Underneath the icy exterior of Pluto’s giant moon, there may have been a warm, life-hosting interior that’s now long gone. According to a new NASA study, analyzing the cracks on the surface could reveal if Charon’s interior was warm enough to have maintained an underground ocean of liquid water.
Orbiting the sun more than 29 times farther than Earth, Pluto and its moons have an estimated surface temperature of minus 229 degrees Celsius. In July 2015, NASA’s New Horizons spacecraft will visit Pluto and Charon -- the first ever to do so.
"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," Alyssa Rhoden of NASA's Goddard Space Flight Center explains in a news release. "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."
Eccentricity refers to how much an orbit deviates from a perfect circle. Jupiter's moon Europa and Saturn's moon Enceladus, for example, have cracked surfaces with evidence for ocean interiors. A gravitational tug-of-war between their parent planets and neighboring moons keep the orbits of Europa and Enceladus oval shaped. These eccentric orbits raise daily tides that flex the interior and stress the surface, and scientists believe that tidal heating extended the lifetimes of subsurface oceans by keeping the interior of those moons warm.
Right now, Charon’s orbit is stable and circular. But according to the new study, past high eccentricity could have generated large tides that caused friction and surface fractures. Charon is unusually massive compared to its planet -- about one-eighth of Pluto’s mass, a solar system record -- and it’s thought to have formed close to Pluto after a giant impact ejected material off the planet’s surface, which went into orbit and coalesced to form several moons.
Gravity between Pluto and Charon would have caused their surfaces to bulge toward each other, generating friction in their interiors and creating strong tides immediately. But that friction would have caused the tides to lag behind their orbital positions, slowing Pluto’s rotation, while transferring that rotational energy to Charon. That would have made Charon speed up and move 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," Rhoden explains. According to models of the interior structure, it wouldn't have taken much eccentricity (less than 0.01) to generate surface fractures like those seen on Europa.
She adds: "Since it's so easy to get fractures, if we get to Charon and there are none, it puts a very strong constraint on how high the eccentricity could have been and how warm the interior ever could have been.” At the moment, its circular orbit means that if there was an ancient underground ocean, it would be frozen by now.
The work was published in Icarus.
Images: NASA, ESA and G. Bacon (STScI)