Pluto ranks among the coldest, most remote worlds in the solar system — temperatures plunge to nearly –230°C, and the surface was long assumed to be geologically frozen in time. Yet images captured by NASA’s New Horizons reveal something harder to dismiss: strange dark markings etched across Sputnik Planitia, a glacier of frozen nitrogen larger than Texas and Oklahoma combined, that scientists are only now beginning to explain.
Whatever left those marks may still be moving.
Dark lines on a frozen world
Sputnik Planitia’s northern edge isn’t the featureless expanse you might expect. New Horizons images from 2015 and 2016 revealed city-sized convection cells separated by thin dark lines, along with broader, more diffuse dark patches scattered across the glacier’s surface. For years, no clear explanation existed for what produced them.
The puzzle deepened when researchers considered Pluto’s conditions. Liquid nitrogen can’t fall as rain there — temperature and atmospheric pressure simply don’t allow it. So whatever darkened those patches didn’t arrive from above. A Southwest Research Institute-led team, publishing in the peer-reviewed Planetary Science Journal, now proposes a different answer: it came from below.
A clue hiding in Greenland’s ice
To make sense of the markings, the SwRI team looked somewhere unexpected — Earth. They compared New Horizons images of Sputnik Planitia with NASA Landsat 9 imagery of Greenland’s ice sheet.
The comparison caught researchers’ attention. On Greenland, narrow dark markings form where liquid water sits on top of ice and snow — either from rain or from water emerging beneath the surface. The patterns on Sputnik Planitia looked strikingly similar, similar enough that researchers proposed the same basic process might be at work, with liquid nitrogen rising from below and wetting Pluto’s frozen surface.
“The surface of Sputnik Planitia is quite young, probably less than one million years based on modeling of the surface overturn, and thus these features that we are looking at must have formed since then,” said SwRI Principal Scientist Dr. Kelsi Singer, a co-author of the study. “Pluto has many unique terrains seen nowhere else in the solar system, and this area of Sputnik Planitia is one of them.”
That youth matters. A geologically young surface means the dark markings are recent — not ancient relics of a warmer era, but signs of something that may still be happening now.
How liquid could rise through miles of ice
The mechanism behind this process isn’t simple. Sputnik Planitia is several miles deep, and getting liquid from its base to the surface requires more than a lucky crack in the ice.
Computer simulations led by Dr. Orkan Umurhan, a senior research scientist at the SETI Institute, offer a plausible path. The models show that nitrogen ice at the glacier’s base can melt under a combination of pressure and mechanical stress, with the resulting liquid migrating upward through narrow channels — driven by buoyancy or pressure from below — in a process resembling how material moves through lava tubes or geyser conduits.
Once it reaches the surface, the liquid nitrogen wouldn’t instantly freeze. It could remain liquid long enough to flow downhill across the glacier, wetting surrounding ice and leaving behind the dark markings New Horizons photographed.
“I think the great significance of these findings is a great motivation and reason to further examine solid-state nitrogen physics at very low temperatures,” Umurhan said. “Specifically, it’s important to examine the physics taking place in solid nitrogen materials under stress and strain, which can cause them to melt. These processes have never been studied in real detail in the laboratory.”
A more geologically alive Pluto than anyone expected
Earlier work, including research led by Dr. Alan Stern, had suggested that liquid once moved across Pluto’s surface in the distant past. This new analysis goes further, raising the possibility that liquid nitrogen exists beneath Sputnik Planitia today — or did so very recently, in geological terms.
That’s a meaningful shift. A world long characterized as frozen and inert may instead be quietly active, cycling material between its depths and surface on timescales that overlap with human history.
There’s also reason to think this isn’t the whole picture. More than half of Pluto has never been imaged at high resolution, so similar processes could be occurring in regions New Horizons never mapped closely. The same basal-melting mechanism may help explain geysers that NASA’s Voyager 2 observed erupting from Triton, Neptune’s largest moon — suggesting the phenomenon isn’t a quirk unique to Pluto.
“Pluto never stops surprising us,” said lead author Dr. Alan Stern. “This new result certainly does that. In addition to suggesting that liquids have recently expressed themselves on Pluto’s surface, it also suggests a new kind of time-variable feature on Pluto.”
What comes next for Pluto exploration
Confirming this discovery will require more data. Scientists say high-resolution observations of Pluto and other Kuiper Belt worlds are needed to determine how widespread these processes might be. Right now, the evidence is compelling but limited to one region of one dwarf planet.
Laboratory work is equally urgent. Understanding how solid nitrogen behaves under extreme cold and pressure — how it deforms, melts, and moves — is essential for testing whether the proposed mechanism works at Pluto’s scale. That research, Umurhan noted, has never been done in meaningful detail.
New Horizons was designed, built, and operated by the Johns Hopkins Applied Physics Laboratory, with mission direction from SwRI under Dr. Stern. It remains one of the most productive planetary missions in NASA history, its data continuing to yield surprises years after the Pluto flyby.
Those dark lines may ultimately point to something worth sitting with. A world at the edge of the solar system, billions of miles from the Sun, with surface temperatures near absolute zero, may still be moving liquid through its interior and onto its surface — making the question of where else quiet geological activity might persist harder to dismiss.
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