New Horizons Data Reveals First Evidence of Flowing Liquid on Pluto

Scientists studying data from NASA's New Horizons spacecraft have found the first evidence that liquid may have recently flowed across Pluto's surface, a discovery suggesting the dwarf planet's frozen nitrogen glacier is more geologically active than researchers had believed.

The study, led by planetary scientist Alan Stern of the Southwest Research Institute and published by SwRI in the Planetary Science Journal, focuses on dark linear and diffuse markings along the northern edge of Sputnik Planitia, the heart-shaped nitrogen-ice glacier that dominates one hemisphere of Pluto and is larger than Texas and Oklahoma combined. Stern, who also serves as principal investigator for the New Horizons mission, is joined on the paper by SwRI principal scientist Kelsi Singer and by Orkan Umurhan, a senior research scientist at the SETI Institute who led the computer modeling behind the findings.

"Pluto never stops surprising us, and this new result certainly does that," Stern said in a statement released by NASA. "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."

Nitrogen rising through cracks

The team's models indicate that liquid nitrogen, generated by heat at the base of the glacier, can rise through narrow conduits resembling lava tubes, driven upward by buoyancy and pressure from below. Once it reaches the surface through cracks at the glacier's edge, the researchers argue, the liquid can remain fluid long enough to travel downhill across the ice before evaporating or refreezing, darkening the surrounding nitrogen ice and leaving the markings that New Horizons photographed during its July 2015 flyby. Because the surface in that region is geologically young, estimated at under a million years old, the researchers say the process appears to be ongoing rather than a relic of Pluto's distant past.

The finding adds to a growing body of evidence, built almost entirely from the roughly nine days of close-range data New Horizons gathered during its single flyby, that Pluto retains enough internal heat to drive active surface processes despite orbiting more than 3 billion miles from the sun, in a region so cold and dim that nitrogen itself freezes solid across most of the surface. Previous New Horizons results had already revealed nitrogen-ice glaciers in slow convective motion, a possible subsurface ocean, and evidence of past cryovolcanism elsewhere on the surface; the new study is the first to point specifically to liquid, rather than solid or slush-like ice, moving across the surface in the recent geological past.

New Horizons, launched in 2006 and operated by the Johns Hopkins University Applied Physics Laboratory with SwRI leading the science team, has no ability to return to Pluto and is now more than 5 billion miles from Earth, continuing to study small bodies in the Kuiper Belt. That means researchers are still mining the same dataset more than a decade after the flyby for insights like this one, a reflection of how much information the spacecraft's brief pass collected and how long it continues to reshape scientists' understanding of the outer solar system. The full paper, published in The Planetary Science Journal, lays out the modeling in detail for other researchers to test against future observations.