Now Nasa has informally named the distinctive crater that appears to have a peak and a letter C shape within it the Eliott Crater and the faults running away from it Virgil Fossa.
A new image released today to mark the discovery shows a 280 miles across section showing where water was found.
A spokesman said: “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.”
Alan Stern, Nasa New Horizons principal investigator, from Southwest Research Institute (SwRI), Boulder, Colorado, said: “Who would have expected a blue sky in the Kuiper Belt? It’s gorgeous.”
The haze particles themselves are likely grey or red, but the way they scatter blue light has drawn the attention of the New Horizons science team.
He added: “Understanding why water appears exactly where it does, and not in other places, is a challenge that we are digging into.”
The New Horizons spacecraft is currently 3.1 billion miles (5 billion kilometers) from Earth, with all systems healthy and operating normally.
“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,” she said.
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 colouring.