Radio active heart
Two teams have looked at the information from that encounter and have come to broadly the same conclusion - that only overturning ice, driven by the dwarf planet's internal heat, can produce the cellular terrain.The polygons are typically about 10-40km across, tiling a deep basin that is surrounded by high mountains.Each cell is domed, standing some 50m above its edges. Those edges then give way to troughs that can reach 100m in depth.
Sluggish lid
There are, however, scaling laws for convection that describe the relationship between the width and depth of convective cells. Taken at face value, these might imply the basin in which the polygons are sitting to be 10-20km deep.
It is the place where geologically recent activity on Pluto is most evident.Glaciers of nitrogen ice are observed to move away from the water-ice mountains, into the plain.
Giant boulders are carried by some of these ice streams. And because water-ice floats on nitrogen ice, the mountain fragments tend to collect in the polygons' troughs, unable to sink with the downward flow of convection. This gives the appearance of chains of hills.
Although no impact craters are seen on Sputnik Planum, there are fields of kilometre-scale pits, particularly in its eastern and southern regions. These are sectors where there is no convection.
The stagnant ice here likely vaporises over time to produce the pits. It is all part of the process that cycles nitrogen between the plain, the atmosphere, the mountains, and back into the plain via the glaciers.