Back in 2015, the New Horizons space craft showed us that Pluto looks nothing like anyone imagined. The most iconic feature of this distant world is a heart-shaped plane named Sputnik Planitia. This feature, like Texas, is slightly bigger than France. Looked at closely, Pluto’s heart is made of polygons of nitrogen ice. This shape is generally associated with convection but explaining how an icy plane convects like oil in a hot pan has been a challenge.

Researcher Adrien Morrison explains: We know that the surface of the ice exhibits remarkable polygonal features – formed by thermal convection in the nitrogen ice, constantly organizing and renewing the surface of the ice.  However, there remained questions behind just how this process could occur.

Based on a lack of craters, Sputnik Planitia appears to be about 40,000 years old. To put that in perspective, neanderthals had just gone extinct and homo sapiens were settling Australia when this giant crater was formed. This means that we aren’t seeing the legacy of some ancient convection; we are seeing a modern process shaping a geologically active world.

That’s another thing that wasn’t expected: we didn’t think Pluto would be geologically active. It is too tiny to have held on to much internal heat left over from its formation, and it takes heat to drive activity. As put by Morison: [New Horizons] showed that Pluto is still geologically active despite being far away from the Sun and having limited internal energy sources. This included at Sputnik Planitia,  where the surface conditions allow the gaseous nitrogen in its atmosphere to coexist with solid nitrogen.

Over the past half-decade, folks have put forward a lot of possibilities, including tidal heating from Charon, a core rich in radiative materials, and residual heat from impacts like the one that formed the plane in question. But these explanations may not be necessary. 

In a new paper in Nature, Morison points out that models show the ability of nitrogen to go from solid to gas as it sublimates at Pluto may be enough to drive the convection. Numerical models based on sublimation are able to reproduce the New Horizons observations, including the size of the polygons, their topography, and even the surface velocities.

Nitrogen – it alone may be able to explain at least this one heart-shaped mystery of Pluto.

Since flying past Pluto in 2015, New Horizons has continued its journey outwards, and in April, it passed the 50 AU mark, putting it fifty times the distance of the Earth from the Sun. New Horizons’ mission is not yet over, and there is enough fuel and power remaining for the little spacecraft to visit at least one more target if one can just be found. The New Horizons team used the Subaru Telescope to search for targets last May. If that data provides no targets, all hope is not lost. The Vera Rubin Observatory offers new promise, and since it is on the ground, we know it will be completed and functional in the not-too-distant future.

More Information

CNRS press release

University of Exeter press release

“Sublimation-driven convection in Sputnik Planitia on Pluto,” Adrien Morison, Stéphane Labrosse and Gaël Choblet, 2021 December 15, Nature