Discovered in June 2005, distant Pluto’s outermost moon Hydra it thought to have formed during the same collision four billion years ago that created the Pluto-Charon system that we see today. Yet despite its age this 31-mile (50-km) -long moon appears remarkably clean and bright, as witnessed by New Horizons during its close pass through the Pluto system in July 2015.
Measured with the Linear Etalon Imaging Spectral Array (LEISA) on New Horizons’ Ralph instrument, the spectral signature of water ice on Hydra is even purer than that seen on Pluto’s much larger satellite Charon, implying larger ice particles with less dusty, dark material.
“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). “This process would have been ineffective on the much larger Charon, whose much stronger gravity retains any debris created by these impacts.”
Only a small point of light in even the best Hubble images prior to July 2015, Hydra was seen by New Horizons to be 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. Read more here.