When NASA's New Horizons spacecraft reached Pluto last July, it gave scientists their first detailed look at one of the most mysterious objects in the solar system. In addition to wonders like soaring mountains, ice volcanoes and a giant heart-shaped basin, images beamed back by the probe revealed a surface marred by a network of fissures and a notably spherical shape.
For some scientists, those last two discoveries are hints that something even wilder may be hidden inside the tiny world, because they are the first direct clues that Pluto could host a subsurface ocean beneath its thick, icy crust. If confirmed, an ocean on Pluto would have profound implications, because it would increase the likelihood that other icy bodies could host liquid water—and possibly life.
"The fact that even cold, distant Pluto could have a subsurface ocean means that there are potential habitats even in apparently unpromising locations," says Francis Nimmo, a New Horizons scientist based at the University of California, Santa Cruz.
Aside from Earth, no bodies in the solar system have large amounts of liquid water on their surfaces. That's a bummer for astrobiologists, as most scientists believe that water is a necessary ingredient for life to arise.
Still, space probes have been collecting evidence for decades that icy moons around Jupiter and Saturn hold vast oceans beneath their crusts. Saturn's moon Enceladus spews geysers that are tantalizingly rich with water and carbon, while Jupiter's Europa is covered in fractures and ridges that hint at a subsurface ocean melting through the ice. These worlds are currently considered some of the best places to look for life elsewhere in the solar system.
Pluto is similarly icy, but the difference is that those moons have more obvious sources of heat to keep internal water liquid: the gravitational kneading they receive as they swing around their massive parent planets. Pluto has no massive companion and orbits between 3 and 5 billion miles from the sun, so astronomers mostly thought it must be too cold for a modern ocean.
Some theoretical models suggested that radioactive decay in Pluto's rocky interior could heat things up enough to create a subsurface ocean at some point in its history, maybe even enough heat that waters persist today, but there was no real evidence, says Nimmo—until now.
Speaking at a recent meeting of the American Geophysical Union (AGU) in San Francisco, Nimmo outlined two key clues from New Horizons. Neither one alone is a slam dunk, he says, but together, they're suggestive.
First, New Horizons revealed the presence of extensional tectonics, faults and fissures across the face of Pluto that could indicate the surface has undergone expansion in the recent past.
"An easy way of doing that is if you have an ocean that's starting to refreeze," Nimmo says, because water expands in volume as it changes from a liquid to a solid. "As the liquid water freezes back into ice, the outer surface of Pluto has to move outward, and you get expansion."
The second piece of evidence has to do with Pluto's shape, in particular, the notable lack of a bulge around its equator like the one found on Earth, its moon and other rounded celestial bodies.
As spherical bodies spin, the rotational forces push material toward the equator, flattening them out somewhat. The moon's equatorial bulge is even greater than it should be given its current rotation rate, and scientists think that's because it was spinning faster earlier in its history, when lunar rock was more ductile. By contrast, although Pluto is spinning faster than our moon, it has no bulge at all.
"The moon is recording an ancient spin state," Nimmo says. "Pluto shows no evidence of that. There are different ways of destroying a fossil bulge, and one of them is to have an ocean." That's because water has more freedom of motion than ice, so a global liquid layer sloshing around inside would help counteract the spinning forces, reducing such a bulge.
So far, the New Horizons team is making a pretty solid case for an ocean on Pluto, says Amy Barr Mlinar, an expert in the formation and evolution of solid planetary bodies at the Planetary Science Institute in Tucson, Arizona.
“It's based on a basic planetary-science type of analysis. It doesn't require a lot of fancy modeling where there are 45 different input parameters that can be messed up," says Barr Mlinar.
But not everyone is convinced just yet, even other members of the New Horizons team. Pluto's surface cracks could be explained by other internal changes in the ice's temperature or structure, says Bill McKinnon, a planetary scientist at Washington University in St. Louis.
“Likewise, the collapse of a fossil bulge is consistent with an ocean on Pluto," McKinnon says. "But an ocean is not required. Nor does it mean the ocean, even if it did exist, has to exist today. The collapse of the fossil bulge could have occurred billions of years ago."
New Horizons performed a single flyby of Pluto. For more concrete proof of Pluto’s ocean, “we would need to go back with an orbiter mission, maybe later in this century,” McKinnon says.
If future tests do confirm the presence of an ocean on Pluto, McKinnon thinks there could be even more hidden seas waiting to be discovered in the fringes of the solar system. Pluto is part of the Kuiper belt, a ring of similar bodies that could also be generating internal heat from radioactive decay.
"Other large Kuiper belt objects are similarly or even more rock-rich, so these worlds could also have oceans," he says.
Such distant oceans would be very different from what we're accustomed to on Earth, notes Nadine Barlow, an astronomer at Northern Arizona University. Besides being locked beneath dozens of feet of ice, a Plutonian ocean would almost certainly have a different composition than Earth's seas.
"We have to remember that the ices out at Pluto not only include water ice but also carbon dioxide and methane ices," says Barlow. Compared to our seas, Pluto's potential ocean would also likely be especially briny, rich in dissolved salts and ammonia that would help reduce its freezing point and keep it in a liquid state.
Those extra ingredients would make Pluto's seawater unappealing to astronauts, but it's still possible some forms of extreme life could call such an ocean home. And while New Horizons has already sped away from Pluto towards its next Kuiper belt target, NASA's planned mission to the Jovian moon Europa might be a crucial testing ground for studying subsurface oceans on icy bodies and determining their feasibility for hosting life.
That means the Europa mission and any future treks to explore Pluto will need to take precautions so as not to contaminate any potentially life-supporting environments with terrestrial organisms, says Barlow.
Barr Mlinar agrees: "We may have to think of clever ways to explore the chemistry of Pluto's ocean from the surface," she says. "We have to learn more about the geology of these bodies and how material from the ocean can be expressed on the surface."