Pluto has a blue sky

New Horizons delivers something amazing to the atmosphere of the dwarf planet

Color image of the atmospheric veil of Pluto by the probe New Horizons: They shine blue. © NASA / JHUAPL / SwRI
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Pluto brings new surprises: The dwarf planet has a blue sky. The veils in its atmosphere glow bluish, because small particles scatter the sunlight - much like in the atmosphere of our earth. On Pluto this is done by Tholine, reddish colored, organic macromolecules, as they are also known from Saturn's moon Titan. They could hit the Pluto surface like snow.

The dwarf planet Pluto always has new surprises in store. After a mountain of water ice, an unusually vast atmosphere and flowing glaciers, the NASA New Horizons probe has now sent out more amazing data from this outpost of the solar system.

Blue by the light scattering

The first color photographs of Pluto's atmospheric veil revealed that these veils are bright blue in color. "Who would have expected a blue sky on an object in the Kuiper Belt?" Says Head of Mission Alan Stern from the Southwest Research Institute (SwRI) in Boulder. "That's terrific!"

The color comes from the light scattering through the haze particles in the Pluto atmosphere, not from the color of the suspended particles themselves. The particles are probably gray or red, but they scatter the blue portion of the light more than the rest, similar to the Earth's atmosphere. "A blue sky often results from the scattering of sunlight by very small particles, " explains Carly Howett from SwRI. "On Earth, these are the nitrogen molecules, on Pluto, there are slightly larger, soot-like particles that we call Tholine."

Sites with exposed water ice are marked in blue here. Selsamerweise there seems to be the most Tholine there. NASA / JHUAPL / SwRI

Snow from tholin particles

Tholines are created in the atmosphere when the sun's UV light causes nitrogen and methane molecules to break down. The degradation products react with each other to form more complex macromolecules. Such tholines were first detected in the atmosphere of the Saturn moon Titan. There they contribute to the circulation of organic molecules, which have stations similar to our water cycle. display

On the dwarf planet Pluto it could be similar, as the researchers explain. Because even there, the tholins gradually accumulate in the haze and thus form ever larger particles. These scatter the light and color the atmosphere blue. However, for a certain size, they serve as condensation nuclei for other gases and form a kind of frost on them. The particles become too heavy and fall as "snow" on the surface. This layer of frozen tholins probably contributes to the reddish color of many surface regions on Pluto.

R tsel around red water ice

The new data also give new riddles. For it is precisely the red areas of the Pluto surface that are the ones in which New Horizons instruments registered the most exposed water ice. "I'm surprised that this water ice is so red, " says Silvia Protopapa of the University of Maryland. Why the Tholine seem to be enriching in these areas, is so far unclear.

Why the water ice on Pluto only appears freely in some smaller places is another open question. "Large parts of Pluto show no water ice on the surface, because it is apparently masked by other types of ice, " says James Cook from SwRI. "Explaining why water only appears where it does so is a scientific challenge that we are now working on."

(NASA, 12.10.2015 - NPO)