A Southwest Research Institute (SwRI)-led research team has discovered evidence of liquid nitrogen rising to the surface of Pluto through cracks in the dwarf planet’s heart-shaped glacier, Sputnik Planitia.
According to the researchers behind the detection, the liquid nitrogen evidence captured by NASA’s New Horizons Spacecraft is the first evidence of any liquid flowing on Pluto. They also suggest that the discovery could help scientists better understand other solar system features such as the geysers on Neptune’s moon Triton.
New Horizons Mission Gave First Hint of Liquid Nitrogen on Pluto
When New Horizons captured images of Pluto’s surface between 2015 and 2016, it gathered extensive imagery of a massive surface feature called Sputnik Planitia. Larger than Texas and Oklahoma combined, the frozen nitrogen lake has long fascinated planetary scientists.
When examining the spacecraft’s photos, researchers spotted thin, dark, linear features separating the city-sized geologic convection cells on Sputnik Planitia’s northernmost portions. Critically, the thinner, separating features were more diffuse than the larger ones. Although some previous studies suggested that Pluto may have a liquid water ocean beneath its frozen outer shell, the surface temperatures would prevent any of that water from flowing on the surface long enough to cause the features.
Some theories proposed a liquid nitrogen explanation for the thinner features, even though the research team behind the discovery said that Pluto’s atmospheric and thermal conditions make liquid rain physically impossible. Still, the team notes, the surface patterns on Pluto’s northern Sputnik Planitia “have been darkened in ways that resemble glacial features on Earth that have been wetted by water rain or the subsurface emergence of liquids to the surface.”
To better understand the data, SwRI Associate Vice President and principal investigator on the New Horizons mission, Dr. Alan Stern, reexamined the data, focusing on similar features on Earth.
Sputnik Planitia Evidence “Very Similar” to Greenland Ice Sheet Features
To start, Dr. Stern and colleagues compared the New Horizons images with Earth imagery collected by NASA’s Landsat 9. The researchers focused on the Greenland Ice Sheet, since similar surface features caused by liquid water separating ice and snow have been previously observed.
When focusing on topography, the team said the two regions appeared “very similar.” This similarity suggested that subsurface liquids in the form of nitrogen were rising to the surface and wetting areas of Sputnik Planitia’s nitrogen ice.
SwRI Principal Scientist Dr. Kelsi Singer, one of the study’s co-authors, said Sputnik Planitia’s surface is “quite young” in geological terms, with most estimates placing it at less than one million years. As a result, the dark, thin surface features must have formed during that short window of time.
“Pluto has many unique terrains seen nowhere else in the solar system, and this area of Sputnik Planitia is one of them,” Dr Singer explained. “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.”
To support the visual evidence of liquid nitrogen on Pluto, Dr. Orkan Umurhan, a senior research scientist at the SETI Institute, oversaw the creation of computer models designed to simulate the dwarf planet’s local environment. According to the study authors, the models “show that nitrogen ice at the base of Pluto’s kilometers-deep Sputnik glacier can melt, forming liquid nitrogen.”
The same models also revealed that buoyancy and pressure from underneath can cause liquid nitrogen to migrate to Pluto’s surface via a series of small conduits that resemble lava or geyser tubes. Although the dwarf planet’s distance from the Sun makes it an extremely frigid environment, the models also suggested that once liquid nitrogen reaches Pluto’s surface, it can maintain liquid form long enough to flow down the nitrogen glacier slopes and cause the features seen in the images.
“Pluto Never Stops Surprising Us”
When discussing the significance of the findings, Dr. Umurhan pointed to the study’s “tantalizing picture that it promotes,” including potentially motivating a further examination of solid-state nitrogen physics at very low temperatures.
“Specifically, it’s important to examine the physics taking place in solid nitrogen materials under stress and strain, which can cause them to melt,” the study author explained, adding that these processes “have never been studied in real detail in the laboratory.”
The SwRI-led team also noted that over half of Pluto remains unmapped in high resolution, meaning there is still a lot of room for future discoveries. This includes further mapping of the other Kuiper Belt planets “to determine if similar processes are occurring elsewhere in that region of the solar system.”
Finally, the SwRI-led team said the findings could offer some assistance in explaining other events across the solar system, such as the geysers observed by NASA’s Voyager 2 on Neptune’s moon Triton.
“Pluto never stops surprising us, and this new result certainly does that,” Stern concluded. “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.”
The study “Evidence for possible N2 basal flow beneath Pluto’s northern Sputnik Planitia” was published in The Planetary Science Journal.
Christopher Plain is a Science Fiction and Fantasy novelist and has spent the last six years as Associate News Editor and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him at christopher@thedebrief.org