Revisiting a Famous Flyby
The story begins with data captured by NASA’s New Horizons spacecraft, which made its historic flyby of Pluto back in 2015. For the first time, we saw the dwarf planet up close, revealing a world of towering ice mountains and vast frozen plains. One of the most
iconic sights was Sputnik Planitia, the western lobe of a giant, heart-shaped glacier made of nitrogen ice. At the time, it was clear Pluto was more complex than we knew, but scientists are still uncovering its secrets more than a decade later. By re-examining high-resolution images, a team of researchers found features that just didn’t add up to a completely frozen world.
The Telltale Streaks
On the northern stretches of the Sputnik Planitia glacier, scientists spotted a network of dark, narrow lines and wider, softer patches. These markings traced the edges of huge, city-sized polygons of ice that are formed as the nitrogen glacier slowly churns, or convects. The patterns looked strangely familiar. Researchers compared them to images of glaciers on Earth, specifically in Greenland. On our planet, similar dark features appear where meltwater has emerged from beneath the ice, wetting the surface and changing its texture or depositing darker materials. The resemblance was striking, leading to a bold hypothesis: something liquid had been flowing on Pluto.
A Different Kind of Liquid
Before you imagine rivers and lakes, it's crucial to understand what kind of liquid we’re talking about. It is almost certainly not water. Pluto’s surface temperature is incredibly cold, so much so that its water ice is as hard as rock. The liquid in question is believed to be nitrogen. Under normal conditions on Pluto, nitrogen is a frozen solid. However, the new theory suggests that under specific conditions, this nitrogen ice can melt. Pluto’s atmosphere is far too thin and cold for nitrogen rain to be possible, so the liquid couldn’t have come from above. Instead, scientists believe it came from deep below the surface.
Pluto's Subsurface Plumbing
So how does solid nitrogen melt on Pluto? The answer likely lies in the dwarf planet’s own internal heat, left over from its formation. The Sputnik Planitia glacier is several kilometres thick, and computer models show that the immense pressure at its base, combined with modest heat from Pluto’s core, could be enough to melt the nitrogen ice. This liquid nitrogen, being less dense than the solid ice around it, would then be forced upward through cracks and fissures. It’s a process researchers have compared to volcanic dike systems on Earth, where magma pushes through rock. Once it reached the surface, the liquid nitrogen would have briefly flowed before refreezing, leaving behind the dark, 'wetted' stains that New Horizons observed.
A Geologically 'Recent' Event
The headline-making part of this discovery is the word “recently.” In human terms, this doesn't mean it happened last week. In geology, “recent” can have a very different meaning. Scientists estimate the surface of Sputnik Planitia is less than a million years old, a mere blink of an eye in the 4.5-billion-year history of the solar system. This is because the slow convection of the glacier is constantly renewing the surface. Therefore, the features seen by New Horizons must have been formed within that timeframe. This is a radical shift from the idea of Pluto as a geologically dead world. It implies that Pluto is dynamic, with ongoing processes that continue to shape its landscape. As Alan Stern, the lead investigator for the New Horizons mission, stated, “Pluto never stops surprising us.”
