Pluto probably has a liquid ocean sandwiched between a rocky core and an icy shell.
When NASA鈥檚 New Horizons spacecraft flew by the tiny world in July 2015, it captured the sharpest-ever images of the planet鈥檚 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.
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鈥淚f 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,鈥� Hammond says. This would make Pluto鈥檚 surface buckle, like the skin of an overripe peach wrinkling as it dries.
But that鈥檚 not what New Horizons saw. Instead, it saw deep cracks. That suggests the dwarf planet is slowly growing bigger, with normal ice 鈥� which has a larger volume than liquid water 鈥� still forming slowly. If this is the case, then no ice-II will have formed, and something must be keeping the ocean wet 鈥� probably heat from the decay of radioactive elements in Pluto鈥檚 rocky core.
Long-lasting liquid
It鈥檚 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鈥檚 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鈥檚 outer edge, isn鈥檛 as dry as we thought, Nimmo says.
The moons of gas giants, like Jupiter鈥檚 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鈥檚 tidal pull. 鈥淵ou don鈥檛 need tidal heating in order to have an ocean 鈥� that鈥檚 an important lesson,鈥� Nimmo says. 鈥淚t means that other big Kuiper Belt objects out there could have oceans, too.鈥�
What鈥檚 more, rocky bodies like Pluto could be more hospitable to life than watery moons, Nimmo says. On Ganymede, the ocean鈥檚 bottom is another layer of ice. Pluto probably has a rocky seabed, which could provide the chemicals needed for life.
And the habitat isn鈥檛 drying up anytime soon. 鈥淚t鈥檚 probably existed for maybe 2 to 3 billion years, and it probably has another billion years to go, give or take,鈥� he says.
Journal reference: Geophysical Research Letters, DOI:
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