“Who would have expected a blue sky in the Kuiper Belt? It's gorgeous.” That was New Horizons mission principal investigator Alan Stern's response to the surprising blue haze of Pluto’s sky, which was revealed earlier this week thanks to images returned by the spacecraft, which made a historic flyby of Pluto on July 14.

The first color photos of the atmosphere have scientists like Stern, quoted above in a NASA press statement, wondering just what’s happening in the air above the dwarf planet. 

The high-altitude layer of blue is believed to be similar in nature to that seen at Saturn’s moon Titan. Both are likely caused by chemical reactions of nitrogen and methane, sparked by sunlight.

“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,” said science team researcher Carly Howett in the statement.

The actual particles are probably red or gray, and the blue comes from the way they disperse blue light. The image above was generated by software that replicates color as it would be perceived by a human eye.

That’s not all New Horizons relayed this week. Other images it transmitted back to Earth reveal several instances of water ice on Pluto’s surface, and now scientists are trying to figure out why water exists where it does. There seems to a connection between the ice and areas of the planet that are bright red due to tholin colorants. In the composite image below, the areas with water are highlighted in blue.

Image credit: NASA 

"I’m surprised that this water ice is so red,” science team member Silvia Protopapa said in the NASA statement. “We don’t yet understand the relationship between water ice and the reddish tholin colorants on Pluto's surface.”

When you combine this find with the announcement last week that Mars has liquid water, it’s been a good autumn so far for water in the Solar System.