Mercury is the closest planet to the Sun. It is a rocky, or terrestrial, planet like Venus, Earth and Mars.
Mercury has the second highest density of all the
planets. Only the Earth is denser. Part of the high density of the
Earth is caused by gravitational contraction. The actual materials of
Mercury include a high proportion of dense materials like Iron. It is
believed that both Mercury and the Earth have molten Iron cores. Both
planets have significant magnetic fields although Mercury’s magnetic
field is not as strong as the Earth's.
Mercury does not have much atmosphere. It has a very thin one consisting mainly of Helium.
Mercury in Antiquity
Because it is so close to the Sun there are some
difficulties in seeing this planet. However, it can be seen with the
naked eye at some times of the year, either in the early morning or in
the evening.
Humans must have seen Mercury in the sky from the time
that they first started to look up and wonder about the lights in the
sky. The ancient Greeks were aware of Mercury, but did not realise that
the object that appeared in the sky in the morning was the same as the
one they sometimes saw in the evening.
Rotation and Revolution
Mercury revolves round the Sun every 88 Earth days.
For every two orbits of the Sun, Mercury rotates on its axis three
times. That is, Mercury’s year is one and a half of Mercury’s days.
Mercury’s orbit is more elliptical than any other
planet. Pluto’s orbit is more elliptical, but Pluto is considered a
dwarf planet in most places now. Mercury’s orbit is also about 7
degrees out of the planetary plane.
Probes
Mercury has only been visited twice by Earth spacecraft.
In 1975, Mariner 10 succeeded in doing flybys of the planet and mapped 45 percent of its surface.
Launched in 2004, the MESSENGER space probe has already done flybys of Mercury, as well as the Earth and Venus, and is due to go into orbit round Mercury in 2011.
The Future
So far, not much attention has been directed to
Mercury. There are obvious difficulties in using a planet so close to
the Sun. However, this does not mean this planet will never be used.
There have been several science fiction stories including the idea that
Mercury will be settled and the apparently abundant minerals used, as
well as making use of the high intensity solar energy.
These stories include Rendezvous with Rama by Arthur C Clarke, and the
Red Mars, Green Mars, Blue Mars trilogy by Kim Stanley Robinson. These
are both very good books, not only having excellent stories, but also
they both have real ideas about the future.
Population Growth & Evolution 
