Mercury, named after the Roman messenger to the gods, is the closest planet to the Sun and the second smallest in the Solar System after Pluto, the most distant planet. It orbits the Sun in 88 days, meaning that for every year we have on Earth, four years have past on Mercury. However, it takes 176 days for the Sun to completely rise and set over the planet's horizon, compared with the 24 hours it takes for the same thing to happen on Earth. It is because of Mercury's speed across the face of the Sun that it is known as the "Messenger" planet. Mercury is so small and so close to the sun that it is almost impossible to see. With telescopes, just after dawn or before sunset, the planet can be seen as a small spot in front of the Sun's surface.
Mercury in front of
the Sun. The other dark spots are sunspots
Mercury's surface looks very similar to the Moon's. It is covered in craters, although does not have the large dark 'oceans' that we see on the Moon's surface. This is because the planet has no atmosphere or air. If small rocks (asteroids and meteors) head for Earth, most burn up in the planet's atmosphere and do not hit the surface. But, because Mercury and the Moon don't have atmospheres (explaining why their skies are always black), meteors heading for them hit the surface and leave craters.
Craters on Mercury
Life on Mercury is impossible. Temperatures on the planet can be extremely hot or extremely cold depending on which side of it is facing the Sun (From 375°c to -200°c). Mercury spins on its axis very slowly, so one side of the planet can be in complete darkness for days. The planet is very heavy for its size. Astronomers believe that it has a large iron core which may explain why the planet weighs so much. Earth is a geologically living planet. Its continents still move, and hot lava is churned out of its core (centre) through volcanic eruptions. Mercury is geologically dead. The surface shows no sign of volcanic activity or any other form of geological activity. It is for this reason that astronomers believe Mercury's core to be extremely cold.
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