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Pluto's Largest Moon May Have Had an Underground Ocean

Theorized cracks on Charon's surface point to the possibility of subsurface water.
Artist's conception of Pluto from one of its moons' surfaces. Image: NASA, ESA, and G. Bacon

Just because Pluto isn’t a “proper” planet anymore doesn’t mean the dwarf planet and its system of moons is any less fascinating. Case in point: a new NASA-funded study suggests that theorized cracks on the surface of Pluto’s largest moon Charon might be evidence that the small body’s interior is warm, perhaps warm enough to have once maintained a subsurface ocean.

Think for a second how insane that would be. Pluto is more than 29 times further from the Sun than the Earth. It’s average surface temperature is estimated to be 380 degrees below zero Fahrenheit (which is about 229 degrees below zero Celcius). That’s way too cold for water to exist as a liquid on the surface, and Pluto’s five moons are all in the same frigid environment.

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A subsurface ocean is not an impossible idea. There are moons in our solar system—like Jupiter's Europa and Saturn's Enceladus—whose cracked icy surfaces cover such bodies of liquid water. In these two moons’ cases, it’s the gravitational tug of war between their respective parent planet and their neighboring moons that cracks the surface. The dueling gravitational environments keep the moons’ orbits from becoming circular, which in turn leads to increased tidal forces. These forces stretch and heat the moons' interiors, stressing their surfaces.

Charon's case is a little different: The moon’s gravitational environment is less extreme than those surrounding the gas giants Jupiter and Saturn. It's only Charon’s past that researchers think might have led to a subsurface ocean. If the moon once sat in a highly eccentric orbit around Pluto, the gravitational environment could have generated tides large enough to cause internal friction and surface fractures.

Charon has about one-eighth the mass of Pluto, a record relationship between a host (dwarf) planet and its main satellite. And it’s thought that Charon might have formed far closer to Pluto than it now orbits, the result of some giant impact ejecting material off the nascent dwarf planet's surface. The ejected material would have gone into orbit around Pluto and coalesced to form Charon and the other, smaller moons.

"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," said Alyssa Rhoden, a scientist from NASA’s Goddard Space Flight Center.

And the evolution of Charon’s orbit would also have affected possible tidal forces on Charon. "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."

Of course, this is all theoretical for the moment. We can’t see any details of Charon’s surface from the Earth. But we’re going to get a good look at the tiny body soon. In July of 2015, the New Horizons mission will make a close pass by Pluto’s system, making the first ever dedicated observations of the dwarf planet and its kin. "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.”

If it turns out that there once was a subsurface ocean on Charon, it might raise the body’s standing on the intrigue scale in the solar system. Life needs liquid water, a useable energy source, and a healthy supply of key elements like carbon, nitrogen, and phosphorus, and surface cracks might indicate that all these things once existed on Charon. We just have a year to wait.