Pluto's polygons explained

Abstract

Imagine sunrise on a frozen plane. Shadows withdraw as the Sun climbs above distant mountains that rise from below the horizon. The ice sheet itself is largely featureless, with a difference in elevation of only some tens of metres over distances of many tens of kilometres - nothing slows the shadows' retreat. The sky overhead remains black, and it stays chilly, only about 35 degrees above absolute zero. This is morning on Sputnik Planum, Pluto.