Pluto Might Be More Geologically Alive Than Anyone Realized

Pluto May Have Liquid Nitrogen Erupting Through Its Ice, Scientists Say

Something unexpected may have happened, and could still be happening, on Pluto’s massive ice sheet. A new study suggests liquid nitrogen may have pushed up from deep underground, broken through frozen nitrogen ice, and spilled across the surface nearly 4 billion miles from the Sun.

A team publishing in The Planetary Science Journal has put forward evidence that liquid nitrogen may have risen, and possibly erupted, from beneath Pluto’s largest glacier, a sprawling frozen plain called Sputnik Planitia. If the hypothesis holds up, it would be the first known case of liquid nitrogen flowing across a planet’s surface in modern times, a picture of Pluto as far more geologically active than most people imagine.

Sputnik Planitia covers more than a million square kilometers, roughly twice the size of Texas, filling an ancient impact basin with nitrogen-rich ice. NASA’s New Horizons spacecraft, during its 2015 flyby, spotted something puzzling along the region’s northern edges: strange, dark features unlike anything else on the planet.

Strange Dark Marks Nobody Could Fully Explain

Images from New Horizons showed two types of unusual dark features clustered along the northern edge of Sputnik Planitia. One type appears as sharp, dark, narrow lines running for tens of kilometers, often following the edges between the giant slow-churning cells that cover much of the glacier’s surface. Surrounding them are wider, hazier dark zones stretching several to roughly 20 times farther out from the narrow core. Against the glacier’s otherwise blinding white surface, both look almost like scorch marks, dark scars on what should be a clean sheet of ice.

Previous explanations for the darkening, changes in ice crystal size, contamination from dark atmospheric particles, or ice converting directly to gas, all fall short the same way. They would produce gradual, diffuse darkening spread evenly across the surface, not the sharp-edged patterns actually observed, with the darkest deposits sitting in low points between cells and fading outward like something flowed there.

Why Liquid Nitrogen Fits the Evidence for Pluto’s Glacier Activity

Liquid nitrogen on Pluto isn’t as far-fetched as it sounds. Not far beneath Sputnik Planitia’s surface, nitrogen can exist in liquid form if the temperature is warm enough, and plausible estimates of heat rising from Pluto’s interior suggest that threshold could be reached somewhere between a few hundred meters and just over a mile down.

Liquid nitrogen is also less dense than solid nitrogen ice, which makes it naturally buoyant. Given a path to travel, it will push upward through a solid glacier the same way molten rock forces its way up through solid crust to erupt as a volcano on Earth. Researchers calculate that path would have to be a narrow vertical crack, far too thin to spot in any existing Pluto imagery.

Pluto’s Glacier May Erupt Like a Volcano

Once on the surface, the liquid would flow outward and downhill along the gentle slopes of the cell surfaces, pooling in the low areas at cell boundaries. As it spreads and cools, it could refreeze and leave the surface darker there, producing patterns like the narrow features and surrounding hazy zones seen by New Horizons.

There is a catch, though. The glacier likely can’t produce liquid nitrogen fast enough to keep a steady flow moving all the way to the surface without it freezing along the way. So the liquid probably builds up in underground reservoirs first, then bursts through periodically once enough pressure gathers, more like an outburst than a leak.

Pinpointing what drives the melting in the first place is harder. A faint trickle of heat from Pluto’s interior is one candidate. A more novel possibility involves Pluto’s climate cycles, spanning millions of years and predicting nitrogen ice gradually evaporating from Sputnik Planitia’s northern portion, thinning the glacier there until its base warms enough to melt. Friction from ice flowing north across the basin floor was also considered, but proved too weak to matter.

Could Other Worlds Have the Same Thing?

Sputnik Planitia has no impact craters at all, not even tiny ones, a sign that something keeps resurfacing the ice there, wiping the slate clean again and again rather than letting it sit untouched for ages.

Similar processes might operate elsewhere. Triton, a moon of Neptune with its own nitrogen ice and mysterious geysers seen by Voyager 2, is one candidate, and Eris, a distant dwarf planet with strong nitrogen ice signals and unusually high brightness, is another. Neither has been photographed in the kind of detail New Horizons captured at Pluto, so for now they remain hunches rather than confirmed matches.

More than half of Pluto itself was never mapped at high resolution, so whether liquid nitrogen features exist elsewhere on the planet is an open question. For now, the dark streaks on Sputnik Planitia’s northern edge stand as the most compelling evidence yet that even on the coldest, most distant worlds in the solar system, the ground may still move and liquids may still flow.

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