Pluto Is Not a Cold, Dead World Afterall

June 6, 2016 | Joanne Kennell

Sputnik Planum
Photo credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

It has a "roiling sea of volatile nitrogen ice."

Pluto: some call it a dwarf while others still consider it the ninth planet of our solar system. But either way you slice it, Pluto is one interesting place.

Although the surface of Pluto is primarily covered in water ice, in a region known as the Sputnik Planum lies a sea of nitrogen ice. Since nitrogen at the temperatures found on Pluto (-380 degrees Fahrenheit) is structurally weak with a low viscosity, it deforms and flows like a fluid, but that is not the case for water. At these frigid temperatures, water has a very high viscosity and can form tall, hard mountains.

According to data from NASA’s New Horizons mission, Pluto has some unusual terrain in the Sputnik Planum region.

"Within this pool of nitrogen ice, there are mountains of water ice," said Jay Melosh, a professor of earth, atmospheric, and planetary sciences at Purdue University and co-author of the study, in a Purdue news release. "The way they have collected suggests they have moved or floated like icebergs with the convection current."

The Purdue University team, led by graduate student Alex Trowbridge under the guidance of Melosh, proposed that the icebergs could be the result of individual Rayleigh-Bénard convective cells. This type of convection is driven by the buoyancy of a fluid that is heated from below and cooled from above.

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"If this is true, we can calculate how deep the pool would need to be for the icebergs to float freely without catching on the bottom," said Melosh. "Evidence suggests this could be a roiling sea of volatile nitrogen ice."

Melosh compared the process to boiling oatmeal on the stove. It doesn’t produce one bubble for the entire pot as the heated oatmeal rises to the surface while the cooler oatmeal sinks to the bottom. Rather, there are lots of small bubbles across the pot, creating a quilted pattern on the surface. This same pattern is seen on Pluto, just not as fast. It likely happens "at a rate of maybe 2 centimeters per year," explained Melosh.

According to the team’s data, the polygons are 12 to 19 miles (20 to 30 kilometers) in diameter and only the tips of the water icebergs can be seen. By assuming the icebergs are spheres — a shape that requires the shallowest volume of liquid to float — the research team made a conservative estimate that the depth of the nitrogen sea must be at least 6 miles (10 kilometers).

This convection could explain why this region of Pluto’s surface is so young, Melosh said. Convection would erase impact craters and renew this area of the dwarf planet’s surface, which fits with the data New Horizons provided. In fact, the surface of Pluto’s Sputnik Planum is estimated to be between one and ten million years old.

"Many people expected Pluto to be a cold, dead world," Melosh said. "What we've discovered through this mission is that cold worlds like Pluto have a different kind of activity that involves materials we think of as gases. This understanding offers a new perspective that cold worlds can be just as active and interesting as our own."

The study was published online in the journal Nature on June 2.

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