A heart-shaped glacier on Pluto has been a favorite since NASA's New Horizons spacecraft flew by in 2015. Scientists now think the heart may be slowly leaking liquid onto the surface.

Researchers say dark streaks on the heart are stains from liquid nitrogen that seeped through cracks and refroze. This suggests Pluto may have working 'plumbing' under its ice.

Alan Stern, the principal investigator of the New Horizons mission, said Pluto never stops surprising us. The new evidence suggests liquids have recently flowed on Pluto's surface.

Recently means roughly within the last million years, not last Tuesday. This activity could still be happening today.

Despite the New Horizons flyby happening a decade ago, scientists are still studying the data. They are now looking at what's happening at the base of the heart-shaped glacier.

The surprising findings are published in The Planetary Science Journal.

The heart is a region called Sputnik Planitia, a deep basin filled with frozen nitrogen. It's larger than Texas and Oklahoma combined.

Kelsi Singer, a study co-author, said Pluto has unique terrains. Exploring Sputnik Planitia helps us understand how materials behave in difficult environments.

New Horizons revealed that the ice plain is smooth and almost crater-free. The surface is divided into broad cells that look like a slow boiling pot.

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

The thin dark streaks on Pluto have been visible for years, but their origin was unclear. Scientists compared them to satellite images of glaciers on Earth.

Similar dark lines appear on Earth's glaciers when meltwater trickles across and leaves wet trails. This raised a question: what could be 'wetting' Pluto's surface?

It's not rain, as Pluto is too cold and thin-aired. If something is wetting the surface, it's coming from beneath.

The team modeled what happens at the bottom of Pluto's heart. Computer simulations suggest the nitrogen ice can slowly melt into liquid nitrogen.

The liquid can rise through cracks like slow-motion lava, pushing its way up. This is similar to how ice melts at the base of some glaciers on Earth.

Pluto's heart is essentially pumping. Researchers say the new work has implications beyond Pluto.

Similar processes could help explain geysers on Neptune's moon Triton and features on distant dwarf planets like Eris.

For a world many people still root for to be reinstated as a 'real' planet, the research is another point for Pluto: a scientifically interesting place.