Pluto’s Heart Glacier Still Pushes Liquid Nitrogen Upward

NASA scientists now argue that liquid nitrogen has recently wet the northern rim of Pluto’s heart glacier, Sputnik Planitia. The stains sit on a nitrogen ice sheet larger than Texas and Oklahoma combined.

The July 31 paper in The Planetary Science Journal treats those marks as the first close look at melt that can rise from the base of a Kuiper Belt glacier, a process that may also run on Triton and Eris.

Dark Streaks on a Glacier the Size of Two States

Southwest Research Institute announced the result on August 5 from San Antonio, and NASA’s science office posted the same day. Alan Stern, New Horizons principal investigator and the study’s lead author, said liquid nitrogen is rising to Pluto’s surface through cracks along the northern edge of Sputnik Planitia. The agency called it the first evidence of recently flowing liquid on Pluto.

Pluto's heart is keeping a secret. 💗

NASA's New Horizons found the first evidence that liquid nitrogen recently reached Pluto's surface beneath Sputnik Planitia, the iconic heart-shaped glacier. The discovery suggests Pluto is more active than imagined. https://t.co/RtSZPSNofg pic.twitter.com/F4k89LkXPS

— NASA Solar System (@NASASolarSystem) August 5, 2026

The glacier fills the western lobe of Tombaugh Regio, the bright heart New Horizons photographed in 2015. The paper describes a basin with a surface area of one million square kilometers, among the most striking features on the dwarf planet. A color mosaic used in the NASA note spans about 700 by 350 kilometers, with a red box drawn around the dark markings the team now ties to wetting from below.

Stern’s group includes Kelsi Singer, a principal scientist at SwRI, and Orkan Umurhan, a senior research scientist at the SETI Institute who ran the melt models. The author list also names Gary D. Clow, Robert S. Anderson, and Alan Howard, terrestrial glaciology specialists brought in because the Pluto marks look like wet ice on Earth. Johns Hopkins Applied Physics Laboratory built and operates the spacecraft; Marshall Space Flight Center oversees the mission for NASA.

SPUTNIK PLANITIA IN HARD NUMBERS

• Basin scale: NASA likens the nitrogen glacier to Texas and Oklahoma combined, and the paper gives an area of one million square kilometers.
• Cell size: Northern Sputnik is broken into convection cells typically 20 to 35 kilometers across, city-scale polygons with troughs at their edges.
• Surface age: Singer said the surface is quite young, probably less than one million years, based on modeling of ice overturn.
• Craters: The paper reports a complete lack of detected craters in Sputnik Planitia down to about 80 meters per pixel, which is why the team treats the churning as ongoing.

Those cells already looked restless in 2015. Nitrogen ice appears to rise in the middle of each polygon and sink at the edges, a slow boil that wipes the plain clean. The new claim is that the thin dark lines and wider dark patches along those edges are not just old dirt. They are stains left when liquid reached the surface, flowed a short way, and froze.

Why Rain Cannot Explain the Stains

Early ideas for the markings included ice flowing around buried bumps, or wind and frost carving the pattern. The new work compared the Pluto images with NASA Landsat 9 scenes of Earth’s ice, including the Greenland ice sheet, where meltwater darkens snow in narrow wet trails. The match is close enough that the team asked what could wet Sputnik at all.

Pluto’s air is too thin and too cold for nitrogen rain. NASA’s note is blunt on that point: atmospheric and thermal conditions make liquid nitrogen rain physically impossible, so if the ice was wetted, the liquid did not fall from the sky. It came from underneath the glacier.

Solar heating and other surface-only ideas also fail the shape test. The dark lines are sharp. Diffuse sunlight would smear a stain. Melt that wells up through cracks, then runs into topographic lows before it freezes, can leave a hard edge and a softer apron beside it, which is what the northern cells show.

“Recently” here means within that million-year resurfacing window, not last week. Singer said the features must have formed since the surface overturned. By geologic clocks that is young enough that the same plumbing could still be working.

Melt Rises Through the Ice Like Slow Lava

The team set out its case for possible N2 basal flow beneath Sputnik Planitia in a 42-page paper with 13 figures and two tables, published July 31 as Planet. Sci. J. 7, 185. The preprint, posted June 25, states the chain in one breath: basal melt of the ice sheet occurs, that melt reaches the surface before it freezes on the way up through colder ice, and it stays liquid long enough on top to fill lows.

Umurhan’s models say nitrogen ice at the base of the kilometers-deep Sputnik glacier can melt into liquid. That liquid can then move upward through small conduits, similar to lava or geyser tubes, driven by buoyancy or pressure from below. Once it reaches the surface, it can remain liquid long enough to run downslope on the glacier, wetting the ice and making the dark features.

WHAT THE BASAL FLOW HYPOTHESIS REQUIRES

• Melt at the bed: Heat from Pluto’s interior has to liquefy nitrogen ice at the base of the thick sheet, where pressure and warmth can coexist.
• A path that stays open: The liquid has to climb through colder ice in cracks or tubes and still arrive as liquid, not as a frozen plug.
• Minutes of flow on top: After it spills out, it needs time to run into troughs and wet a visible patch before it freezes again.

