Charon – the largest moon of the dwarf planet Pluto – may once have had a warm subterranean ocean, suggests a team of planetary scientists led by Dr Alyssa Rhoden of NASA’s Goddard Space Flight Center.
This artist’s impression shows the surface of Pluton, Charon and the Sun. Image credit: ESO / L. Calçada.
Discovered in June 1978 by United States Naval Observatory astronomer James Christy, Charon is the largest of Pluto’s five moons.
It has a diameter of about 1,207 km, just over half that of Pluto, and a mass about one-eighth that of Pluto.
Unlike Pluto, which is surrounded by a thin envelope of nitrogen, methane and carbon monoxide gases, Charon seems to have no atmosphere.
In 2007, observations of patches of ammonia hydrates and water crystals on the Charon’s surface suggested the presence of cryo-geysers.
NASA’s New Horizons mission is scheduled to visit Charon in July 2015.
“By comparing the actual New Horizons observations of Charon to the various predictions, we can see what fits best and discover if Charon could have had a subsurface ocean in its past, driven by high eccentricity,” said Dr Rhoden, who is the first author of a paper published in the journal Icarus.
“Our model predicts different fracture patterns on the surface of Charon depending on the thickness of its surface ice, the structure of the moon’s interior and how easily it deforms, and how its orbit evolved.”
The scientists found that a past high eccentricity could have generated large tides, causing friction and surface fractures.
Charon is unusually massive compared to its planet. It is thought to have formed much closer to Pluto, after a giant impact ejected material off the planet’s surface. The material went into orbit around Pluto and coalesced under its own gravity to form Charon and several smaller moons.
Initially, there would have been strong tides on both objects as gravity between Pluto and Charon caused their surfaces to bulge toward each other, generating friction in their interiors. This friction would have also caused the tides to slightly lag behind their orbital positions.
The lag would act like a brake on Pluto, causing its rotation to slow while transferring that rotational energy to Charon, making it speed up and move farther away from Pluto.
“Depending on exactly how Charon’s orbit evolved, particularly if it went through a high-eccentricity phase, there may have been enough heat from tidal deformation to maintain liquid water beneath the surface of Charon for some time,” Dr Rhoden said.
“Using plausible interior structure models that include an ocean, we found it wouldn’t have taken much eccentricity to generate surface fractures like we are seeing on Europa.”
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Alyssa Rose Rhodena et al. The interior and orbital evolution of Charon as preserved in its geologic record. Icarus, published online April 30, 2014; doi: 10.1016/j.icarus.2014.04.030
