(CNN) When NASA's New Horizons spacecraft flew past Pluto in 2015, researchers hoped that its data would help them unravel some of the dwarf planet's mysteries. Instead, the discoveries made during the close-up look at Pluto and its moon Charon revealed more questions that needed answering.

One of the big revelations from the flyby was the discovery of an ocean beneath the icy shell encapsulating Pluto. The ice shell was thin in a spot near the equator that's about the size of Texas, known as Sputnik Planitia, which helped researchers notice Pluto's odd topography and suggest the ocean's existence.

But this created a conundrum. Given the age of Pluto, estimated to be between 4.4 billion and 4.6 billion years, the ocean should have frozen hundreds of millions of years ago. And rather than forming a bubble, the ice shell should have flattened over the frozen ocean.

But Pluto's ocean isn't frozen, so researchers used New Horizons data and computer simulations to model what might keep the ocean from freezing. Their study was published Monday in the journal Nature Geoscience

The most likely scenario is gas between the ocean and the ice shell that acts like an insulating later. This layer of gas, probably methane that originated in Pluto's rocky core when the dwarf planet was forming, would be thick and have low thermal conductivity.

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