Charon, the largest of Pluto’s moons, might have an ocean beneath its crust, according to a new study by Alyssa Rhoden and colleagues published in the planetary science journal Icarus.
The dwarf planet and its moon, located between 2.7 billion and 4.5 billion miles from Earth (the variation is due partly to Pluto’s highly eccentric orbit) form a kind of double-planet system similar to the Earth and Moon. Charon’s diameter is more than half that of Pluto, and like the Earth and Moon, the system is thought to have originated with a giant impact, according to Amy Barr and Geoffrey Collins, writing in another recent Icarus paper.
Charon’s orbit around Pluto is nearly circular, but in the past, when it was more elliptical, huge tidal forces must have affected Charon’s interior. This would have created conditions very similar to what we see today at Jupiter’s moon Europa and Saturn’s moon Enceladus. Rhoden and her colleagues created computer models of the evolution of the Pluto-Charon system, which showed that if Charon’s orbit had only a minimal eccentricity early in its lifetime, enough heat would have been created from tidal stresses to create an ocean of liquid water under the surface.
Charon might in fact be more interesting than Pluto from an astrobiological viewpoint, because the moon’s surface appears to be covered by water ice, as compared to the nitrogen ice and methane ice on Pluto’s surface. Observations with the Gemini Observatory also have suggested geysers and patches of ammonia hydrates and water crystals on the surface of Charon. If the moon’s orbit was even a little more eccentric in the past, the tidal stresses likely would have generated cracks in the ice similar to the ones we see on Europa. The fracture pattern would be different depending on the thickness of the ice. Luckily, since Charon has no significant atmosphere, we should be able to see these fractures once the New Horizons spacecraft arrives at the Pluto-Charon system a year from now.
One of the most ambitious NASA missions ever launched, New Horizons has been speeding toward the outer solar system since January 2006. Since last July its cameras have been able to distinguish Pluto and Charon as separate objects, and around this time next year we may finally get our answer about an ocean on Charon. After its Pluto fly-by in July 2015, the spacecraft will continue on into the Kuiper Belt. The project just received a go-ahead to use the Hubble Space Telescope to search for a small Kuiper Belt object New Horizons could investigate up close.