Out of the entire Solar System, where would you least expect to find water ice?  Venus, of course, but if we discount that particular hell-planet I’d expect most people to say Mercury.  After all, it’s so close to the Sun that it’s surface temperature at midday is a full 400°C!  Yet NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) robotic spacecraft has found huge reserves of ice at the bottom of craters in Mercury’s polar regions.

Like all the inner planets, Mercury was heavily bombarded by asteroids and comets falling inwards from the outer solar system, billions of years ago.  Even today, it receives a constant pattering of much smaller meteroids, many of which were originally ejected by passing comets.  Since comets are made from a mixture of dust, water and complex carbon molecules (tar, for example), you expect to find a lot of these things on all the inner planets.  On Earth they’re preserved by our moderate climate and thick atmosphere, but Mercury has no such protection.  So how does it manage to accumulate in large enough amounts to be seen from orbit?

Mercury is pockmarked with large craters, just like the Moon, and it’s axis of rotation is aligned almost perfectly with it’s orbit around the Sun.  Together, these two factors mean that the craters near the North and South poles of Mercury are in permanent shadow and are never exposed to the fierce heat of the nearby Sun.  Throw in several million years worth of space-dust to make a nice insulating blanket, and it turns out that the accumulated water from all those ancient comets manages to freeze into quite a thick crust of ice.  Of course, by planetary standards it’s actually not that much.  But I think astronauts standing in one of these craters would be impressed to know that the ice sheet buried beneath their feet was the size of a major city, even if it is less than 20cm thick!