Pluto’s icy plain known as Sputnik Planum appear mostly flat. But, shortly after New Horizons close encounter with Pluto – images were beamed back to Earth showing clumps of hills. Here’s an image released right after last July’s flyby.

Sputnik Planum

At the time, scientists were blown away by the vast icy plains of Pluto. They didn’t really have an explanation for the hills either. After all, they were just seeing the images for the first time too. Early indications pointed to the hills rising above the surrounding terrain.

Today, New Horizons gives us another explanation. The hills are ‘floating.’ Nitrogen ice glaciers appear to be carrying large chunks of water ice from Pluto’s uplands.

Pluto water ice hills

The image above shows us the surprising geological activity at play here. How are nitrogen ice glaciers carrying huge chunks of water ice? Nitrogen ice is denser than water ice. So, the water ice floats on top of the sea of nitrogen ice. NASA compares it to how icebergs float in Earth’s Arctic Ocean.

‘Hill chains’ highlight the flow path water ice hills take along the glaciers before clumping together. If you look closely, you can see exactly where the hills interact with the convective motions of the nitrogen ice and are pushed towards the edges of the ‘cells.’

Here’s another image without all the annotations.

Pluto floating hills

It’s hard to grasp the sheer size of the hills based on this image, but NASA says the clusters are up to 12 miles across. Each hill measures anywhere from one to several miles across.

At the top right, a feature named Challenger Colles (honoring the crew of the space shuttle Challenger) spans 37 by 22 miles. Its location and size offer scientists clues as to what’s going on. They believe these hills are being ‘beached’ because the nitrogen ice is shallow there.

Fly Me to Pluto

When New Horizons caught up with Pluto last year, it opened a whole new world to us. And with it, plenty of mysteries. Today, the New Horizons team scratches one of the mysteries off their list and continues to show us just how amazing a small rock at the edge of our solar system is.

Image credits: NASA

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