Just like Earth has icebergs that float in icy oceans, Pluto also has hills of floating water ice glaciers. What makes these large chunks of water ice on the dwarf planet different from Earth's icebergs is that they float on top of frozen nitrogen.
As scientists pore over images beamed back by NASA's New Horizons, which flew by and captured images of the dwarf planet in July last year, they saw a series of hills in a vast plain called Sputnik Planum.
The region is known to have large quantities of nitrogen ice that give the region a smooth appearance against a series of craggy mountains. Each of the hills in this region is about a couple of miles across and appears to move.
According to NASA, the phenomenon has something to do with ice floating on ice. Pluto has rock-hard water ice glaciers that float atop frozen nitrogen, which abound in the frigid world, but how does ice float on top of ice?
The ice-on-ice movement seen on Pluto is made possible by involving two different types of ice: the nitrogen ice and the more familiar water ice.
Nitrogen does not freeze until it reaches -346 degrees so it is icy but flowing. Pluto's minus-380 degrees Fahrenheit temperatures likely make the water ice glaciers as hard as mountain rocks on Earth but these structures are still less dense compared to frozen nitrogen.
Because nitrogen ice is denser compared to the standard ice, the ice water hills can float in their position. Scientists believe that these hills break apart and are carried on top of the nitrogen flow, causing them to move based on the flow's pattern. NASA scientists likened this movement to those of the icebergs found in Earth's Arctic Ocean.
"'Chains' of the drifting hills are formed along the flow paths of the glaciers," NASA reported. "When the hills enter the cellular terrain of central Sputnik Planum, they become subject to the convective motions of the nitrogen ice, and are pushed to the edges of the cells, where the hills cluster in groups reaching up to 12 miles (20 kilometers) across."