New analysis of decade-old spacecraft images suggests that liquid nitrogen may still be rising to Pluto's surface today, the strongest evidence yet that the dwarf planet remains geologically active rather than a frozen, inert world. The findings, based on data collected by NASA's New Horizons mission, focus on the northern edge of Sputnik Planitia, the vast nitrogen-ice glacier that forms the western lobe of Pluto's famous "heart."
Sputnik Planitia is larger than Texas and Oklahoma combined. Images New Horizons captured during its 2015 flyby show dark, thin linear features and more diffuse dark patches cutting across the glacier's northernmost reaches. Researchers led by Southwest Research Institute scientist Alan Stern, the mission's principal investigator, compared those patterns with satellite images of glaciers on Earth — captured by NASA's Landsat 9 — that have been darkened by seeping meltwater or rain. The resemblance, the team argues, suggests Pluto's markings were made the same way, only with liquid nitrogen instead of water.
Co-author Orkan Umurhan of the SETI Institute built computer models showing that nitrogen ice at the base of the glacier could melt under pressure and travel upward through narrow conduits before refreezing near the surface. That mechanism, the team says, could explain the dark streaks without requiring nitrogen "rain," which is not physically possible given Pluto's thin, cold atmosphere. The study was published in The Planetary Science Journal.
A model, not a direct sighting
It is important to note what the study does not show. No spacecraft has directly detected liquid nitrogen on Pluto's surface; the conclusion rests on matching surface patterns to a computer simulation of subsurface melting, a process the authors acknowledge "has never been studied in detail in the laboratory." The dark features are real and observed; the liquid-flow explanation for them is an inference, not a confirmed measurement.
Even with that caveat, the result adds to a growing picture of Pluto as more dynamic than expected for a small, cold world nearly 40 times farther from the sun than Earth. Sputnik Planitia already displays slow-churning convection cells, roughly city-sized, that resurface the glacier every few hundred thousand years. If confirmed, near-surface liquid nitrogen would suggest heat is escaping from Pluto's interior more efficiently, or more recently, than most models assumed.
"Pluto never stops surprising us, and this new result certainly does that," Stern said of the findings, according to the Southwest Research Institute.
New Horizons flew past Pluto in July 2015 and has not returned; the spacecraft, now well beyond Pluto's orbit, continues to send back data from the Kuiper Belt. Researchers say confirming the liquid-nitrogen hypothesis will likely require future laboratory work on nitrogen-ice physics and, eventually, a new mission capable of imaging the region at higher resolution than New Horizons could achieve during its brief flyby.
