New Horizons captures blue skies and water ice on Pluto

In a significant find, the first colour images of Pluto's atmospheric hazes taken by NASA's New Horizons probe reveal that the hazes are strikingly blue like the blue sky we see on Earth.

The spacecraft has also detected numerous small, exposed regions of water ice on Pluto.

"Who would have expected a blue sky in the Kuiper Belt? It's gorgeous," said Alan Stern, New Horizons principal investigator from the Southwest Research Institute, Boulder, Colorado.

The haze particles themselves are likely gray or red but the way they scatter blue light is amazing.

"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," Howett added in a NASA statement.

Scientists believe the tholin particles form high in the atmosphere, where ultraviolet sunlight breaks apart and ionizes nitrogen and methane molecules.

The probe also spotted water ice on Pluto. "Understanding why water appears exactly where it does, and not in other places, is a challenge that we are digging into," said science team member Jason Cook.

The areas showing the most obvious water ice spectral signatures correspond to areas that are bright red in the colour images.

"I'm surprised that this water ice is so red. We don't yet understand the relationship between water ice and the reddish tholin colorants on Pluto's surface," asked Silvia Protopapa, science team member from the University of Maryland.

New Horizons is currently five billion kilometres from Earth with all systems healthy and operating normally.