"Who would have expected a blue sky in the Kuiper Belt? It's gorgeous," said Alan Stern, principal investigator of the New Horizons team at the Southwest Research Institute in Boulder, Colorado.
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.
"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."
Scientists believe the tholin particles form high in the atmosphere, where ultraviolet sunlight breaks apart and ionizes nitrogen and methane molecules, allowing them to react with each other to form 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 major 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.
"Large expanses of Pluto don't show exposed water ice," said science team member Jason Cook, "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."
A curious aspect of the detection is that the areas showing the most water ice correspond to areas that are bright red in recently released color images.
"I'm surprised that this water ice is so red," says Silvia Protopapa, a science team member from the University of Maryland, College Park. "We don't yet understand the relationship between water ice and the reddish tholin colorants on Pluto's surface."
The New Horizons spacecraft is currently 3.1 billion miles from Earth, with all systems healthy and operating normally.
New Horizons is part of NASA's New Frontiers Program, managed by the agency's Marshall Space Flight Center in Huntsville, Alabama. The Johns Hopkins Applied Physics Laboratory designed, built, and operates the New Horizons spacecraft and manages the mission for NASA's Science Mission Directorate. The Southwest Research Institute leads the science mission, payload operations, and encounter science planning.