Ů��С��Ƶ

Pluto probably has a liquid ocean sandwiched between a rocky core and an icy shell.

When NASA’s New Horizons spacecraft flew by the tiny world in July 2015, it captured the sharpest-ever images of the planet’s surface. That close-up helped Noah Hammond of Brown University in Rhode Island and colleagues show that Pluto never formed a bizarre phase of ice that would solidify its ocean for good.

If Pluto ever had a liquid sea that froze solid, the pressure from the heavy outer ice shell would squish the subsurface ocean into a denser phase called ice-II, which has a smaller volume than liquid water.

“If the oceans were to freeze completely, soon after that you form this ice-II, which would cause all of Pluto to undergo a huge volume contraction,” Hammond says. This would make Pluto’s surface buckle, like the skin of an overripe peach wrinkling as it dries.

But that’s not what New Horizons saw. Instead, it saw deep cracks. That suggests the dwarf planet is slowly growing bigger, with normal ice – which has a larger volume than liquid water – still forming slowly. If this is the case, then no ice-II will have formed, and something must be keeping the ocean wet – probably heat from the decay of radioactive elements in Pluto’s rocky core.

Long-lasting liquid

It’s not the first time someone has suggested Pluto has a subsurface sea. In 2011, of the University of California at Santa Cruz argued that the dwarf planet’s icy shell could insulate an ocean. The Brown study strengthens that idea, and also suggests that the Kuiper belt, the region of icy worlds at the solar system’s outer edge, isn’t as dry as we thought, Nimmo says.

The moons of gas giants, like Jupiter’s moon Ganymede, have subsurface oceans because tidal forces from the planet keeps them sloshing around. In contrast, Pluto seems to have a liquid ocean despite not experiencing a large planet’s tidal pull. “You don’t need tidal heating in order to have an ocean – that’s an important lesson,” Nimmo says. “It means that other big Kuiper Belt objects out there could have oceans, too.”

What’s more, rocky bodies like Pluto could be more hospitable to life than watery moons, Nimmo says. On Ganymede, the ocean’s bottom is another layer of ice. Pluto probably has a rocky seabed, which could provide the chemicals needed for life.

And the habitat isn’t drying up anytime soon. “It’s probably existed for maybe 2 to 3 billion years, and it probably has another billion years to go, give or take,” he says.

Journal reference: Geophysical Research Letters, DOI:

Read more: New Horizons shows Pluto sporting blue skies and red water ice