It's Just a Phase

Explanation: Nitrogen, methane, and carbon monoxide are detected as solid ices on Pluto's surface, and will be mapped by the instruments aboard New Horizons. But they cannot melt and become liquid, because Pluto's surface pressure is too low - Pluto's ices sublimate directly to gas.

Pluto

Image credit: NASA/JHUAPL/W. McKinnon/Nature.