When the New Horizons spacecraft flew past Pluto in 2015, it revealed a striking feature dominating the visible hemisphere: a near-perfect, heart-shaped glacier known as Sputnik Planitia. This immense plain of nitrogen ice is not a frozen relic from the early solar system. Instead, it is the visible expression of a working geological engine. The surface is remarkably free of impact craters, organized into vast convecting cells that slowly churn and renew the landscape. The question that immediately follows is how a tiny, distant world could remain active when it should have frozen solid billions of years ago.

The answer lies in the unique gravitational relationship between Pluto and its largest moon, Charon. The two bodies are locked in a mutual tidal embrace that maintains a permanent deformation in Pluto's interior. This steady-state grav ...