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Why Is The Sky Blue? (On Pluto, That Is)

Pluto's atmosphere has a haze of blue, in this color picture taken by the New Horizons spacecraft that was released by NASA.

Color images of Pluto released by NASA this year show the dwarf planet has a reddish brown surface. But an even newer photo shows that despite those colors, Pluto's atmosphere has a blue haze.

The discovery results from the New Horizons probe's fly-by of Pluto, which also captured data showing that the planet contains "numerous small, exposed regions of water ice," NASA says.

But first things first: Why would a planet that's been known to be reddish — even pink — have a blue sky?

Scientists attribute the color disparity to tholins, particles formed after sunlight sparks chemical reactions between nitrogen and methane in the atmosphere. The process was first seen on Titan, Saturn's moon; in the case of Pluto, the particles are likely gray or red — but they scatter blue light, making it the most visible to the human eye.

"A blue sky often results from scattering of sunlight by very small particles. On Earth, those particles are very tiny nitrogen molecules. On Pluto they appear to be larger — but still relatively small — soot-like particles we call tholins."

Those tholins fall to the surface, forming what NASA has called "a reddish 'gunk.'"

In addition to the blue-sky report, NASA says New Horizons detected exposed regions of water ice on Pluto.

A "curious" aspect of the new data, the space agency says, is that the sections of Pluto's surface with the most obvious water ice "correspond to areas that are bright red" in a recent "false color" image of the planet that enhances its composition and texture.

Figuring out that relationship is one of the research scientists' next tasks.

"I'm surprised that this water ice is so red," says science team member Silvia Protopapa of the University of Maryland. "We don't yet understand the relationship between water ice and the reddish tholin colorants on Pluto's surface."

The images and data were recently sent back from the New Horizons spacecraft, which buzzed Pluto as it sped through the outer edge of our solar system in July. The craft is now 3.1 billion miles away from Earth.

Regardless of the explanation, the researchers are enjoying the view.

"Who would have expected a blue sky in the Kuiper Belt? It's gorgeous," says New Horizons principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colo.

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