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If you went on a trip to Pluto, you could mail folks back home a scenic postcard that featured white-capped mountains. That's because Pluto is the only place in the solar system other than Earth that is known to have mountains with white peaks. As NPR's Nell Greenfieldboyce reports, these mountaintops aren't white because of fallen snow.
NELL GREENFIELDBOYCE, BYLINE: A tourist on Pluto could take in all the sights that NASA's New Horizons probe discovered five years ago, like a big heart-shaped glacier made of frozen nitrogen, methane and carbon monoxide. And just to the west of Pluto's icy heart, there's mountains. Tanguy Bertrand is an astronomer at NASA's Ames Research Center. He says these mountains are made of water ice.
TANGUY BERTRAND: Water ice on Pluto is so cold that it's hard, just like a rock on Earth. So that's why you can make mountains of water ice on Pluto.
GREENFIELDBOYCE: The mountains are about 2 1/2 miles tall, like the Alps. They're mostly dark brown and red because they're covered in particles from Pluto's haze. The peaks, however, are bright white. And because they look so much like white-capped mountains on Earth...
BERTRAND: Initially, it seemed logical that this high-altitude frost could form like on the Earth.
GREENFIELDBOYCE: But now he and his colleagues say in the journal Nature Communications that Pluto's mountaintops got their white in an alien way. On Earth, snow collects at mountaintops because at higher altitudes, the air and ground are colder. As a moist wind approaches a mountain...
BERTRAND: It rises upslope, and it cools. And the water condense there to form snow on clouds on top of the mountains.
GREENFIELDBOYCE: On Pluto, it's completely different.
BERTRAND: It's not snowing on Pluto.
GREENFIELDBOYCE: He says there, the atmosphere actually gets warmer at higher altitudes because of heating from the sun. The surface of these water ice mountains remains super cold, however. And because these tall mountains peak in a region that's full of methane gas, the methane becomes frost on the cold surface.
BERTRAND: So you have no formation of clouds, and then you have no formation of icy particles in the atmosphere. Every single (unintelligible) is directly at the surface.
GREENFIELDBOYCE: He says something similar might happen on Triton, a moon of Neptune. But Triton is kind of flat. So Pluto and Earth may be the only spots in the solar system to enjoy white-capped mountain scenery.
Nell Greenfieldboyce, NPR News.
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