Pluto: Blue Sky in New NASA Images

Pluto has blue skies. Or at least, it has a blue atmospheric haze. 

That is, images returned last week by NASA's New Horizons mission reveal that to be the case. "Who would have expected a blue sky in the Kuiper Belt? It's gorgeous," said Alan Stern, New Horizons principal investigator from Southwest Research Institute (SwRI), Boulder, Colorado, in a release from NASA.

The effect has everything to do with how light scatters, of course, just as it does regarding Earth's skies. In this case, the haze particles are likely gray or red, but they scatter blue light in an attention-getting way.

"That striking blue tint tells us about the size and composition of the haze particles," said researcher Carly Howett, also with SwRI. "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."

The general theory is that tholin particles -- those soot-like things -- form high in the atmosphere. At that point, ultraviolet sunlight is breaking apart and ionizing nitrogen and methane molecules. This allows those to interreact to build very complex, charged ions with negative or positive charges. When those recombine, they form macromolecules, which are also complex. This process was first found by scientists in Saturn's moon Titan's upper atmosphere. As more complex molecules grow, they become small particles. Then gases condense and coat them with ice frost, and eventually they fall to the surface and add to Pluto's redness, as the release confirmed. 

Along with blue skies, Pluto has many small, exposed areas of water ice, New Horizons researchers have found after looking at data from the Ralph spectral composition mapper.

However, the scientists are still working on learning why water ice is exposed in only certain spots on Pluto. Jason Cook, of SwRI, said in the release, "Understanding why water appears exactly where it does, and not in other places, is a challenge that we are digging into."

The water ice areas on Pluto are very red in the images. Said Silvia Protopapa, a science team member from the University of Maryland in the release: "We don't yet understand the relationship between water ice and the reddish tholin colorants on Pluto's surface."

At the moment, New Horizons is still about 3.1 billion miles from Earth.