Pluto is cold; the average surface temperature is just forty degrees Celsius above absolute zero. So how is there glacial activity on the distant body?

On Earth, glaciers form when enough snow builds up year after year to become ice. The compression of accumulated snow forces it to re-crystallize, forming larger grains. These grains grow larger, crushing out air pockets and the whole structure becomes more dense. Over the course of years, this snow will eventually become glacier ice.

This ice is soft compared to rock. So under the immense weight of its own structure, the glacial ice can become plastic. This means a glacier can flow down mountains or across plains.

A similar thing is happening on Pluto, though the materials are different. The bulk of the glaciers on Pluto are made of nitrogen, which is a soft solid ice. Water ice on the surface is much harder. At temperatures as low as Pluto’s surface, water ice is hard as rock. Not only can water ice support the weight of a nitrogen ice glacier, the relatively softer nitrogen ice can flow over water ice like a water ice glacier does over rock on Earth.

But there's another factor at play, and that’s density. Water ice is far less dense than nitrogen ice, so the big blocks of water ice can float and move in the soft nitrogen ice, and in fact, water ice hills and mountains are seen floating in Pluto’s icy heart.

Credits: NASA/JHUAPL/SwRI