visited Mercury to this date. However a new spacecraft has been sent to Mercury, and is scheduled to arrive 2011. The probe is called MESSENGER. Mariner 10 only mapped about 45% of the Mercurian surface, but with MESSENGER astronomers are hoping not only to map the entire planet, but also to gain an understanding of how our solarsystem was formed.
More about the exploration of Mercury:
Universe Today article
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Previous: The Sun.
Average distance from sun:
57 909 175 km.
Equatorial Radius: 2 439.7 km.
Mass: 3.3022 x 10^23 kg.
Density: 5.427 g/cm^3.
Escape Velocity: 4.25 x 10^3 m/s.
Length of day: 58.646 Earth days.
Length of year: 87.97 Earth days.
Mean Orbit Velocity: 47.873 km/s.
Equatorial Inclination to Orbit: 0 degrees.
Min./Max. Surface Temperature: -173/427 °C.
Atmosphere: Helium 42%, Sodium 42%, Oxygen 15%, other 1%.
Space Art 1: Mercury's surface is heavily scarred with craters, reminding of our own moon. This is a hypothetic view.