Complex hazes rise over Pluto's surface, which is backlit by the setting sun in this image from New Horizons.
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Complex hazes rise over Pluto's surface, which is backlit by the setting sun in this image from New Horizons.

Pluto at Dusk: Spectacular, and Spookily Earthlike

There’s a world near the solar system’s fringe that, in many ways, is more Earthlike than anyone could have imagined. Here, frosted glaciers slowly drain moisture from icy mountains that tower over smooth, lowland basins. Overhead, the sky is filled with haze – lots and lots of it, carved into multiple layers.

That world is Pluto.

In July, the New Horizons spacecraft flew within 8,000 miles of the dwarf planet’s surface, furiously snapping photos and gathering data about the composition of the world’s frosty veneer. Then, 15 minutes after its closest approach, New Horizons flipped around and caught a Pluto, just before sunset. Today, NASA released the spacecraft’s dusky postcard.

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A near-sunset view of Pluto’s rugged, icy mountains and flat plains. The heart, Sputnik Planum (right), is flanked by rugged mountains up to 11,000 feet (3,500 meters) high, including the informally named Norgay Montes in the foreground and Hillary Montes on the skyline. The scene is 780 miles wide. (NASA/JHU/SwRI)

If you randomly stumbled across the image, you’d be forgiven for thinking you’d found a black-and-white image of the Earth.

The image, “reminds me of the Transantarctic Mountains along the Ross Ice Sheet, because of the tall mountains looming over a flat open expanse of ever-changing ice,” says New Horizons team member Simon Porter of the Southwest Research Institute.

On Earth, we have a pretty good idea of how mountains form. On Pluto, it’s not clear yet. There are no obvious volcanoes, eroding plateaus, or colliding tectonic plates. “The formation of blocky mountains on Pluto may be more analogous to the breakup of sea ice on Earth, but on a much grander scale,” says team member Alan Howard of the University of Virginia.

And then there are those glaciers, which scientists assume flow as glaciers do on Earth, advancing inch by inch and draining moisture from the mountains. Except Pluto’s glaciers are made from nitrogen, not water ice. Scientists think it’s likely Pluto has something analogous to Earth’s hydrological cycle, where water evaporates from the oceans, rains or snows back down, and returns to the seas by rivers and glaciers. But Pluto’s chemistry and temperature are different, perhaps dominated by soft nitrogen ice, and also featuring methane and carbon monoxide ices.

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Ice, probably frozen nitrogen, that accumulated on the highlands (right) is draining from Pluto’s mountains into the informally named Sputnik Planum through a series of valleys (red arrows). The flow front of the ice moving into Sputnik Planum is outlined by the blue arrows. (NASA/JHU/SwRI)

“What happens is that these ices evaporate from the icy plains informally called Sputnik Planum [the heart], are transported through the atmosphere tens to hundreds of kilometers, and then are deposited on the surface, probably as frosts,” Howard says. “Where enough frost has accumulated, it flows like glaciers back to the source, which we think is Sputnik Planum.”

So, a nitrological cycle, perhaps fueled by Pluto’s beating heart?

Among the features in the image that have scientists most excited is the haze shrouding Pluto’s surface – haze that is more complex than anticipated, and more voluminous than on Earth. “I still haven’t managed to wrap my head around the way that Pluto’s haze looks,” says Sarah Hörst of the The Johns Hopkins University. “Everything I know and believe about haze formation told me Pluto would have haze, but I really wasn’t sure we would be able to see it in the images.”

On Earth, hazes tend to form near the ground, when the gases in car exhaust (for example) reacts with sunlight to produce particles. On Pluto — and around Saturn’s moon Titan — haze forms all over, including high in the atmosphere, and is compositionally different than on Earth. For Hörst, the most remarkable thing about the haze in this new image are its spectacular layers, the origin of which is still a mystery. “The incredibly detailed structure will be able to tell us a lot about the atmospheric processes that are occurring here,” she says.

On Pluto, not everything hazy is alien, though. “If you were standing on the top of one of the mountains rising maybe 1.5 miles above the surrounding terrain, and looked towards the setting (or rising) Sun, you could see ground-hugging hazes like you often see on Earth,” Howard says.

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The setting sun illuminates a fog or near-surface haze, cut by the parallel shadows of many local hills and small mountains on Pluto. (NASA/JHU/SwRI)

As surprising as it is for a small, frozen world to look rather Earthy, there’s really nothing special about sharing characteristics with our home world. The more we look, the more we find familiar faces and features on alien worlds – things like volcanoes on Io, geysers on Enceladus, methane rivers and seas on Titan, plate tectonics on Europa. And, there’s water, water everywhere. That’s why, by better understanding these exotic places in our solar neighborhood, we end up learning more about this planet we call home.