Liquid nitrogen may have recently flowed across Pluto’s heart-shaped glacier

A new study led by Southwest Research Institute (SwRI) provides evidence that liquid nitrogen is rising to Pluto’s surface through cracks at the northern edge of Sputnik Planitia, part of the massive heart-shaped glacier on the dwarf planet’s surface. This is the first evidence of liquid flow occurring recently on Pluto. The study is based on data from NASA’s New Horizons spacecraft and led by SwRI Associate Vice President Dr. Alan Stern, the 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.”

Sputnik Planitia is a vast, frozen nitrogen glacier on Pluto, larger than Texas and Oklahoma combined. In 2015 and 2016, New Horizons images of the northernmost portions of this region revealed city-sized geologic convection cells on Sputnik Planitia, separated by thin, dark linear features and more diffuse dark features. New analyses indicate that these dark, linear and diffuse features appear to be occasionally and temporarily wetted, perhaps from time to time, by a liquid—most likely liquid nitrogen.

This result was published in Planetary Science Journal.

Earth-like clues in Pluto ice

Pluto’s atmospheric and thermal conditions make liquid nitrogen rain physically impossible. Nonetheless, the surface patterns on northern Sputnik Planitia have been darkened in ways that resemble glacial features on Earth that have been wetted by rainwater or the subsurface emergence of liquids.

The SwRI-led team of Pluto researchers compared the New Horizons images with NASA Landsat 9 imagery of locations on Earth, including the Greenland ice sheet. There, dark, narrow surface features have been identified in areas where liquid water occurs on ice and snow. The Sputnik Planitia images from New Horizons appear very similar, suggesting that subsurface liquid nitrogen is rising and wetting Pluto’s nitrogen ice.

“The surface of Sputnik Planitia is quite young, probably less than 1 million years old 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.”

Models point to buried melt

The new work provides the first evidence of recent liquid flows to Pluto’s surface. Earlier research, including some led by first author Stern, suggested ancient liquid flows, but the new work suggests there is currently or recently liquid nitrogen beneath the glacier’s surface.

Computer models led by Dr. Orkan Umurhan, senior research scientist at the SETI Institute, show that nitrogen ice at the base of Pluto’s kilometers-deep Sputnik glacier can melt, forming liquid nitrogen. Furthermore, these computer models revealed that this liquid can be transported upward to the surface through small conduits, similar to lava or geyser tubes, driven by buoyancy or pressure from below.

The researchers found that once it reaches the surface, the melted nitrogen can remain liquid long enough to flow downward on slopes on Sputnik Planitia’s nitrogen glacier, wetting the icy surface and producing the observed dark features.

“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.”

Unmapped terrain leaves open questions

While no other regions of Pluto show evidence of basal flow, more than half of Pluto remains unmapped in high resolution. The melting process and the liquid’s upward movement on Pluto could also potentially explain other events seen across the solar system, such as the geysers observed by NASA’s Voyager 2 on Neptune’s moon Triton. Further high-resolution mapping of Pluto and other Kuiper Belt planets is needed to determine if similar processes are occurring elsewhere in that region of the solar system.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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