A Heart of Active Ice
The focus of the new research is Sputnik Planitia, the vast, basin-like glacier that makes up the western lobe of Pluto's iconic heart-shaped feature. This enormous plain isn't made of water ice like glaciers on Earth, but of frozen nitrogen, methane,
and carbon monoxide. Ever since NASA's New Horizons spacecraft flew by Pluto in 2015, scientists have been captivated by Sputnik Planitia. Its surface is remarkably free of impact craters, which is a tell-tale sign that the surface is young in geological terms and is being constantly renewed. The glacier's surface is broken into large, city-sized polygonal cells that slowly churn in a process called solid-state convection, much like a cosmic lava lamp. This process, where warmer, less dense ice from below rises and cooler, denser ice from the surface sinks, was the first major clue that Pluto is far from a dead world.
Reading the Glacial Tea Leaves
The latest study, published in the Planetary Science Journal, takes a closer look at images of the northern part of Sputnik Planitia. Researchers, led by New Horizons Principal Investigator Alan Stern, identified dark streaks and patches along the boundaries of the convection cells. These features bear a striking resemblance to patterns seen on Earth's own ice sheets where liquid water has wetted the surface, either from rain or from subsurface melt. However, Pluto's frigid temperatures and thin atmosphere make liquid nitrogen rain physically impossible. This led the team to an astonishing conclusion: the liquid must be coming from beneath the glacier.
What Lies Beneath?
The findings suggest that liquid nitrogen may be seeping up through cracks and fissures in the ice, temporarily wetting the surface before freezing again. This implies that there is a source of heat deep beneath Sputnik Planitia, enough to melt the nitrogen ice at the base of the kilometers-deep glacier. Computer models support this theory, showing that liquid nitrogen could form at the base and be forced upward through conduits by pressure or its own buoyancy, much like magma on Earth. This marks the first evidence of recently flowing liquid on Pluto's surface, a groundbreaking finding for a world nearly six billion kilometres from the Sun. “Pluto never stops surprising us,” said Alan Stern in a statement accompanying the study.
A New Kind of Active World
This discovery fundamentally changes our perception of Pluto. The idea that a dwarf planet so far from solar warmth could harbor enough internal heat to drive such activity was once considered unlikely. The source of this internal heat is still debated—it could be residual warmth from Pluto's formation or heat generated by the decay of radioactive elements in its core. This internal engine not only drives the slow convection of the glacier but also appears to be creating pockets of liquid nitrogen. These findings place Pluto in a category of surprisingly dynamic worlds, suggesting that the necessary conditions for geological activity may be more common in the cosmos than previously believed. It forces us to reconsider what makes a planet 'alive' and expands the potential for active geology to the coldest, most distant corners of our solar system.
