The nitrogen ice glaciers on Pluto appear to carry an intriguing cargo: numerous, isolated hills that may be fragments of water ice from Pluto’s surrounding uplands. These hills individually measure one to several miles or kilometers across, according to images and data from NASA’s New Horizons mission.
The hills, which are in the vast ice plain informally named Sputnik Planum within Pluto’s ‘heart,’ are likely miniature versions of the larger, jumbled mountains on Sputnik Planum’s western border. They are yet another example of Pluto’s fascinating and abundant geological activity.
Because water ice is less dense than nitrogen-dominated ice, scientists believe these water ice hills are floating in a sea of frozen nitrogen and move over time like icebergs in Earth’s Arctic Ocean. The hills are likely fragments of the rugged uplands that have broken away and are being carried by the nitrogen glaciers into Sputnik Planum. ‘Chains’ of the drifting hills are formed along the flow paths of the glaciers. 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.
At the northern end of the image, the feature informally named Challenger Colles – honoring the crew of the lost space shuttle Challenger – appears to be an especially large accumulation of these hills, measuring 37 by 22 miles (60 by 35 kilometers). This feature is located near the boundary with the uplands, away from the cellular terrain, and may represent a location where hills have been ‘beached’ due to the nitrogen ice being especially shallow.
http://www.nasa.gov/feature/pluto-s-mysterious-floating-hills
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