Dark lines on the heart
A study led by the Southwest Research Institute offers the first evidence that liquid nitrogen has recently flowed across Pluto's surface. Researchers examined images collected by NASA's New Horizons spacecraft of Sputnik Planitia, the frozen nitrogen glacier that forms the western lobe of the dwarf planet's heart-shaped region. The glacier is larger than Texas and Oklahoma combined.
In the northern part of the glacier, thin dark lines separate city-sized convection cells. New analysis suggests those lines, and broader dark patches, mark places where a liquid has occasionally wetted the ice.
Comparing Pluto with Greenland
The team, led by Alan Stern, New Horizons principal investigator and an associate vice president at SwRI, compared the spacecraft images with NASA Landsat 9 imagery of Earth. On the Greenland ice sheet, narrow dark features appear where liquid water sits on snow and ice. The Pluto features look much the same, pointing to liquid nitrogen rather than water.
Earth's surface temperatures are far too cold for liquid water to sit on Pluto's surface anyway; at those temperatures water ice behaves like rock, while nitrogen can melt, freeze and flow.
How nitrogen could melt
The glacier may be deep enough for pressure and internal heat to melt nitrogen near its base, possibly between 300 meters and 1.4 kilometers below the surface. A fracture or a change in pressure could then let liquid rise through cracks. Another possibility is that sublimation slowly thins the northern ice, weakening convection so that heat builds up underneath.
Earlier work by Stern and others suggested ancient liquid flows on Pluto. The new paper argues the process may be happening now, or happened recently, making the dark markings a type of feature that changes over time. "Pluto never stops surprising us," the researchers said. The study appears in The Planetary Science Journal.