BLUE SKIES ON PLUTO: Earth isn't the only planet with blue skies

BLUE SKIES ON PLUTO: Earth isn't the only planet with blue skies. Pluto has them, too. The first color images of Pluto's atmosphere were beamed back to Earth by NASA's New Horizons spacecraft just last week, and the sky looks a lot like home:

New Horizons took the picture just after it sped by Pluto on July 14, 2015. The spacecraft's cameras were looking back at Pluto's nightside as sunlight illuminated the fringe of blue around Pluto's circumference.

"Who would have expected a blue sky in the Kuiper Belt?" says Alan Stern, principal investigator of the New Horizons mission. "It's gorgeous."

Carly Howett of the New Horizons science team explains the phenomenon: "A blue sky often results from scattering of sunlight by very small particles. On Earth, those particles are nitrogen molecules. On Pluto they appear to be soot-like particles we call tholins."

The term "tholin" was coined by Carl Sagan and Bishun Khare to describe organic substances they obtained in Miller-Urey experiments on gas mixtures akin to atmosphere of Saturn's moon Titan. On Pluto, tholins form high in the atmosphere where UV sunlight breaks apart nitrogen and methane molecules. The fragments re-combine to form complex macromolecules. These macromolecules continue to combine and grow until they become "tholins."

Ironically, tholins themselves are not blue. They merely scatter blue light. When tholins fall to the ground they show their true colors: gray or red. At least some of Pluto's patchy red coloring is thought to result from a gentle rain of these particles from the planet's atmosphere.