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Liquid nitrogen may be rising through cracks beneath Pluto’s surface and reaching the dwarf planet’s surface near Sputnik Planitia, its vast heart-shaped glacier, according to a new study led by the Southwest Research Institute (SwRI).
The findings provide the first evidence that liquid may have flowed on Pluto relatively recently. The study, led by SwRI Associate Vice President and New Horizons principal investigator Alan Stern, was published in the peer-reviewed Planetary Science Journal.
Sputnik Planitia is a huge glacier made largely of frozen nitrogen and covers an area larger than Texas and Oklahoma combined. Images taken by NASA’s New Horizons spacecraft during its 2015 flyby show city-sized convection cells in the glacier’s northern region, separated by narrow dark lines and broader dark patches.
Researchers now suggest that some of these dark features could form when liquid nitrogen temporarily reaches the surface and wets the frozen nitrogen ice.
Unlike Earth, Pluto cannot receive liquid nitrogen as rainfall because of its extremely cold conditions and thin atmosphere. However, the patterns observed on Sputnik Planitia resemble markings found on terrestrial glaciers where liquid water reaches or moves across ice.
To investigate the similarities, the researchers compared New Horizons images with NASA Landsat 9 observations of icy regions on Earth, including Greenland. They found that some dark markings on Greenland occur where liquid water is present on the surface, providing a possible analogue for Pluto.
“The surface of Sputnik Planitia is quite young, probably less than one million years,” said SwRI Principal Scientist Kelsi Singer, a co-author of the study. She said the features must therefore have formed relatively recently.
Computer simulations led by SETI Institute senior research scientist Orkan Umurhan provide a possible mechanism. The models indicate that nitrogen ice several kilometres beneath Sputnik Planitia could melt and produce liquid nitrogen.
The liquid could then move upward through narrow channels, driven by buoyancy or pressure from below. Once reaching the surface, it could flow downhill across the glacier and temporarily wet the surrounding nitrogen ice, producing the dark patterns observed by New Horizons.
Previous research had suggested that liquid may have flowed on Pluto in the distant past. The new study raises the possibility that liquid nitrogen exists beneath Sputnik Planitia today or was present there in the geologically recent past.
Researchers have not yet found clear evidence of similar basal liquid flows elsewhere on Pluto. More than half of the dwarf planet, however, has never been mapped at high resolution.
The findings could also have implications for other icy worlds. A similar process may help explain some activity on Triton, Neptune’s largest moon, where NASA’s Voyager 2 observed geyser-like eruptions during its 1989 flyby.
The researchers say laboratory studies of nitrogen ice under extremely low temperatures and pressure will be important for testing how such melting and subsurface movement could occur on Pluto.