NASA's Messenger probe has already taken thousands of photos of Mercury's surface after coming into orbit around the planet earlier this week. And it's got plenty still to do, including searching for hidden pockets of ice on Mercury's boiling surface.
Mercury's surface temperatures can reach well over 800 degrees Fahrenheit, and the planet's slow but steady rotation means that pretty much everywhere on the planet is eventually covered in the Sun's fierce heat. And yet, for all these extremes, there's good reason to think that Mercury has deposits of frozen ice hidden in its poles.
The key is the existence of polar craters that are deep enough for their bottoms to never come into direct contact with sunlight. These areas would have absolutely freezing temperatures, perhaps reaching as low as minus 300 degrees Fahrenheit. That's obviously more than cold enough for ice to "survive" down there, safely tucked away from the Sun's rays.
The reasons astronomers suspect Mercury has ice on it in the first place comes from data gathered during earlier fly-by missions. Water is highly reflective of radio waves, and previous radio probes turned up some unusually reflective regions in the poles. These don't have to be water deposits, but astronomers consider that the most likely possibility.
If there is water on Mercury, there isn't an awful lot of it. Astronomers estimate there's between 10^14 and 10^15 kilograms of ice on the planet. This may sound like quite a bit, but Antarctica's ice sheet alone has more than 10^18 kilograms, and even Mars's polar regions clock in at 10^16 kilograms. We also don't know where the water has come from, but the most likely possibilities are that it was either ejected from within the planet's core or comets deposited the stuff into the planet's craters, with the comet impacts elsewhere on the planet quickly melting into nothing.
Though the search for water on Mercury is one of Messenger's most exciting tasks, it's far from the only one. For a look at some of the other aspects of its orbital mission, check out Space.com.