When the New Horizons probe zipped past Pluto last July, it spotted weird, kilometers-wide polygonal features (lower right of image above) within a broad icy plain dubbed Sputnik Planum. Now, researchers have come up with an explanation for these odd shapes: Each one is an immense mass of ice that’s picking up the dwarf planet’s internal heat at its base, rising toward the surface at its center, and then circulating outward from there and downward at its edges, à la huge versions of tabletop “lava lamps” popular in the 1960s. Using images gathered during the flyby, scientists estimate that the generally flat surface of each circulating mass is up to 50 meters higher in elevation than average at its center, where material is upwelling, and is surrounded by a trough as much as 100 meters deep where material circulates downward, as they report online today in Nature. Other data have shown the icy plains are predominantly made of nitrogen but also include carbon monoxide and methane, all of which are soft enough to flow at Pluto’s frigid 37 K (–236°C). The team’s computer simulation of the glacierlike flow within each mass suggests that the surface ice moves horizontally at no more than a few centimeters each year, which nevertheless is quick enough to totally resurface each polygonal cell every 500,000 years or so. This repaving helps explain why the surface is relatively crater-free. It may also explain why other bodies in the Kuiper belt are unusually bright, the researchers say: Fresh ice is typically much brighter than an ancient stagnant surface on which dust and substances produced by interactions with sunlight or other radiation have built up through the ages.