A detective story
To come to the conclusion of an underground liquid ocean on Pluto, Dalle Ore's team first had to put together a story likely to explain it. The color of Pluto was his first clue. When New Horizons began to return images of Pluto, the red spots that covered large regions of the world jumped on the researchers. They were a clear sign that the frozen surface of Pluto is not all water ice, but contains other elements. Virgil Fossae was one of the particularly red features in the so-called Cthulhu region of Pluto, the dark zone to the left of Pluto's bright and famous heart. So the researchers badyzed the spectral information of that region to know what types of materials are present.
They found a strong sign of ammonia. But ammonia is easily broken by the ultraviolet light of the sun and charged particles, and by cosmic rays. Then, according to the calculations, the researchers believe that the ammonia of the surface must have been deposited in the last billion years. That may seem like a long time to humans, who only existed a few hundred thousand years ago. But it's not terribly long in a geological sense.
Perhaps most importantly, ammonia is found in a region that looks fairly young, since the ice pattern around it looks like it emerged from an ice volcano and has not been destroyed by meteorites or covered since then.
And while none of the clues is conclusive on its own, together they form a much stronger argument. The ammonia mixed in water decreases its freezing point. Planetary researchers have long argued that a liquid ocean with a healthy portion of ammonia, something like a third of the mixture, would remain liquid even though Pluto's surface, far from the sun, is cold and far below of normal freezing temperatures.
The resistant appearance of Pluto from the first images of New Horizon has implied that cryovolcanism is a recent or ongoing activity on the dwarf planet. And active ice volcanoes require something below the surface of Pluto to be liquid or at least muddy, able to squeeze and flow through cracks to the surface.
The ammonia that leaked to the surface could provide an explanation of how cold Pluto has kept liquid deposits underground, perhaps to this day.