NASA’s New Horizons spots signs of liquid nitrogen flowing on Pluto

The findings provide the first evidence that liquid has flowed on Pluto in relatively recent times. The research is based on observations from NASA's New Horizons spacecraft and was led by SwRI Associate Vice President Dr. Alan Stern, principal investigator of the New Horizons mission.

"Pluto never stops surprising us," Lead Author Stern said, "and 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."

Strange Dark Features on Pluto's Giant Glacier

Sputnik Planitia is a vast glacier made largely of frozen nitrogen and is bigger than Texas and Oklahoma combined. Images collected by New Horizons in 2015 and 2016 showed that its northern region contains city-sized geologic convection cells separated by thin dark lines and broader, more diffuse dark patches.

A new examination of those features suggests that they may occasionally become temporarily wet. The most likely source is liquid nitrogen rising from below the surface. The findings have been published in the peer-reviewed Planetary Science Journal.

Liquid nitrogen cannot fall as rain on Pluto because the dwarf planet's temperature and atmospheric conditions do not allow it. Even so, parts of northern Sputnik Planitia have been darkened in patterns that resemble features seen on terrestrial glaciers after they are exposed to rainwater or to liquid emerging from beneath the ice.

Clues From Greenland's Ice Sheet

To investigate the resemblance, the SwRI-led team compared images from New Horizons with NASA Landsat 9 images of icy regions on Earth, including Greenland's ice sheet.

On Greenland, narrow dark markings appear in places where liquid water is present on top of ice and snow. Similar patterns appear on Sputnik Planitia, leading researchers to propose that subsurface liquid, specifically nitrogen, may be rising upward and wetting Pluto's frozen nitrogen 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, one of the study's co-authors. "Pluto has many unique terrains seen nowhere else in the solar system, and this area of Sputnik Planitia is one of them. Its surface provides a different set of conditions compared to what we are used to on Earth, and exploring that allows us to better understand how materials behave in environments that are difficult to produce on Earth."

Earlier studies, including research led by Stern, had proposed that liquid once flowed on Pluto in the distant past. The new findings go further by suggesting that liquid nitrogen may exist beneath Sputnik Planitia today or may have been present there very recently.

How Liquid Nitrogen Could Reach the Surface

Computer simulations led by Dr. Orkan Umurhan, senior research scientist at the SETI Institute, offer a possible explanation for how this could happen.

The models indicate that nitrogen ice at the bottom of Sputnik Planitia, which is several kilometers deep, can melt and produce liquid nitrogen. That liquid could then rise through narrow channels toward the surface, much like material moving through lava or geyser tubes. Buoyancy or pressure from below could help push the liquid upward.

Once it reaches the surface, the liquid nitrogen could remain in liquid form long enough to travel downhill across the glacier. As it moves, it could wet the surrounding nitrogen ice and create the dark markings observed by New Horizons.

"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," 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."

Implications Beyond Pluto

So far, researchers have not identified clear evidence of this type of basal liquid flow elsewhere on Pluto. However, more than half of the dwarf planet has never been mapped at high resolution, leaving open the possibility that similar processes could be occurring in other regions.

The same mechanism involving melting and the upward movement of liquid may also help explain activity elsewhere in the solar system. One possible example is Triton, Neptune's largest moon, where NASA's Voyager 2 spacecraft observed geysers erupting from the surface.

Scientists say additional high-resolution observations of Pluto and other Kuiper Belt planets will be needed to determine whether similar processes occur elsewhere in the distant solar system.

The Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, designed, built, and operates the New Horizons spacecraft and mission for NASA's Science Mission Directorate. The Planetary Missions Program Office at Marshall Space Flight Center (MSFC) in Huntsville, Alabama, provides NASA oversight for New Horizons. Southwest Research Institute, based in San Antonio, directs the mission via Principal Investigator Dr. Alan Stern, who leads the science team, payload operations, and science planning. New Horizons is part of the New Frontiers Program managed by NASA's MSFC.