Pluto is turning out to be quite the (dwarf) planet. After the very pleasant surprise that our farthest neighbor has mountains of ice 11,000 feet high comes news that the skies over Pluto are blue. Photos taken by the New Horizons probe reveal blue hazes in the planet’s atmosphere. “Who would have expected a blue sky in the Kuiper Belt? It’s gorgeous,” said Alan Stern, New Horizons principal investigator.
“That striking blue tint tells us about the size and composition of the haze particles,” said science team researcher Carly Howett. “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 tholins form high up in the atmosphere, where they are exposed to ultraviolet rays which ionize the methane and nitrogen. As they settle towards the planet’s surface they grow bigger, and appear red, hence Pluto’s ruddy appearance.