“Who would have expected a blue sky in the Kuiper Belt? It’s gorgeous,” Alan Stern, New Horizons principal investigator, said in a statement.
While the haze is probably made up of red or gray particles, the blue tint means they are small enough to scatter blue light in the same way our own sky does. The particles are probably a bit bigger than the nitrogen that gives our sky its color, but they are in the same ballpark.
Enter tholins, which you've probably heard of at least once during this epic year of Plutonian science. These tar- or soot-like particles, which are formed when nitrogen and methane break apart in ultraviolet light and recombine in new, complex macromolecules, are responsible for the red coloring on Pluto's surface — and probably for the great red spot on its moon Charon, as well. It's most likely these particles, formed high up in Pluto's atmosphere, that create the brilliant blue scatter. You can read more about tholins here.
Blue skies aren't the only cool find announced today: NASA scientists also report finding small patches of exposed water ice. There's ice all over the chilly planet, but in most places any H2O is covered up by ices made from other molecules. Intriguingly, the team reports, the water ice seems to correspond with the parts of the planet most strongly tinted red by tholins. They are not sure why the water ice shows up on some parts of the surface and not others, but it may very well have to do with these exciting macromolecules.