Data released from NASA's New Horizons on Thursday probe showed that the distant dwarf planet has a blue atmospheric haze.
The space agency released an image that showed a blue layer surrounding Pluto, taken by the spacecraft's camera.
Blue skies often results from scattering of sunlight by very small particles. "On Earth, those particles are very tiny nitrogen molecules," said science team researcher Carly Howett, from the Southwest Research Institute (SwRI). "On Pluto they appear to be larger - but still relatively small - soot-like particles we call tholins."
The particles are thought to accumulate condensed gases on their surfaces, eventually falling to the planet's surface and adding to its red coloring.
New Horizons launched from Earth in 2006 to study the Pluto system, as well as other objects in the Kuiper Belt zone - at the frozen distant edge of our solar system.
"Who would have expected a blue sky in the Kuiper Belt? It's gorgeous," said Alan Stern, one of the principal investigators on the New Horizons project.
Puzzle of water ice distribution
Researchers were also able to map out areas of water ice on various parts of Pluto's surface, and will now puzzle out why it appears to be distributed where it is.
"Large expanses of Pluto don't show exposed water ice," said team member Jason Cook, also of SwRI. "Understanding why water appears exactly where it does, and not in other places, is a challenge that we are digging into."
Last week, NASA released the best color pictures it has obtained so far of Pluto's largest moon, Charon, which has a diameter more than half that of Pluto itself.
The images showed canyons that stretched more than 1,000 miles (1,600 kilometers) above the equator. Each surface fracture appeared to be about four times as long as the Grand Canyon and, in places, twice as deep.
rc/jr (AFP, AP)