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Blue Pluto
A picture from NASA’s New Horizons probe reveals the blue color of Pluto’s atmospheric haze, as seen in a backlit view. Credit: NASA / JHUAPL / SwRI

The latest pictures from NASA’s New Horizons mission to Pluto reveal for the first time that the backlit dwarf planet is surrounded by a beautiful blue glow – and also pinpoint the location of water ice deposits exposed on the surface.

Thursday’s images were released after a hubbub that suggested an “amazing” discovery would be revealed this week. Although the hype got a bit out of control, the revelations really do raise intriguing questions about Pluto’s weather and geology.

First, about that blue sky: New Horizons captured pictures of the sunlight scattered by Pluto’s thin atmospheric haze shortly after its July 14 flyby – but the color view from the Multispectral Visible Imaging Camera, part of the spacecraft’s Ralph instrument suite, came down only recently.

“Who would have expected a blue sky in the Kuiper Belt? It’s gorgeous,” mission principal investigator Alan Stern said in Thursday’s news release. (The Kuiper Belt is the broad ring of icy objects that lie beyond the orbit of Neptune.)

Stern and his colleagues suspect that the particles creating the blue glow are actually reddish bits of soot-like compounds known as tholins. Such particles would be created when the sun’s ultraviolet radiation sparks reactions between nitrogen and methane in the atmosphere.

So how can reddish material create a blue glow? It all has to do with what happens to light rays as they zip through the edge of the atmosphere. Red light is scattered away by the fine particles, while blue light continues its path to the observer. A similar phenomenon explains why Titan’s orange atmosphere looks blue at the edge, and why sunsets are blue on red Mars.

Then there’s the evidence for water on Pluto: For weeks, New Horizons’ scientists have suspected that the dwarf planet’s towering mountains were built from deposits of frozen H2O, but the latest view gets into the details. Ralph’s spectral composition mapper reveals the areas where water ice is exposed – and where it isn’t.

Water ice deposits
Regions with exposed water ice are highlighted in blue in this composite image from New Horizons’ Ralph instrument, combining visible imagery from the Multispectral Visible Imaging Camera with infrared spectroscopy from the Linear Etalon Imaging Spectral Array. The scene is approximately 280 miles (450 kilometers) across. Credit: NASA / JHUAPL / SwRI

“Large expanses of Pluto don’t show exposed water ice, because it’s apparently masked by other, more volatile ices across most of the planet,” science team member Jason Cook of the Southwest Research Institute is quoted as saying. “Understanding why water appears exactly where it does, and not in other places, is a challenge that we are digging into.”

One curious clue: The places where water ice is exposed seem to correspond with the places where those reddish tholins are concentrated on the surface.

New Horizons will be sending back data from its flyby for at least the next year, so stay tuned for still more amazing discoveries from Pluto – and more amazing mysteries as well.

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