Beneath the immense glacier that forms part of Pluto’s famous “heart,” nitrogen could melt and then rise to the surface. Researchers propose this scenario to explain strange dark traces observed by the New Horizons probe. If their interpretation is correct, this icy region would still be geologically active today.
The studied area lies north of Sputnik Planitia, a plain covering about one million square kilometers and filled mainly with nitrogen ice. New Horizons images show large cells with visible boundaries there. They are linked to the very slow movements of the ice, which transports material between the depths and the surface.
NASA image
But some boundaries are more intriguing. They contain narrow, dark lines bordered by more diffuse areas that are also darkened.
S. Alan Stern and his colleagues propose that liquid nitrogen rising from the base of the glacier could explain these marks. At sufficient depth, pressure and heat from Pluto’s interior could allow a small portion of the nitrogen ice to melt. This liquid, less dense than the surrounding ice, would then tend to rise.
The journey nevertheless poses a problem: near the surface, temperatures are extremely low, and the liquid nitrogen should quickly freeze. The authors therefore calculated the conditions needed for it to cross the glacier before solidifying. Once it reached the open air, it could briefly flow through depressions in the terrain before freezing or evaporating.
The observed marks would then correspond to the paths taken by this liquid and to the deposits left around them. However, this is not a direct observation of liquid nitrogen. The available images date from New Horizons’ 2015 flyby, and the study offers a physical interpretation of the shapes visible on the surface.
Another clue comes from the absence of detected craters in Sputnik Planitia. An ancient surface should accumulate them over time through impacts. Their absence indicates that the ice is renewed and regularly erases the traces, consistent with a region that remains geologically active.
The authors’ calculations indicate that an internal heat flow of a few milliwatts per square meter could help melt nitrogen at the base under certain conditions. The possible presence of an insulating layer above a subsurface ocean could also help retain this heat. A future mission capable of revisiting Pluto at higher resolution would be needed to search for recent changes in these dark marks.