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Pluto's hills of water ice
This image focuses in on a part of Pluto’s heart-shaped region where hills of water ice appear to be floating on top of a nitrogen glacier. Challenger Colles, toward the top of the inset photo, is a wide cluster of water-ice hills. (Credit: NASA / JHUAPL / SwRI)

There’s plenty of evidence that Pluto is a frozen water world, with mountains of ice that rise more than 10,000 feet in height, but here’s something even weirder: Huge chunks of frozen H2O appear to be floating in a sea of frozen nitrogen, like icebergs in Earth’s polar regions.

Those are among the findings reported today in this week’s update from NASA’s New Horizons mission. The piano-sized New Horizons spacecraft collected gigabytes’ worth of observations last year during its July 14 flyby, and it’s been sending back data ever since then.

The Plutonian icebergs dot a vast plain in the bright heart-shaped region that’s informally known as Sputnik Planum. They measure anywhere between a mile to several miles in width.

New Horizons’ science team suggests that the hill-like chunks have been broken away from the neighboring uplands and are being carried along by the flow of Sputnik Planum’s nitrogen glaciers. The blobs of frozen water float on top because they’re less dense than the frozen nitrogen. In some areas, you can see chains on chunks moving along channels in the glaciers.

There’s a particularly large cluster of debris in an area that’s about 37 by 22 miles across, up near the notch in Pluto’s heart. The chunks may have clustered there because they “washed up” on a strand where the nitrogen slush is shallow, New Horizons’ scientists say.

The cluster has been nicknamed Challenger Colles in honor of the crew of the space shuttle Challenger, which blew apart in 1986. Just one week ago, NASA marked the 30th anniversary of the Challenger shuttle disaster. It’s not yet clear whether the Challenger nickname will endure — but the tribute holds true for this year, at least.

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