NASA’s New Horizons spacecraft has identified signs that liquid nitrogen might still be flowing on the surface of Pluto, a discovery that challenges previous assumptions about the dwarf planet's geologic activity. The findings come from an analysis of images taken by New Horizons during its 2015 flyby, focusing on the northern edge of Sputnik Planitia, a massive heart-shaped glacier on Pluto. This region contains dark markings that resemble features seen on Earth’s glaciers after they are exposed to water or liquid from beneath the ice.
The research team compared these patterns with images of Earth’s ice sheets, including Greenland, where similar dark lines appear when liquid water moves across frozen terrain, according to ScienceDaily. These comparisons led scientists to believe that liquid nitrogen is rising from beneath Pluto’s surface through cracks and channels. The study was led by Southwest Research Institute and included contributions from principal investigator Alan Stern of NASA's New Horizons mission.
The research was published July 31 in The Planetary Science Journal. Stern was the lead author of the study. NASA described the work as the first evidence of recently flowing liquid on Pluto.
Stern said the discovery continues to surprise scientists about what Pluto can reveal about planetary processes. The surface of Sputnik Planitia is relatively young, estimated to be less than a million years old based on models of how the glacier overturns. This suggests that the dark features observed by New Horizons must have formed in recent geological time.
The evidence does not mean New Horizons directly photographed liquid nitrogen flowing across Pluto. Researchers instead identified dark linear and diffuse features that appear similar to areas on terrestrial glaciers that have been wetted by liquid water. NASA said Pluto's atmospheric and thermal conditions make liquid nitrogen rain physically impossible. That makes a source beneath the glacier a possible explanation for the markings.
Earlier studies had suggested liquid may have flowed on Pluto in the distant past, but this is the first evidence that such activity could be happening now or very recently. Computer simulations by researchers at the SETI Institute propose how liquid nitrogen might rise from deep within Sputnik Planitia to the surface. These models show that nitrogen ice at the base of the glacier can melt and then move upward through narrow tubes under pressure or buoyancy.
The modeling was led by Orkan Umurhan of the SETI Institute. The models indicate that liquid nitrogen could move through small conduits in the glacier in a process somewhat similar to material moving through lava or geyser tubes. The nitrogen would rise because liquid nitrogen is less dense than the surrounding solid nitrogen ice.
Once it reaches the surface, the liquid nitrogen could travel downhill and wet surrounding ice, forming the dark patterns seen by New Horizons. This process may also explain similar activity elsewhere in the solar system, such as geysers on Neptune’s moon Triton. While no other clear signs of this type of flow have been found on Pluto yet, more than half of its surface has not been mapped in high resolution.
Scientists still have unanswered questions about how solid nitrogen behaves under the extreme conditions found on Pluto. Umurhan said laboratory research is needed to better understand how nitrogen ice responds to stress and strain and when those processes can cause melting. The research therefore provides evidence for the liquid-flow explanation rather than direct proof that liquid nitrogen is currently flowing across Pluto.
Scientists say further observations of Pluto and other Kuiper Belt objects will be needed to confirm if similar activity occurs in other places. The New Horizons spacecraft was built by Johns Hopkins Applied Physics Laboratory, which operates the spacecraft and manages the mission for NASA’s Science Mission Directorate. The mission is part of the New Frontiers Program overseen by NASA's Marshall Space Flight Center in Huntsville, Alabama. NASA says Southwest Research Institute directs the mission through principal investigator Alan Stern and leads its science team.
New Horizons passed Pluto on July 14, 2015. The spacecraft revealed a world with nitrogen glaciers, enormous ice plains and evidence of continuing geological processes. Earlier New Horizons studies had already shown that nitrogen and other volatile ices can move across Pluto's surface like glaciers.
This latest finding adds to the growing understanding that even icy worlds far from the Sun can have surprising geologic activity.