Discovered in June 2005, Pluto's outermost moon Hydra is thought to have formed four billion years ago during a massive impact event that created Pluto and Charon.
Despite its age, this 31-mile-wide moon appeared remarkably clean and bright in New Horizons images during the spacecraft's historic close pass through the Pluto system in July 2015.
Scientists' initial speculation was proved right when data from the spacecraft was analyzed and revealed that Hydra, like its name, is covered in nearly pure water ice.
New Horizons Zooms In on Pluto's Tiny Moons Nix, Hydra
Measured with the Linear Etalon Imaging Spectral Array (LEISA) on New Horizons' Ralph instrument, the spectral signature of water ice on Hydra is even stronger than that seen on Pluto's much larger satellite Charon, indicating a surface coated with bigger ice particles and less dusty, dark material.