What the Images Show
Scientists re-examining high-resolution images taken during the 2015 New Horizons flyby noticed something peculiar on Sputnik Planitia, the vast, heart-shaped glacier of frozen nitrogen that dominates Pluto's surface. They identified a series of dark,
narrow lines and wider, diffuse patches along the boundaries of large, city-sized convection cells that make up the glacier's surface. These features strongly resemble darkened, wetted areas seen on glaciers here on Earth, such as in Greenland, where meltwater flows over ice and snow. The striking similarity led researchers to a startling hypothesis: that these marks on Pluto were created by liquid temporarily wetting the frozen nitrogen ice before refreezing.
A Puzzle on a Frigid World
The idea of liquid on Pluto is deeply counterintuitive. With surface temperatures plummeting to around minus 230 degrees Celsius, its atmosphere is far too cold and thin to allow for anything like nitrogen rain. Any liquid exposed to the surface would flash-freeze almost instantly. This rules out atmospheric or surface-level explanations for the dark features. So, if the liquid isn't coming from above, scientists concluded it must be coming from below. This finding points towards a far more complex and geologically active world than previously imagined, one with processes capable of altering its landscape.
The Subsurface 'Geyser' Theory
According to a study published in the Planetary Science Journal, the most likely explanation involves a process called basal melting. Sputnik Planitia is a glacier of nitrogen ice several kilometres deep. Computer models suggest that the immense pressure at the base of this massive ice sheet could be sufficient to melt the nitrogen ice, creating pockets of liquid. This liquid nitrogen, being slightly more buoyant than the solid ice around it, could then be forced upwards through cracks and fissures in the glacier. Acting like a kind of slow-motion geyser or lava tube, these channels would allow the liquid to reach the surface, where it would briefly flow and spread, 'wetting' the ice and creating the dark patches seen by New Horizons before it freezes again.
A New Kind of Geological Activity
This discovery transforms our understanding of Pluto. Far from being a geologically dead world, it appears to host active subsurface processes. Alan Stern, the principal investigator of the New Horizons mission, noted that Pluto "never stops surprising us" and that these findings suggest a new kind of time-variable feature on the dwarf planet. While the images were taken in 2015, the surface of Sputnik Planitia is considered geologically young—likely less than a million years old—because the nitrogen ice is constantly churning in slow-moving convection cells. This implies that the liquid flows that created these features are a relatively recent phenomenon in geological terms, and the process could still be happening today.
