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Mercury - A planet of extremes
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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!

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Images
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1.  Mosaic of Mariner 10 images
(http://www.solarsystem.f2s.com/mercury/img1a.jpg)

Images from the 1975 Mariner 10 mission have been reprocessed to create 
this dramatic mosaic. It clearly shows the scars of impact cratering 
all over Mercury's battered surface (the smooth band and patches 
represent regions where no image information is available).

© Astrogeology Team, U.S. Geological Survey 

2.  Mercury's Antoniadi ridge
(http://www.solarsystem.f2s.com/mercury/img2a.jpg)

This is an image of a 450 kilometre (280 mile) ridge called Antoniadi. It 
travels along the right edge of the image, and transects a large 80 kilometre 
(50 mile) crater about half way in between. It crosses smooth plains to the 
north and intercrater plains to the south. 

© Calvin J. Hamilton

3.  Close up of Mercury's surface features
(http://www.solarsystem.f2s.com/mercury/img3a.jpg)

This Mariner 10 photograph shows Santa Maria Rupes, the sinuous 
dark feature running through the crater at the centre of the 
image. This is an example of the enormous cliffs (known as 
scarps) which cuts across much of the planet's surface.

© Calvin J. Hamilton

4.  The surface of Mercury from space
(http://www.solarsystem.f2s.com/mercury/img4a.jpg)

This image of Mercury was constructed from photos taken by 
Mariner 10 a few hours before the spacecraft's closest and 
first encounter with the planet on March 29, 1974. It clearly 
shows the heavy cratering of the surface.

© Calvin J. Hamilton