Sputnik Planum Seems to be Undergoing Convection

On Pluto, icebergs floating in a sea of nitrogen ice are key to a possible explanation of the quilted appearance of the Sputnik Planum region of the dwarf planet's surface.

Data reported by NASA's New Horizons New Horizons mission to the Pluto system shows unusual terrain in this region, which features a large deposit of nitrogen ice with a pattern of polygons that are thickest at their centers and dip at their edges. Purdue University researchers have proposed that the polygons seen in the images could be individual Rayleigh-Bénard convection cells. A paper detailing the work will be published in the journal Nature online on Thursday (June 2).

Purdue graduate student Alex Trowbridge, under the guidance of Jay Melosh, a distinguished professor of earth, atmospheric and planetary sciences, and professor of physics and aerospace engineering, led the research.

"Evidence suggests this could be a roiling sea of volatile nitrogen ice," Melosh 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 of Pluto appears to be primarily very cold water ice. However, within the Sputnik Planum region the icy surface drops into a basin that holds a pool of nitrogen ice. Both water and nitrogen are solids at Pluto's temperatures, but the nitrogen ice is structurally weak and has a low viscosity that allows it to deform and flow like a fluid, while the water ice has a very high viscosity and can form tall, hard mountains, Trowbridge said.

"Within this pool of nitrogen ice, there are mountains of water ice that have collected at the edges of the polygons," he said. "The way they have collected suggests they have moved or floated like icebergs with the convection current. 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."

The polygons also can provide information about the depth of the pool of nitrogen through known ratios of width to depth for individual cells of convection, he said.

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