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Sputnik Planitia

Pluto: How did the icy polygons originate? Sublimation could explain the shape of floating ice from Sputnik Planitia

The driving force has been determined: until now it was not clear why the heart-shaped blocks of the bright ice level of Pluto had such a striking angular shape. A model now suggests that nitrogen ice sublimation could be the driving force behind this formation. Accordingly, the evaporation of this ice cools the surface and thus increases the temperature difference to warmer depths. This, in turn, maintains convection and thus the continuation of snowfall.

When NASA’s New Horizons spacecraft flew close to Pluto for the first time in 2015, it also revealed a surprisingly dynamic world. glacier flow, Possible Isvolcanin And maybe once a icy ocean. But especially striking is the bright, heart-shaped plain of Sputnik Planitia. Its surface consists of about 30 km of polygonal masses of nitrogen ice bordered by dark grooves. The nitrogen emitted from this area during the day is the engine for wind Pluto.

The surface of the Sputnik Plain Planitia (light blue-greenish) is divided into polygonal masses of nitrogen ice. ©NASA/JHUAPL/SwRI

What is an ice convection engine?

But how did the polygonal ice floes originate at Sputinik Planitia? And why do they have this roughly regular angular shape? As early as 2016, researchers came to the conclusion that these clumps must have been created by convection: there is a circulating current below the surface of the ice level, causing cold ice to sink into the channel and warmer ice layers from the depths to the top. This creates a roughly regular polygonal pattern of ups and downs – similar to on the surface of the sun Only in slow motion and icy cold.

“We know that the remarkable polygonal structures of the ice surface are formed by convection of the ice,” explains first author Adrian Morrison from the University of Exeter. “But the question arises as to how this process will continue.” Because this requires a sufficient temperature difference between the surface and depth – and whether Pluto’s interior could provide this is questionable.

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Cold sublimation as a driving force?

So Morrison and his team searched for a different convection engine — and it didn’t start in the depths, but on the surface of the ice. Your hypothesis: The temperature gradient may occur because heat is constantly being extracted from the ice from above. They see sublimation as a possible actor: “We suspect that sublimation is the mysterious driver that keeps convection going in Sputnik Planetia,” the researchers wrote.

Reason: Just as evaporation has a cooling effect on the skin or other surfaces, the direct transition from ice to a gaseous state also has a cooling effect on the environment. Morrison and colleagues used model simulations to check whether this effect was enough to trigger a circular current in the Pluto ice sheet, as they reconstructed conditions in the Sputnik Planitia ice sheet.

Sublimation creates polygons

The result confirmed the hypothesis: the cooling effect from nitrogen sublimation on the ice surface is strong enough to move the ice underground and drive convection. In addition, this effect resulted in the same polygonal structures as can be seen in Sputnik Planitita. So the floating ice is closely related to the climate of Pluto and the emission of nitrogen ice.

“We found that convection from sublimation creates these polygonal structures,” the team says. The resulting patterns in size, shape, and topography are perfectly consistent with the ice floats that also make up Pluto’s heart-shaped ice plain. At about 1 million years old, the surface age and rate of convection were also consistent with previous estimates.

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Also possible on other celestial bodies

As the research team explains, convective currents, driven by sublimation rather than internal heat, could exist on other icy celestial bodies. Possible candidates include Neptune’s moon Triton, Uranus’ moon Umbriel, or those outside Neptune’s orbit. in the Kuiper belt It revolves around the orbs Eris and Makemake. (Nature, 2021; doi: 10.1038/s41586-021-04095-w)

Coyle: University of Exeter