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Pluto's Moon Coated in Nearly Pure Water Ice

Despite its age, this 31-mile-wide moon appeared remarkably clean and bright in images.

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.

Spectral data gathered by New Horizons shows Hydra to be coated in a much more pure ice material than the larger Charon. | NASA/JHUAPL/SwRI

"Perhaps micrometeorite impacts continually refresh the surface of Hydra by blasting off contaminants," said Simon Porter, a New Horizons science team member from the Southwest Research Institute (SwRI) in Boulder, Colorado. "This process would have been ineffective on the much larger Charon, whose much stronger gravity retains any debris created by these impacts."

Photos: NASA's Mission to Pluto by the Numbers

Only a small point of light in even the best Hubble images prior to July 2015, New Horizons resolved Hydra as an irregular moon shaped "like the state of Michigan" with at least two relatively large craters and an upper region slightly darker than the lower, implying a difference in surface composition.

Source: NASA

Hydra imaged by New Horizons on July 14, 2015 from a distance of about 143,000 miles (231,000 km).

After several false starts, NASA in 2001 agreed to fund an independent effort to fly a spacecraft to Pluto, the only member of the solar system’s original nine planets that hadn’t been explored. Five years later, New Horizons blasted off to begin a nearly 3 billion mile journey to Pluto, farther than any probe has traveled since the 1970s-era Pioneer and Voyager spacecraft.

Here’s a look at the New Horizons mission by the numbers.

MORE: From the Start, Pluto was a Puzzle: Timeline

Launching a small spacecraft on a big rocket is one way to get going fast. Slingshotting off giant Jupiter’s gravity is another. New Horizons did both, and still the journey to distant Pluto took nearly 10 years. It is zipping along at about 31,000 mph -- fast enough to fly from New York City to Los Angeles in less than 5 minutes.

MORE: Fuzzy to Clear: Space Robots Snap Solar System Into Focus

Image: Viewed from the top of the Vehicle Assembly Building at Kennedy Space Center, NASA’s New Horizons spacecraft roars off the launch pad aboard an Atlas V rocket on Jan. 19, 2006.

At its closest approach, New Horizons will pass about 7,750 miles from Pluto and about 17,900 miles from its orbital mate Charon. The view will be about 500 times better than this image, taken on July 7 when New Horizons was just less than 5 million miles from Pluto. New Horizons will pass through the Pluto system in about 30 minutes. The probe carries seven science instruments, including LORRI, the Long Range Reconnaissance Imager, telescope.

MORE: Once Just a Speck of Light, Pluto About to Be Unveiled

During the encounter, New Horizons will take hundreds of pictures in both visible and near-infrared wavelengths. The best images should depict surface features as small as 200 feet across. With nearly 3 billion miles between New Horizons and Earth, a radio signal, which travels at the speed of light, will take about 4.5 hours to reach Earth.

MORE: New Horizons is Carrying 9 Stowaways to Pluto

Image: An artist's impression of Pluto's surface reveals an icy surface -- we're about to find out what Pluto is really made of.

With just one shot to get a close-up view of Pluto, New Horizons is designed to gather as much data as possible, as quickly as possible. In all, scientists expect the spacecraft to collect 100 times more data during closest approach than it can transmit back to Earth just after the encounter. A few high-priority images and data will be sent back just before and after closest approach, but the rest will trickle in over the next 16 months.

MORE: Pluto Flyby Begins: NASA Probe Enters Encounter Phase

Image: Diagram showing the sequence of events during New Horizons' encounter with the Pluto system.

New Horizons draws electricity from a single radioisotope thermoelectric generator, or RTG, which converts heat given off by the natural decay of about 24 pounds of radioactive plutonium. It runs on less power than a pair of 110-watt light bulbs.

MORE: Student Experiment Will Count Cosmic Particles Around Pluto

Image: Artist's impression of New Horizons flying past Jupiter, with its RTG visible in the lower right of the image.

After its Pluto flyby, New Horizons will continue out into the Kuiper Belt region of the solar system. Scientists hope to extend its mission so it can pass by at least one of the thousands of icy bodies that orbit in this vast domain. Eventually, New Horizons will end up leaving the solar system. It is expected to remain viable until the late 2030s.

MORE: After Pluto, Where Will NASA's New Horizons Go?

Image: Artist's impression of New Horizons encountering a Kuiper Belt object beyond Pluto.