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New detailed images of the distant frozen world of Pluto, have for the first time shown a white, dark-orange and charcoal-black coloured terrain undergoing seasonal changes.

The new NASA images taken by the Hubble Space Telescope indicate the dwarf planet has become significantly redder, with its illuminated northern hemisphere getting much brighter.

Scientists at a NASA press conference say Pluto's colour is believed to be a result of ultraviolet radiation from the distant sun breaking up methane on the surface, leaving behind a dark and red carbon-rich residue.

Compared to earlier Hubble pictures taken in 1994, the new set of images taken in 2002 and 2003, show the northern polar region has become brighter, while the southern hemisphere is now darker.

Dr Simon O'Toole, a planetary scientist with the Anglo-Australian Observatory says "these changes hint at very complex processes affecting the visible surface."
More than ice and rock

Principal NASA investigator Dr Marc Buie from the Southwest Research Institute in Boulder, Colorado says "the new images are allowing planetary astronomers to better interpret more than three decades of Pluto observations from other telescopes."

"The new Hubble pictures show Pluto is not simply a ball of ice and rock, but a dynamic world that undergoes dramatic atmospheric changes."

Ground-based observations, taken in 1988 and 2002, show that the mass of the atmosphere doubled over that time, which may be due to warming and sublimating nitrogen ice.

"These are driven by seasonal changes which are caused by a combination of Pluto's highly elliptical orbit, and its axial tilt, unlike Earth where the tilt alone drives seasons," says Buie.

He says, Pluto's seasons are very different because of its highly elliptical orbit around the Sun, which take 248 years to complete.

"Spring transitions to polar summer quickly in the northern hemisphere because Pluto is moving faster along its orbit when it is closer to the Sun."
New Horizons

The Hubble images will remain the sharpest view of Pluto until NASA's New Horizons spacecraft approaches the planet in July 2015.

"The Hubble pictures will help scientists select where New Horizons will fly when it reaches Pluto in 2015," says O'Toole.

Buie says prominent feature stands out.

"Particularly noticeable in the Hubble image is a bright spot that has been independently noted to be unusually rich in carbon monoxide frost," he says. "It's a prime target for New Horizons."

"Everybody is puzzled by this feature and New Horizons will get an excellent look at the boundary between this bright feature and a nearby region covered in pitch-black surface material."

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