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Icy Hydra outshines its dirty neighbour Charon

NASA's latest New Horizons data haul

NASA's latest data release from its New Horizons probe has confirmed the reason that Pluto's moon Hydra is covered by ice.

The agency says its analysis of the highly-reflective Hydra shows “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 “nearly pristine” water ice is showing absorption bands “even deeper” than that observed on Pluto's largest moon, Charon. Earlier this year, NASA explained that Charon's water ice surface is the reason for surface features snapped by New Horizons.

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

The NASA release speculates that Hydra is cleaner than Charon, because its gravity is too weak to hold onto contaminants.

Instead, when there's a collision with a micrometeorite, New Horizons team member Simon Porter suggests the impact “blasts off” contaminants.

The data was collected by New Horizons in last year's fly-by, but the probe is so distant and the connection so skinny that the data, collected by the probe's Ralph/Linear Etalon Imaging Spectral Array (LEISA) in July 2015, has only just been received and analysed. ®

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