Hot and Cold

This view shows Mercury's north polar region, colored by the maximum biannual surface temperature, which ranges from >400 K (red) to 50 K (purple). As expected for the Solar System's innermost planet, areas of Mercury's surface that are sunlit reach high temperatures, and hence most of this image is colored red!

In contrast, some craters near Mercury's poles have regions that remain permanently in shadow, and in these regions even the maximum temperatures can be extremely low. Evidence from MESSENGER and Earth-based observations indicate that water ice deposits are present in these cold craters. The craters nearest Mercury' poles have surface temperatures less than 100 K (-173°C, -280°F), and water ice is stable on the surface, such as in Prokofiev. However, many craters near but somewhat farther from Mercury's poles have cold, permanently shadowed interiors, but the maximum temperature is too high for water ice to persist at the surface. In these craters, water ice is present but is buried beneath a thin, low-reflectance volatile layer likely consisting of organic-rich material, such as in Berlioz crater.

http://photojournal.jpl.nasa.gov/catalog/PIA19247