Pluto's famous heart is bleeding, according to new research, hinting that the distant world is more alive than it looks.

That bright, heart-shaped glacier has been a fan favorite ever since NASA's New Horizons spacecraft flew past the cold outer world in 2015. Scientists now say the heart isn't just the dwarf planet's special cutie mark, it may be slowly leaking liquid — yes, liquid — onto the surface. 

Researchers say dark streaks along the heart's rim are actually stains left by liquid nitrogen that recently seeped through cracks, flowed, and refroze. The new evidence makes a compelling case that Pluto isn't a frozen, dead world — it may have working "plumbing" under its ice.

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"Pluto never stops surprising us," said Alan Stern, principal investigator of the New Horizons mission, in a statement. "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."

"Recently" here means roughly within the last million years, not last Tuesday. Still, by geological standards, that could mean this activity is ongoing today.

Despite the fact that the New Horizons flyby happened a decade ago, scientists are still unpacking the enormous, once-in-a-generation dataset that came from it. Early work tackled some of the big-picture questions, like what is Pluto made of. Now teams are undertaking more complicated investigations, like what's really happening at the base of the heart-shaped glacier. 

Their surprising findings, the result of many years of study, are published in The Planetary Science Journal. 

The heart is a region called Sputnik Planitia, a deep basin filled with frozen nitrogen. Nitrogen is the same gas that makes up most of Earth's air, but on Pluto's frigid surface, it behaves like rock. The basin is larger than Texas and Oklahoma combined and sits lower than the surrounding highlands.

"Pluto has many unique terrains seen nowhere else in the solar system, and this area of Sputnik Planitia is one of them," said Kelsi Singer, one of the study's co-authors, who works with Stern at Colorado-based Southwest Research Institute. "Exploring that allows us to better understand how materials behave in environments that are difficult to produce on Earth."

New Horizons revealed that this ice plain is strangely smooth and almost crater-free, which means it must be getting resurfaced in relatively short periods of time. The surface is divided into broad, city-sized cells that look like a very slow boiling pot. Scientists think the nitrogen ice rises in the middle of each cell and sinks at the edges, constantly churning, through the convection process.

So Pluto's heart already looked young and restless. The new work suggests it also bleeds.

The thin dark streaks and more diffuse dark patches have been visible in the images for years, but their origin was unclear. Some early ideas involved ice flowing around buried bumps or patterns carved by winds and frost.

In the new study, scientists compared those Pluto markings with satellite images of glaciers on Earth. Similar dark lines appear, for example, when meltwater trickles across a glacier and leaves wet trails. That raised an obvious question: If the patterns resemble damp ice, what could be "wetting" it?

If you were thinking rain, Pluto is far too cold and thin‑aired for any sort of precipitation, even nitrogen. So if something is wetting the surface, it's not falling from the sky — it's coming from beneath.

To explore that idea, the team modeled what happens at the bottom of Pluto's heart. Computer simulations suggest the nitrogen ice at the base of the thick glacier can slowly melt into liquid nitrogen, a bit like how ice melts at the base of some glaciers on Earth. That liquid can rise through cracks like slow-motion lava, pushing its way up through cracks and tubes.

In that way, Pluto's heart is a-pumpin'.

Researchers say the new work has implications beyond Pluto. Similar processes could help explain geysers seen on Neptune's moon Triton and features on distant dwarf planets like Eris, which also shows signs of nitrogen ice.

For a world many people still root for to be reinstated as a "real" planet, the research is another point for Pluto: a far more scientifically interesting place than once thought.