The bright nitrogen ice plain forming the left lobe of Pluto’s heart has not a single impact crater anywhere on it, which tells planetary scientists the surface is being churned from below and completely replaced on a cycle short enough that parts of that ground are younger than our own species

The bright plain on the image-left side of Pluto’s heart is Sputnik Planitia, a basin roughly 1,000 kilometres across filled mainly with nitrogen ice. At Pluto’s surface temperature, nitrogen ice is far softer than the water ice that forms the surrounding crust and mountains. In a 2016 Nature model led by William McKinnon, nitrogen ice more than about one kilometre thick could convect under plausible conditions on Pluto. Warm, buoyant nitrogen ice rises towards a cell centre, spreads sideways and eventually descends along the margins after cooling. The source of the convection remains under studyThe first convection models emphasised heat entering the nitrogen ice from Pluto’s interior.