A heart bigger than Texas
On the surface of Pluto, 5.91 billion kilometers from the Sun, is a vast, heart-shaped basin of ice named Tombaugh Regio. Its prominent western lobe is a 1,200 by 2,000-kilometer impact basin called Sputnik Planitia, an area so large it dominates an entire hemisphere of the dwarf planet. This plain is not made of water ice, but of frozen nitrogen, with smaller amounts of solid carbon monoxide and methane. Data from the New Horizons spacecraft in 2015 revealed that the surface of Sputnik Planitia is remarkably young, estimated to be less than 10 million years old, and possibly as young as 180,000 years in some places. This youth is evident from the complete lack of impact craters, which would typically cover an ancient surface in the outer solar system. The basin itself is an ancient impact feature, likely 3 to 4 kilometers deep, that has since acted as a cold trap, collecting these volatile ices over billions of years.
A cosmic lava lamp
The most striking feature within Sputnik Planitia is its division into a network of irregular polygons, typically 16 to 48 kilometers across. These are the tops of massive convection cells, where solid nitrogen ice behaves like a fluid over geological timescales. Warmed by a faint heat source from Pluto's interior, the nitrogen ice slowly rises in the center of the cells, spreads out, cools, and then sinks back down along the troughs that define the cell boundaries. This process, similar to a lava lamp, renews the surface. This geological activity was unexpected for such a small, cold world. Along the edges of the plain, enormous mountains of water ice—such as the Hillary Montes and Tenzing Montes, which rise 1.6 to 3.4 kilometers high—float on the denser nitrogen ice like icebergs.
The engine driving this slow-motion churning is being studied by scientists. Pluto is too far from the sun for solar energy to be a factor, and its small size suggests its primordial heat from formation should have dissipated long ago. The leading hypothesis is that Pluto's rocky core contains enough radioactive elements, such as uranium, thorium, and potassium-40, to generate a small but persistent amount of heat through their decay. This modest internal warmth, trapped beneath Pluto's water-ice crust, would be sufficient to power the sluggish convection in the overlying nitrogen ice sheet. Other possibilities, such as heat from a subsurface liquid water ocean, are also debated, but the exact mechanism is still unconfirmed.