Pluto: A World of Blue Skies and Red Ice

posted: 10/09/15
by: Jason Major for Discovery News
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A tiny frozen world three billion miles away from the sun isn't the place where you might expect to find a brilliant blue sky, but that's exactly what NASA's New Horizons team has discovered on far-off Pluto.

Of course, if you were to somehow stand on Pluto and look up, the sky above your head would still look black. Pluto's atmosphere is much too thin to actually fill in with scattered light. But at sunset and sunrise -- which, on Pluto, are about 3.2 Earth-days apart -- you might see the horizon illuminated by a lovely blue glow.

Blue skies on Pluto banner crop

The image above is a color-composite made from data acquired by New Horizons' Ralph/Multispectral Visible Imaging Camera (MVIC) as the spacecraft was traveling away from Pluto after its historic close encounter on July 14. It shows the silhouette of Pluto backlit by the sun, surrounded by a sky-blue multilayered haze.

"Who would have expected a blue sky in the Kuiper Belt? It's gorgeous," said Alan Stern, principal investigator for the New Horizons mission from the Southwest Research Institute in Boulder, Colorado.

Here on Earth, the daytime sky appears blue because of the way molecules of nitrogen and oxygen in the atmosphere disperse visible light from the sun, an effect called Rayleigh scattering. Shorter blue wavelengths are scattered more effectively, giving the sky its blue color.

On Pluto, 33 times farther from the sun than Earth is, light from our star is scattered through layers of atmospheric haze in much the same way -- except there it's not by molecules of oxygen or nitrogen but by very tiny soot-like particles called tholins.

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