When NASA’s New Horizons spacecraft looked back at Pluto following its historic July 14, 2015, flyby, sunlight scattering through a tenuous nitrogen atmosphere revealed a striking blue twilight. Over a decade later, researchers analyzing those same high-resolution images have discovered evidence that liquid nitrogen may have risen from beneath Pluto’s frozen surface and briefly flowed across Sputnik Planitia.
Backlighting Pluto: How a Departed Spacecraft Revealed a Blue Twilight
The piano-sized New Horizons spacecraft had already passed the dwarf planet when it performed one of the mission’s most revealing observations. Turning its instruments back toward Pluto, the probe placed the world directly between the camera and the Sun. The surface became a dark disc, while a thin, luminous atmosphere edged in a brilliant blue appeared around the silhouette.
That departure view changed the geometry. New Horizons reached closest approach on July 14, 2015, after a journey of more than nine years. While sunlit views mapped mountains, plains, and volatile ice during the pass, the high-phase geometry of the retreat turned the atmosphere into a bright rim. Haze particles strongly scatter light in particular directions, and seen around a silhouette, aerosol particles send sunlight toward the camera along long slant paths through the atmosphere.
The blue result came from Ralph, one of the remote-sensing instruments aboard New Horizons. Its Multispectral Visible Imaging Camera, or MVIC, measured Pluto through several wavelength bands. NASA combined blue, red, and near-infrared information to reproduce colour as closely as possible to human perception, revealing more than a dozen atmospheric layers and low-lying haze crossed by shadows.
Sputnik Planitia and the Mystery of the Nitrogen Glaciers
Pluto’s heart-shaped region features a vast nitrogen glacier known as Sputnik Planitia, an ice sheet larger than Texas and Oklahoma combined. This bright western basin holds nitrogen ices accumulated to a thickness of at least 2.5 miles (4 kilometers). The heavy mass of the glacier, combined with the tidal pull of the moon Charon, forced the dwarf planet to reorient itself in a process known as true polar wander.
James Tuttle, a planetary scientist at the Jet Propulsion Laboratory in Pasadena, California, explained that planetary bodies change their spin axis in response to large geologic processes. The constant imbalance of Sputnik Planitia literally tipped the dwarf planet so the basin aligned more closely with the tidal axis between Pluto and Charon.
Beyond surface reorientation, New Horizons data indicated a heavier mass beneath the basin, which scientists suspect is a subsurface water ocean.
Subsurface Flow: Could Liquid Nitrogen Have Wet Pluto’s Surface?
More than a decade after the initial encounter, scientists revisiting the flyby images published a peer-reviewed analysis in The Economic Times titled Evidence for possible N2 basal flow beneath Pluto’s northern Sputnik Planitia.
The study suggests liquid nitrogen may have risen from beneath the frozen surface to briefly flow across the glacier.
Because Pluto’s surface is far too cold for nitrogen rain, researchers used computer modeling to demonstrate that nitrogen ice deep beneath the glacier could melt under unusual pressure, stress, and strain. The liquid could then push upward through narrow pathways like geyser or lava conduits, reaching the surface long enough to travel downhill and wet nearby ice.
To test the visual patterns, the research team examined Landsat 9 images of Greenland’s ice sheet, discovering narrow dark features where liquid water had reached snow and ice surfaces. These patterns resemble the markings observed in northern Sputnik Planitia.
Dwarf Planet Dynamics and the Tholin Haze Mystery
This physical complexity matches the overall behavior of the dwarf planet. The surface of Sputnik Planitia is estimated to be less than one million years old because convection continually overturns the nitrogen ice, meaning any surface markings formed within that young geological window.
“Pluto never stops surprising us.”
Continuing the Journey Through the Outer Solar System
Following its encounter with Pluto and a subsequent flyby of the small Kuiper Belt object Arrakoth on New Year’s Day 2019, the spacecraft continues its journey. Located some 4.6 billion miles (7.4 billion km) from Earth, the mission team is actively seeking a suitable target for a potential third flyby.
New visualisations released by NASA continue to showcase the wealth of data captured during the historic encounter. Moviemaker Paul Schenk of the Lunar and Planetary Institute in Houston used LORRI black-and-white images and MVIC color data to construct simulated flights over Pluto and Charon, mapping surface details as small as 230 feet across on Pluto and revealing Vulcan Planitia’s low-lying volcanic plains on Charon.
