Friday's Astronomy Picture of the Day from NASA (link) may be the most remarkable picture yet released of those taken by the New Horizons spacecraft of the distant dwarf planet Pluto. New Horizons shot a huge number of pictures as it flew at tremendous speed past Pluto and its moons July 14 and has since been transmitting them back to Earth.
The mountains are thought to be made of frozen water, what we on Earth usually call just plain "ice" but astronomers refer to as "water ice" to distinguish it from the frozen forms of other substances. In fact, much of the frozen surface you can see above is likely made of nitrogen ice and carbon monoxide ice.
Click on the picture for a larger version, and click the APOD link at the top for more information.