Mercury

Mercury is a lump of rock, barely larger than the Moon, living under the fierce heat of the burning Sun. As it quietly spins out a day it experiences extremes in temperature from over 400°C at noon (hot enough to melt solid zinc) to less than -150°C at night.

At first, Mercury sounds like the last place in the Solar System you would go searching for ice. Scorched by the burning heat of the Sun any exposed frozen water would instantly be evaporated. Recently however, a lunar probe found evidence that ice is stored in craters in the north and south poles on the Moon. This data suggests that deep craters near Mercury's poles may also contain ice deposits.

Remember that when the surface is not lit by the Sun, the temperature plummets to as low as -150°C. Because Mercury does not have an atmosphere, heat is not conducted from the hot sunlit surface to these so called "cold traps". This opens up the possibility that substantial pools of frozen water may exist in permanently shaded regions.

On the surface Mercury is very similar to our Moon, a battered world, scarred by aeons of impacts by meteorites. The surface is also wrinkled, with great ridges hundreds of kilometres long called scarps, which probably appeared when Mercury cooled and shrunk soon after it formed. By measuring its weight we can tell that inside, Mercury is much more like the Earth than the Moon. It has a core made of iron, and the centre may be molten, kept hot by radiation deep within the core.

It has been known for centuries that Mercury revolves around the Sun once every 88 days, and for a long time it was thought that length of a day was the same. This century scientists bounced a radar beam off the planet and found the true period to be 58.6 days, exactly two-thirds the orbital period. This strange relationship between the orbit and the spin means that on Mercury, a single day lasts twice as long as a year!