New Horizons Finds Blue Skies and Water Ice on Pluto

NASA | OCTOBER 19, 2015

The first color images of Pluto’s atmospheric hazes, returned by NASA’s New Horizons spacecraft last week, reveal that the hazes are blue.

Pluto’s haze layer shows its blue color. Credits: NASA/JHUAPL/SwRI

The haze particles themselves are likely gray or red, but the way they scatter blue light has gotten the attention of the New Horizons science team. 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.

Scientists believe the tholin particles form high in the atmosphere, where ultraviolet sunlight breaks apart and ionizes nitrogen and methane molecules and allows them to react with one another to form more and more complex negatively and positively charged ions. When they recombine, they form very complex macromolecules, a process first found to occur in the upper atmosphere of Saturn’s moon Titan.

The more complex molecules continue to combine and grow until they become small particles; volatile gases condense and coat their surfaces with ice frost before they have time to fall through the atmosphere to the surface, where they add to Pluto’s red coloring.

In a second significant finding, New Horizons has detected numerous small, exposed regions of water ice on Pluto.

Water Ice on Pluto. Credits: NASA/JHUAPL/SwRI

A curious aspect of the detection is that the areas showing the most obvious water ice spectral signatures correspond to areas that are bright red in recently released color images.

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