The paper’s own title keeps the claim honest. This is evidence for possible basal flow, not a photograph of a river. No New Horizons frame shows liquid sitting on the ground. The argument is morphological: the stains look wet, rain is impossible, and the physics of a deep nitrogen sheet allows melt to form and rise.

Greenland’s Wet Ice Gave the Team Its Clue

The Earth comparison is doing real work, not decoration. On Greenland, liquid water darkens ice by enlarging grains or dropping dark impurities. Landsat 9 images show narrow dark lines in those wet zones. Northern Sputnik’s discrete dark lines and more diffuse dark aprons sit in the same places a hydrologist would expect water, along cell boundaries and in lows.

That is why the glaciologists are on the paper. Stern’s Pluto group has spent a decade on New Horizons geology. Clow, Anderson, and Howard read ice the way field teams read a wet glacier on Earth. The joint read is that Sputnik is not only convecting solid nitrogen. It is occasionally leaking.

Pluto never stops surprising us. In addition to suggesting that liquids have recently expressed themselves on Pluto’s surface, this result also suggests a new kind of time-variable feature on Pluto.

Alan Stern, New Horizons principal investigator, NASA science note

A time-variable stain is a practical headache. A single 2015 flyby froze one lighting geometry, one season, one view of the northern cells. If the dark rims come and go as liquid wets and refreezes, later visits would see a different heart. The 2015 mosaic is a snapshot of a leaky system, not a finished map of a dead plain.

The Same Plumbing May Run on Triton and Eris

The SwRI release says the melt path could help explain geysers NASA’s Voyager 2 saw on Neptune’s moon Triton in 1989. Triton carries nitrogen ice and has thrown dark, geyser-like plumes. One long-standing idea is that heat at the base of a nitrogen cap makes fluid that then blows out. Pluto now supplies a worked example of basal nitrogen melt reaching a surface, even if Pluto’s stains are seeps rather than plumes.

Eris, a distant dwarf planet with nitrogen ice, is the other named target. The authors wrote that Eris could be a good candidate for basal melt or liquid flows to its surface, and that no other Kuiper Belt dwarf planet has yet shown nitrogen ice on its surface in the same way. They also wrote that proof on Eris, or on any later world with a deep nitrogen reservoir, would have to wait for high-resolution mapping on the scale New Horizons gave Pluto.

NITROGEN WORLDS WITH A POSSIBLE BASAL MELT PATH

Further high-resolution mapping of Pluto and other Kuiper Belt planets is needed, the SwRI team said, before anyone can say the same process is running elsewhere. Until that happens, Sputnik is the calibration plate. Every later claim of wet nitrogen ice will be compared with those northern cell rims.

Eleven Years On, the Flyby Photos Keep Working

New Horizons flew past Pluto in July 2015 and, in that pass, first showed craterless plains in Pluto’s heart. The mission’s early notes already described a broken surface of irregular segments about 20 kilometers across, bordered by shallow troughs, some of them filled with darker material. The 2026 paper is a second look at those same troughs, not a new spacecraft.

FROM FLYBY TO BASAL FLOW PAPER

1. July 2015: New Horizons flies past Pluto and images the bright western heart, later named Sputnik Planitia.
2. 2015 to 2016: Downlinked frames of the northern cells show thin dark linear features and more diffuse dark patches between city-sized polygons.
3. June 25, 2026: Stern and colleagues post the 42-page preprint arguing for liquid molecular nitrogen sourced from beneath the glacier.
4. July 31, 2026: The Planetary Science Journal publishes the peer-reviewed study.
5. August 5, 2026: SwRI and NASA release the finding as the first evidence of recently flowing liquid on Pluto.

The long gap is the story’s quiet engine. A flyby that lasted hours is still paying out papers because no follow-on orbiter is in flight. Early work asked what Pluto is made of. This round asks what is happening at the base of one glacier, using Landsat 9 as a field analog and computer melt tubes as a stand-in for a lander that will not arrive.

The louder cultural fight this result will feed is whether Pluto counts as a planet. The sharper scientific gap is colder. Solid nitrogen under stress and strain, Umurhan said, has never been studied in real detail in the laboratory. The plumbing model is outrunning the freezer physics.

Half of Pluto Has Never Been Mapped This Well

No other region on the mapped half of Pluto shows this basal-flow pattern, and more than half the globe was never imaged at this resolution. The northern Sputnik stains could be a local trick of ice thickness, heat, and cracks. They could also be the one place the camera caught a process that is common wherever nitrogen sits kilometers deep.

I think the great significance of these findings, and the tantalizing picture that it promotes, is a great motivation and reason to further examine solid state nitrogen physics at very low temperatures. 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.

Orkan Umurhan, senior research scientist, SETI Institute, SwRI release

Singer put the exploration case in plainer words. Pluto has terrains seen nowhere else in the solar system, and this part of Sputnik Planitia is one of them. Its surface offers conditions that are hard to produce on Earth, and reading it teaches how materials behave in the cold. The next test is not another press image of the heart. It is a lab bench that can squeeze nitrogen ice until it melts, and a camera that can look again at those dark rims.