And as our images cleared up, it became apparent that Pluto's heart — a large frozen plain informally known as Sputnik Planum — was something worth exploring. It was void of impact craters, which is a sign that some kind of geological process resurfaced it fairly recently. Instead, the plain was peppered with mysterious polygonal shapes between six and 24 miles in diameter.
Data from the short flyby is still being beamed back to Earth as we speak, so we're still being treated to amazing, unprecedented pictures of Pluto and its moon Charon. And according to a pair of papers published Wednesday in Nature, that broken, frozen heart might still be "beating" — replenishing itself so often with new ice that it could be one of the youngest surfaces in the solar system.
“For the first time, we can determine what these strange welts on the icy surface of Pluto really are,” William B. McKinnon of Washington University, who led one of the studies, said in a statement.
Based on models made by the two research teams, the mysterious polygons of Pluto's heart are actually rising bubbles created by some kind of heating system below. Throughout Sputnik Planum, a layer of mostly-nitrogen ice — likely several miles deep in some spots — is being heated ever-so-slowly by the scant convection of sluggishly decaying radioactive elements inside the dwarf planet.
On Pluto, "solid" nitrogen is actually quite malleable, and can flow viscously. As the flowing frozen nitrogen slowly warms up, parts of it rise up to the surface like blobs in a lava lamp. Over time, these bubbles cool down and sink to the bottom again — leaving room for new bubbles to rise and take their place. The researchers say they believe that these cells sometimes fuse together in sets before they sink down, which could help create some of the more complex line work observed on the planum.
They believe the resulting bubbles reform every 500,000 to 1 million years or so. In geological terms, that's quite rapid — especially for a world so far from the sun's energy.
“We found evidence that even on a distant cold planet billions of miles from Earth, there is sufficient energy for vigorous geological activity, as long as you have ‘the right stuff,’ meaning something as soft and pliable as solid nitrogen," McKinnon said in a statement.
And this process would do a perfect job of smoothing over any crater marks inflicted on Pluto's surface in the intervening years, explaining how its heart has stayed so pure.
“Not only is it the heart of Pluto, it’s the beating heart,” McKinnon told National Geographic. “There are actually things happening. If we were to come back in 100,000 years, the pattern would be markedly altered.”
The two papers agree on quite a lot, which makes a much stronger case for these beating-heart bubbles than a single paper could on its own. The only major point they disagree on is how deep the nitrogen reserves under these bubbles go, which could make a big difference in how much nitrogen covers the planet.
“Sputnik Planum is one of the most amazing geological discoveries in 50-plus years of planetary exploration," New Horizons Principal Investigator Alan Stern said in a statement.
Exactly how much nitrogen is flowing beneath Pluto's icy heart — and what that means for the history of the little world — remains to be seen. With any luck, we'll soon have another mysterious dwarf planet to look at: New Horizons is already on track to visit the Kuiper Belt object 2014 MU69 on New Year's Day, 2019. Pending extended funding from NASA and a whole lot of luck, the team could see similarly shocking geological processes on that alien world.