Washboard and fluted terrains on Pluto as evidence for ancient glaciation

Distinctive landscapes termed ‘washboard’ and ‘fluted’ terrains^1,2, which border the N₂ ice plains of Sputnik Planitia along its northwest margin, are among the most enigmatic landforms yet seen on Pluto. These terrains consist of parallel to sub-parallel ridges that display a remarkably consistent east-northeast–west-southwest orientation—a configuration that does not readily point to a simple analogous terrestrial or planetary process or landform. Here, we report on mapping and analysis of their morphometry and distribution as a means to determine their origin. Based on their occurrence in generally low-elevation, low-relief settings adjacent to Sputnik Planitia that coincide with a major tectonic system, and through comparison with fields of sublimation pits seen in southern Sputnik Planitia, we conclude that washboard and fluted terrains represent crustal debris that were buoyant in pitted glacial N₂ ice that formerly covered this area, and which were deposited after the N₂ ice receded via sublimation. Crater surface age estimates indicate that this N₂ ice glaciation formed and disappeared early in Pluto’s history, soon after formation of the Sputnik Planitia basin. These terrains constitute an entirely new category of glacial landform.