pluto and moons
June 14, 2014

New Horizons Mission To Analyze Pluto’s Moon For Evidence Of Underground Ocean

redOrbit Staff & Wire Reports - Your Universe Online

If there are cracks on the ice-covered surface of Charon, an upcoming analysis of those fractures could help determine whether or not the interior of Pluto’s moon was warm enough to have been home to a subterranean ocean of liquid water.

In April, research funded by the US space agency and published in the journal Icarus reported how tidally-driven fractures could have formed on the moon if its orbit was previously eccentric. Likewise, other moons in the outer solar system, including Jupiter's moon Europa and Saturn's moon Enceladus, also have surface fractures with evidence for ocean interiors.

Given Charon’s distance from the Earth, it has been difficult to make the detailed observations required to determine whether or not this is the case. However, that will change next July, as NASA's New Horizons spacecraft will travel to Pluto and its moon for the first time, providing the most detailed observations of each to date.

“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,” explained Alyssa Rhoden of NASA's Goddard Space Flight Center in Maryland and lead author of the Icarus research paper.

“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,” she added.

Pluto orbits the sun at a distance more than 29 times farther than Earth, and has a surface temperature estimated to be approximately 380 degrees below zero Fahrenheit. As such, this extremely distant world and its moons are much too cold for liquid water to exist on the surface – but underneath the surface might be a different story.

“As Europa and Enceladus move in their orbits, a gravitational tug-of-war between their respective parent planets and neighboring moons keeps their orbits from becoming circular,” NASA explained. “Instead, these moons have eccentric (slightly oval-shaped) orbits, which raise daily tides that flex the interior and stress the surface.”

It is believed tidal heating helped keep the interiors of those two moons warm, thus extending the lifespan of their subsurface oceans. However, that is not the case with Charon, as the study authors found prior high eccentricity may have produced large tides that resulted in friction and surface fractures.

Charon, which is about one-eighth of Pluto’s mass, is described as unusually massive in comparison to the planet it orbits. It is believed to have formed much closer to Pluto, having formed (along with several smaller moons) from material ejected from the planet’s surface following a giant impact that coalesced under its own gravity.

At first, there would have been strong tides on both Pluto and Charon as the gravity between them caused their surfaces to bulge towards one-another, creating friction beneath the surface. This friction, in turn, would have caused the tides to lag slightly behind their orbital positions – slowing Pluto’s rotation and transferring rotational energy to Charon. That would cause the moon to increase in velocity and move further 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,” said Rhoden. “Using plausible interior structure models that include an ocean, we found it wouldn't have taken much eccentricity (less than 0.01) to generate surface fractures like we are seeing on Europa.

“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,” she added. “This research gives us a head start on the New Horizons arrival – what should we look for and what can we learn from it. We're going to Pluto and Pluto is fascinating, but Charon is also going to be fascinating.”