The icy dwarf planet Pluto undergoes dramatic seasonal changes, according to images from the Hubble Space Telescope.
The pictures from Hubble revealed changes in the brightness and the colour of Pluto's surface.
Mike Brown, from the California Institute of Technology, suggested Pluto had the most dynamic surface of any object in the Solar System.
Hubble will provide our sharpest views of Pluto until the New Horizons probe approaches in 2015.
The researchers note that Pluto became significantly redder in a two-year period, from 2000 to 2002.
When Hubble pictures taken in 1994 are compared with a new set of images taken in 2002 to 2003, astronomers see evidence that Pluto's northern polar region has become brighter.
Referring to the striking changes on Pluto, Professor Brown commented: "Imagine the Moon changing by that much. We're used to looking at the Moon and it being the same night after night. This thing has changed dramatically in that time.
"If you look around the entire Solar System, the only things that change their surfaces by any noticeable amount are the Earth, where ice caps come and go. There is Mars, where ice caps come and go. That's it.
 | Dramatic changes on Pluto 
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"[With Pluto] you are looking at the surface in the Solar System that has the biggest changes of anything we've ever seen."
Pluto is one of a population of icy objects which inhabit an outer region of the Solar System called the Kuiper Belt.
At some 2,360km (1,467 miles) across, Pluto is smaller than several moons in the Solar System. Its eccentric orbit carries it around the Sun every 248 years.
Marc Buie, co-author of the research from the Southwest Research Institute in Boulder, US, said the redness seen on Pluto could be related to carbon. He added that methane (CH4) had been detected on the surface using spectroscopic measurements.
"If you put methane in an environment like that, it's not stable... because you have this dynamic, young surface, you could stir things up and release more methane all the time."
But researchers said it was not possible to relate different colours on Pluto to differences in the surface composition.
The brightening in the northern hemisphere could be the result of nitrogen ice vaporising at the sunlit pole and then refreezing at the opposite pole which is not illuminated.
But Marc Buie said the exact mechanism was a mystery: "What we think is maybe happening, is that as you are vigorously sublimating nitrogen off the lit pole, it has to be changing the texture of the ice and frost on the surface."
He added: "This could be gardening the surface and turning it into a 'fairy castle' structure that is more effective at scattering light back," he said.
Pluto's extreme orbit is one of the reasons behind the dramatic changes observed on its surface.
"Right now, Pluto is nearly in its closest position to the Sun. If you go to the year 2108, it's in its furthest position from the Sun. In that time period temperatures on Pluto will change dramatically," said Professor Brown.
| PLUTO: A QUICK GUIDE Named after underworld god Lost status as planet in 2006 Average of 5.9bn km to Sun Orbits Sun every 248 years Diameter of 2,360km Has at least three moons Rotates every 6.8 days Gravity about 6% of Earth's Surface temperature -233C Nasa probe visits in 2015 |
"It's close to springtime on Pluto. In the fall, it will be so much further away from the Sun, and so much colder. Things that boiled up in the spring will condense."
"We think that these things are driven by seasonal processes on Pluto," said Dr Buie, "But it's a little bit of a surprise that you would see this big of a change this fast because the seasons take 248 of our years to progress."
Co-author Dr Alan Stern, also from the Southwest Research Institute, offered one example of how these changes could be speeded up.
He said computer models had shown "there are a series of closed-loop feedbacks that can force the process to change quickly".
For example, he told BBC News, "as Pluto draws away from the Sun, the sunlight is weaker and the planet wants to cool off. As it cools, the atmosphere must snow to the ground.
New Horizons will shed light on Pluto's composition
"When it snows, the surface becomes brighter, which helps it cool and speeds the process. That feeds on itself and you get more and more cooling."
The images, taken by Hubble's Advanced Camera for Surveys (ACS), will be vital for planning details of the New Horizons spacecraft's flyby of Pluto in 2015.
New Horizons will pass by Pluto so quickly that only one hemisphere will be photographed in the highest possible detail.
Alan Stern, who is principal investigator on the mission, said that with every great planetary reconnaissance mission "we have always learnt that when we get there, we are blown away by how primitive our ideas were from blurry images taken from Earth.
He told BBC News: "When we get there, the odds are very high that we will have so much more information and rich detail that all our views circa 1990 and 2000 and 2010 will appear antiquated. That's why I don't like to make predictions."
He added: "No one predicted river valleys on Mars, or volcanoes on the Galilean satellites, or that Mercury was mostly a core and little else. It's entirely likely that Pluto will be something so surprising that everything we've done so far looks quaint in comparison."
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