Data from the New Horizons mission revealed manyand hinted at others. One tantalizing tidbit was the prospect of an ammonia-rich ocean of water under the surface, a liquid sea beneath the frozen plains.
"Thanks to the incredible data returned by New Horizons, we were able to observe tectonic features on Pluto's surface, update our thermal evolution model with new data and infer that Pluto most likely has a subsurface ocean today," said Noah Hammond, a graduate student in Brown University's Department of Earth, Environmental and Planetary Sciences who was lead author of a paper on the subject. The paper was published in the journal Geophysical Research Letters.
The team updated a thermal model of Pluto based on detailed data from the New Horizons probe. Those include tectonic features such as fault lines hundreds of kilometers long and mountains of water ice that indicate Pluto is slowly expanding over time.
"What New Horizons showed was that there are extensional tectonic features, which indicate that Pluto underwent a period of global expansion," Hammond said. "A subsurface ocean that was slowly freezing over would cause this kind of expansion."
How could the water remain liquid so far from the sun? The slow natural decay of radioactive elements in Pluto's core may have been sufficient to keep the water unfrozen beneath the icy surface. Over time, however, that water could have started to refreeze. Since water expands as it turns into ice, this would have produced the features seen on Pluto's surface.
"We now have half a dozen worlds, like [Saturn's moon] Enceladus, [Jupiter's moons] Europa and Ganymede, and now Pluto, that seem to have oceans in their interiors," said New Horizons lead scientist Alan Stern earlier this year, noting that only Earth wears its oceans on the outside. "From the surface, we don't see them. Who knew that oceans would turn out to be fairly common?"
To determine whether the ocean is still liquid or whether it has already frozen entirely, Hammond and his team ran a thermal evolution model using New Horizons data on Pluto's diameter and density. They found that, if the ocean under Pluto's surface had already frozen completely, it would have changed to a phase called ice II, which has a much more compressed structure than ordinary water ice, aka ice I. Basically, if there was no liquid water under Pluto's surface, it would show signs of contraction, rather than expansion.
This stage would only occur if Pluto's icy shell was more than 260 km (155 miles) thick. Evidence suggests that Pluto's shell is at least 300 km (186 miles) thick. So the case for liquid water on Pluto is strong.
"That's amazing to me," Hammond said. "The possibility that you could have vast liquid water ocean habitats so far from the sun on Pluto ... is absolutely incredible."