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An image captured during the New Horizons flyby in July 2015 shows Hydra, Pluto's outermost moon. (Credit: NASA / JHUAPL / SwRI)
An image captured during the New Horizons flyby in July 2015 shows Hydra, Pluto’s outermost moon. (Credit: NASA / JHUAPL / SwRI)

If you’re looking for a place to chip off an ice cube for a cool interplanetary drink, you can’t do much better than Hydra, Pluto’s outermost moon.

Newly released readings from NASA’s New Horizons mission reveal that Hydra’s surface is dominated by nearly pristine frozen water. It’s significantly purer than the mixed-up ice on the surface of Pluto – or even the water ice found on the surface on Charon, Pluto’s biggest moon.

The findings are drawn from the first compositional data relating to Pluto’s four smaller moons, including Hydra as well as Nix, Styx and Kerberos. The infrared spectral readings were acquired almost 10 months ago, when the piano-sized New Horizons spacecraft flew past the Plutonian system. But the data had to stay stored in the onboard computer’s memory until only recently.

About half of the data from the flyby has been sent back so far, and it will take months longer to send back the rest – including the readings for Pluto’s other small moons.

It’s not surprising that there’s water ice on Hydra. The moon is thought to have been formed from the lighter pieces of a debris disk that was created when an icy object slammed into a proto-Pluto billions of years ago.

But there’s still a mystery for scientists to puzzle over: Why is the stuff on Hydra’s surface so much purer than the water ice on Charon?

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

Charon and Hydra compared
New compositional data from NASA’s New Horizons spacecraft reveal a distinct water-ice signature on the surface of Pluto’s outermost moon, Hydra. The water is purer than it is on the surface of Charon, Pluto’s largest moon. Click on the image for a larger version. (Credit: NASA / JHUAPL / SwRI)

The hubbub over Hydra is just the latest in a string of findings from the New Horizons team. Here are some additional nuggets:

  • Pluto behaves less like a comet than expected and more like Mars or Venus in the way it interacts with the solar wind. Readings from New Horizons show that the solar wind, a continuous stream of electrically charged particles from the sun, is diverted around Pluto more abruptly than expected. Like Earth, Pluto has a long tail of heavy ion that extends “downwind” to a distance of at least 73,800 miles. The findings were published this week in the Journal of Geophysical Research – Space Physics.
  • The New Horizons team issued an updated global map of Pluto that incorporates the highest-resolution imagery currently available. There’s also a new elevation map showing that part of Pluto’s heart-shaped region, known as Sputnik Planum, is on average 2 miles lower than the surrounding terrain.
  • Other imagery captured during the flyby highlights several dozen “halo” craters in a region of Pluto known as Vega Terra. A comparison of the image with compositional data suggests that the floors of the craters and the terrain between them contain water ice, but that methane ice has settled on the rims and walls of the crater. It’s not yet known why.
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