Nitrogen ice can flow on Pluto
The heart-shaped region’s broad western lobe, Sputnik Planitia, is filled with nitrogen ice. At Pluto’s frigid surface, that ice is solid, but “solid” does not mean immobile. Given enough time, nitrogen ice can deform and flow, much as rock does deep inside Earth—only here, the motion reaches the surface.
Heat from Pluto’s interior warms the base of the ice layer. Warmer nitrogen becomes slightly less dense and rises; near the surface it cools, grows denser, and sinks again. This slow overturning is called convection: a circulating movement that carries heat through a material.
A surface that continually renews itself
As the ice circulates, fresh material comes up from below while older surface ice is carried aside and eventually drawn downward. The result is not a glacier sliding as one unchanging slab, but a landscape whose surface is gradually replaced by movement within the ice.
That renewal helps explain why Sputnik Planitia looks unusually smooth and has relatively few visible impact craters. Craters accumulate when space rocks strike a surface; here, circulation can deform and erase those marks over time. The ice’s motion therefore acts like a geological reset, though on a timescale far slower than anything a person could watch.
Why the “heart” is more than a shape
On many worlds, a crater is a record of an old collision, preserved until another impact or geological process alters it. Pluto’s nitrogen-ice plain tells a different story: its surface is being remodeled from below. The heart’s smooth appearance is not simply a frozen snapshot—it is evidence that even in extreme cold, a world can remain geologically active.
