Scientists have made a big discovery on Pluto’s moon Hydra based on compositional data gathered by NASA’s New Horizons spacecraft.

The data shows that Hydra, the outermost of Pluto’s small moons, is covered in almost pristine water ice, which explains why Hydra had such a reflective surface when New Horizons photographed it, according to a NASA statement.

NASA has been receiving data at a slow trickle from New Horizons’ flyby of Pluto last year. The new compositional data was gathered July 14, 2015, at a distance of 150,000 miles by the Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument.

Pluto’s big moon, Charon, is also dominated by crystalline water ice, but Hydra’s water-ice absorption bands are deeper, indicating that the ice grains on Hydra are larger than Charon’s, according to the statement.

Scientists are trying to figure out why Hydra’s ice is so much cleaner than Charon’s. One suggestion is that it has continuous micrometeorite impacts, knocking off contaminants, something that wouldn’t work on the larger CHaron due to its gravity.

“The new data – known as infrared spectra – show the unmistakable signature of crystalline water ice: a broad absorption from 1.50 to 1.60 microns and a narrower water-ice spectral feature at 1.65 microns,” the statement reads. “The Hydra spectrum is similar to that of Pluto’s largest moon, Charon, which is also dominated by crystalline water ice. But Hydra’s water-ice absorption bands are even deeper than Charon’s, suggesting that ice grains on Hydra’s surface are larger or reflect more light at certain angles than the grains on Charon.

“Hydra is thought to have formed in an icy debris disk produced when water-rich mantles were stripped from the two bodies that collided to form the Pluto-Charon binary some 4 billion years ago,” the statement continues. “Hydra’s deep water bands and high reflectance imply relatively little contamination by darker material that has accumulated on Charon’s surface over time.”