Pluto’s Strange Shapes Caused By Churning Nitrogen Ice

Bubble, bubble, toil and trouble

Almost a year after the New Horizons spacecraft buzzed by the dwarf planet, we’re still learning amazing new things about Pluto.

In two papers published this week in Nature, researchers announced that they had an explanation for the strange polygonal shapes that showed up in close-up images of Pluto’s surface. The odd block-like shapes (6-24 miles across) may have been caused by convection inside a roiling layer of nitrogen ice.

Convection is the same process that happens in a pot of boiling water on a stove. A heat source warms up the water on the bottom, which rises to the top of the water in the pot. There, the water cools and sinks back down, only to be warmed up again, continuing the cycle. In Pluto’s case, the heat comes from radioactivity deep under the surface, and instead of water, the medium here is nitrogen ice, which can stay almost fluid even at the extraordinarily low temperatures at the outer regions of our solar system.

“Evidence suggests this could be a roiling sea of volatile nitrogen ice,” Jay Melosh, a planetary geologist at Purdue who worked on one of the papers said. “Imagine oatmeal boiling on the stove; it doesn’t produce one bubble for the entire pot as the heated oatmeal rises to the surface and the cooler oatmeal is pushed down into the depths, this happens in small sections across the pot, creating a quilted pattern on the surface similar to what we see on Pluto. Of course, on Pluto this is not a fast process; the overturn within each unit happens at a rate of maybe 2 centimeters per year.”

The surface turns over about once every 500,000 years. That might seem like forever, but to planetary geologists, that qualifies as a rapid resurfacing of the planet, and could explain another mystery; why researchers don’t see impact craters on the ice sheet. Typically with an older planet like Pluto, researchers would expect to see plenty of evidence of meteorites slamming into the surface, but Pluto has a relatively unblemished crust.