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A detective story
To arrive at the conclusion of an underground liquid ocean on Pluto, the team of Dalle Ore had first to reconstruct a history likely to explain it. The color of Pluto was their first clue. When New Horizons started rendering images of Pluto, the red spots covering large parts of the world exploded to the researchers. They clearly indicated that the ice surface of Pluto was not just water ice, but contained other elements. Virgil Fossae was one of the particularly red features of the Cthulhu region, the dark area to the left of the luminous and famous heart of Pluto. The researchers therefore examined the spectral information for this region to determine the types of materials present.
They found a strong sign of ammonia. But ammonia is easily dissociated by ultraviolet light and charged particles from the sun, as well as by cosmic rays. So, according to calculations, researchers believe that surface ammonia has to be deposited over the last billion years. This may seem like a long time to humans, who have only been born for a few hundred thousand years. But it is not very long from the geological point of view.
Perhaps most importantly, ammonia is located in an area that looks very young, because the ice motif that surrounds it seems to always come from an ice volcano and was not destroyed by meteors neither covered since.
And even if no clue is decisive in itself, together they constitute a much stronger argument. Ammonia mixed with water decreases its freezing point. Planetary researchers have long postulated that a liquid ocean with a good amount of ammonia – about a third of the mixture – would remain liquid, even though Pluto's surface, far from the sun, is icy and well below the normal.
The robust appearance of Pluto since the first images of New Horizon hinted that cryovolcanism is a recent or ongoing activity on the dwarf planet. And active ice volcanoes require something beneath the surface of Pluto to be liquid or at least melting, able to contract and cross the cracks to the surface.
The ammonia spilled on the surface could explain why the freezing ice of Pluto has kept the tanks of liquid underground, perhaps even to this day.
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