"That striking blue tint tells us about the size and composition of the haze particles," said NASA's Carly Howett in a statement. "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." These tholins are likely generated high above Pluto's surface where solar radiation breaks down atmospheric nitrogen and methane into these smaller particles.
Equally intriguing is the discovery of surface water ice, which would also appear red to the naked eye. This is, again, due to the high concentrations of thorin particles covering much of the dwarf planet. "Large expanses of Pluto don't show exposed water ice," Jason Cook, of NASA's Southwest Research Institute (SwRI), said in a statement. "because it's apparently masked by other, more volatile ices across most of the planet. Understanding why water appears exactly where it does, and not in other places, is a challenge that we are digging into." Luckily, the New Horizon's spacecraft is still in tip-top shape, despite being some 3.1 billion miles from Earth.