For a world without weather, without oceans, and with a surface temperature hovering near minus 380 degrees Fahrenheit, Pluto keeps finding new ways to surprise the scientists who study it. A team led by planetary scientist Alan Stern of the Southwest Research Institute reports the first evidence that liquid, not just ice, has recently moved across the dwarf planet's surface.
The finding centers on Sputnik Planitia, the vast nitrogen-ice glacier that forms the western lobe of Pluto's famous heart-shaped feature. It is larger than Texas and Oklahoma combined. Poring back over imagery that NASA's New Horizons spacecraft captured during its single flyby in July 2015, researchers identified narrow, dark surface markings near the glacier's northern edge that closely resemble features on Earth where liquid water wets snow and ice, such as parts of the Greenland ice sheet.
Nitrogen rising from below
The team's explanation is not rainfall, which is physically impossible in Pluto's thin, frigid atmosphere. Instead, they propose that heat trapped beneath the kilometers-thick glacier is periodically melting nitrogen ice from below, a process called basal melting, and pushing liquid nitrogen up through surface cracks. The researchers compared the New Horizons images directly against Landsat 9 satellite pictures of Greenland's melt features to make the case, a cross-planetary comparison detailed in the Planetary Science Journal.
Because Sputnik Planitia's surface is geologically young, estimated at under a million years old based on models of how its icy cells overturn, the dark markings must have formed relatively recently in the glacier's history. That timing matters: it suggests the process is not a one-time relic of Pluto's formation but something closer to an ongoing, if intermittent, phenomenon.
"Pluto never stops surprising us, and this new result certainly does that," said Stern, who was also the principal investigator on the New Horizons mission, in a statement accompanying the paper. Co-author Kelsi Singer noted that this corner of Sputnik Planitia is unlike any other terrain yet documented in the solar system.
Researchers caution that the evidence is indirect. No spacecraft has directly sampled the material, and New Horizons will not return to Pluto; the conclusions rest on interpreting a single, decade-old set of images alongside terrestrial analogues. Orkan Umurhan of the SETI Institute, who was not involved in acquiring the original data but commented on the physics, said the result is a strong motivation to study how nitrogen ice behaves at extremely low temperatures, since the physics remains poorly understood at Pluto's conditions. Confirming the mechanism will likely require future missions capable of observing Sputnik Planitia over time.
