NASA images Pluto's mysterious 'floating hills'

WASHINGTON, Feb. 4 (UPI) -- New imagery from NASA's New Horizons probe showcases a unique feature of Pluto's geography -- "floating hills." These isolated hills are found scattered across Pluto's icy plains, or Sputnik Planum, a portion of the dwarf planet's heart-shaped region.

NASA scientists think the hills are composed of water ice, which is less dense than the nitrogen-dominated ice that makes up Sputnik Planum, causing the hills to float across the icy plains like an iceberg set adrift in the Arctic.

Like icebergs, the hills are fragments of larger deposits of water ice found upland. Pluto's glaciers slowly carry them out into the flat, barren plains. Once in the sea of frozen nitrogen, 'chains' of hills are subject to the convection patterns of the nitrogen ice.

As the hills are pushed towards the edges of these convection cells, they begin to congregate and butt up against each other. Some of the hill clusters stretch 12 miles across.

A separate collection of hills is found farther north. The cluster is called Challenger Colles in honor of the crew of the lost space shuttle Challenger. The group measures 37 miles by 22 miles. Researchers think hills become "beached" in the area as they're pushed into an especially shallow region of nitrogen ice.