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Blue skies and water ice on Pluto

NASA has released the first colour images of Pluto’s atmospheric hazes, providing new insights about the dwarf planet – and well as showing its skies are blue.

And separate imaging has confirmed that there are numerous 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 Southwest Research Institute (SwRI), Boulder, Colorado.

The haze particles themselves are likely gray or red, but scattering blue light.

“That striking blue tint tells us about the size and composition of the haze particles,” said science team researcher Carly Howett, also of SwRI.

“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.”

NASA writes:

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 the second significant finding, New Horizons has detected numerous small, exposed regions of water ice on Pluto. The discovery was made from data collected by the Ralph spectral composition mapper on New Horizons. 

Regions with exposed water ice are highlighted in blue in this composite image from New Horizons' Ralph instrument. [Credits: NASA/JHUAPL/SwRI]  

Regions with exposed water ice are highlighted in blue in this composite image from New Horizons’ Ralph instrument. [Credits: NASA/JHUAPL/SwRI]  

The strongest signatures of water ice occur along Virgil Fossa, just west of Elliot crater on the left side of the inset image, and also in Viking Terra near the top of the frame.

A major outcrop also occurs in Baré Montes towards the right of the image, along with numerous much smaller outcrops, mostly associated with impact craters and valleys between mountains.

“Large expanses of Pluto don’t show exposed water ice,” said science team member Jason Cook, of SwRI, “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.”

Bill Condie

Bill Condie

Bill Condie is a science journalist based in Adelaide, Australia.

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