The first compositional data from NASA’s New Horizons spacecraft about Pluto’s small, irregularly shaped moon Hydra show the moon’s surface is dominated by nearly pristine water ice.

The new data – known as infrared spectra – were gathered by New Horizons’ Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument on July 14, 2015, from a distance of 150,000 miles (240,000 km).

The data 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.

Members of the New Horizons science team said that the Hydra spectrum is similar to that of 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.

According to the scientists, Hydra is 31 miles (50 km) long, 25 miles (40 km) wide, and its irregular shape resembles the state of Michigan.

It 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 small moon’s deep water bands and high reflectance imply relatively little contamination by darker material that has accumulated on Charon’s surface over time.

The New Horizons science team is investigating why Hydra’s ice seems to be cleaner than Charon’s.

“Perhaps micrometeorite impacts continually refresh the surface of Hydra by blasting off contaminants,” said team member Dr. Simon Porter, from Southwest Research Institute.

“This process would have been ineffective on the much larger Charon, whose much stronger gravity retains any debris created by these impacts.”

According to NASA, New Horizons is currently 3.24 billion miles (5.21 billion km) from Earth and 223 million miles (358.9 million km) beyond Pluto, with all systems healthy and operating normally.

The spacecraft is on course for a close flyby of the Kuiper Belt object 2014 MU69 in 2019.