In 2015, NASA's New Horizons flew past Pluto and captured detailed images of its frozen surface. More than a decade later, scientists revisited the images and found evidence that liquid nitrogen may have once flowed across the dwarf planet

When NASA’s New Horizons spacecraft passed Pluto in 2015, it gave scientists their first close look at a distant world that had previously been little more than a blur. More than a decade later, those same images are still revealing new clues. A new peer-reviewed analysis suggests that liquid nitrogen may have risen from beneath Pluto’s frozen surface and briefly flowed across Sputnik Planitia, leaving dark marks behind.

The finding does not mean scientists watched a liquid stream moving across Pluto. Instead, researchers combined unusual surface features, comparisons with Greenland and computer models to build a possible explanation for what happened. The study, titled “Evidence for possible N2 basal flow beneath Pluto’s northern Sputnik Planitia,” was published in The Planetary Science Journal.

What did scientists find on Pluto?

Sputnik Planitia is the bright western section of Pluto’s famous heart-shaped region. Its nitrogen glacier is larger than Texas and Oklahoma combined, and New Horizons photographed large convection cells across its surface.

Among those features are thin dark lines and wider shadowy patches.

Because Pluto’s surface is far too cold for nitrogen rain to create such marks, researchers considered another possibility. Their models suggest nitrogen ice deep beneath the glacier could melt under the unusual pressure, stress and strain found there.

The liquid could then be pushed upward through narrow pathways, somewhat like geyser or lava conduits. Once reaching the surface, it may have remained liquid long enough to travel downhill and wet nearby ice.

That process could explain the darker tracks seen in the New Horizons images.

As Alan Stern, New Horizons principal investigator and lead author of the study, put it: “Pluto never stops surprising us.”

Could liquid nitrogen really move beneath the ice?

Computer modeling provides the main support for the idea.

Sputnik Planitia contains a glacier several kilometers deep. Under the right conditions, nitrogen ice near its base could melt. Buoyancy or pressure from below could then force the liquid upward.

The proposed process is especially striking because Pluto appears frozen and inactive at first glance. Yet the models suggest its nitrogen ice can still move, melt at depth and potentially reach the surface.

The researchers are careful about what the evidence proves. The dark markings are consistent with ice that was temporarily wetted from below, but they are not a direct recording of a liquid flow.

The New Horizons spacecraft made only one close pass of Pluto in 2015. Its images cannot establish exactly when a particular flow happened or how often such events occur.

What did Greenland have to do with it?

Earth provided an unexpected comparison. The research team examined Landsat 9 images of Greenland’s ice sheet and found narrow dark features in places where liquid water had reached the surface of snow and ice.

Some of those patterns resemble the markings observed in northern Sputnik Planitia.

The comparison is not proof by itself. But when combined with the fact that nitrogen rain cannot explain the features and with the results of the computer models, it supports the possibility that liquid nitrogen emerged from beneath Pluto’s glacier and temporarily wet its surface.

That makes the Greenland comparison useful as a visual clue rather than a direct confirmation.

How recently could this have happened?

The surface of Sputnik Planitia is thought to be relatively young. According to the modelling, its surface may be less than one million years old because convection continually overturns the nitrogen ice. If the dark features were created by liquid nitrogen flows, they therefore formed within that relatively young period.

The research also raises the possibility that liquid nitrogen could still exist beneath the glacier today.

There is, however, no way to attach a precise date to an individual event. New Horizons photographed Pluto during a single flyby, leaving scientists with snapshots rather than continuous observations.

Is this liquid water?

No. The proposed liquid is nitrogen, not water. The study does not report an underground river like those found on Earth, nor does it provide evidence of life on Pluto.

The distinction is important because Pluto is extremely cold. Water ice there behaves more like rock, while nitrogen is the material that could potentially melt and move under the conditions modeled by the researchers.

The importance of the finding lies in what it suggests about volatile ice and how it can reshape distant planetary surfaces.

Why are scientists still studying old Pluto images?

New Horizons made history on July 14, 2015, when it became the first spacecraft to explore Pluto up close. It passed roughly 7,800 miles above the surface and revealed glaciers, mountains, haze and possible signs of continuing activity.

But the mission did not answer every question. Scientists are still returning to the images with new models and comparisons. In this case, photographs taken during a single flyby became the starting point for a much later investigation into whether Pluto’s nitrogen glacier might be active beneath the surface.

Researchers now want laboratory experiments examining how solid nitrogen behaves under Pluto-like cold, pressure, stress and strain. More work is needed to understand when melting begins, how long the liquid could survive and how efficiently it could travel upward. There is also a major limitation in the available imagery: more than half of Pluto has not been photographed at high resolution.

What could this mean for Pluto?

The possible nitrogen flows add another layer to a world that already turned out to be far more complicated than expected. Sputnik Planitia may not simply be a giant frozen store of nitrogen. Instead, its ice could circulate, melt at depth and occasionally release liquid onto the surface.

The possibility also raises questions about whether similar processes could help explain activity elsewhere, including the geysers observed on Neptune’s moon Triton. That connection remains an open possibility rather than an established conclusion.

FAQs

Did scientists see liquid flowing on Pluto?
No. The evidence comes from images and computer models.

Was the liquid water?
No. Researchers propose that it was liquid nitrogen.