The most detailed images of Pluto have been captured by the Hubble Space Telescope, revealing a world undergoing seasonal surface colour and brightness changes.

The new images show variations in the surface appearance of Pluto on the order of a few hundred kilometres across. While this is too coarse for understanding the finer details of Pluto's surface geology, the images provide evidence of a complex world comprised of white, dark-orange and charcoal black terrain, colours which are thought to relate to the degrading ultraviolet radiation of the distant Sun breaking up methane present on Pluto’s surface, leaving behind a dark and red carbon-rich residue.

In particular, Pluto became significantly redder between 2000 and 2002, while its illuminated northern hemisphere became brighter and the southern hemisphere got darker. These changes most likely relate to surface ice melting on the sunlit pole and then refreezing on the other pole as the planet enters the next phase of its 248 year long seasonal cycle. Earth's seasons are driven by the tilt of our planet, but it's Pluto's elliptical orbit that defines its asymmetric seasons, with spring transitioning into summer quickly in the northern hemisphere when Pluto is moving faster along its orbit when it is closer to the Sun.

Ground-based images taken in 1988 and 2002 revealed that the mass of Pluto's atmosphere doubled during that time, likely thanks to the warming and melting of nitrogen ice. Putting the ground-based and Hubble images together hints at complex processes influencing the appearance of the visible surface, and provides more information on the seasonal processes and changes occurring in Pluto's atmosphere.

“The Hubble observations are the key to tying together these other diverse constraints on Pluto and showing how it all makes sense by providing a context based on weather and seasonal changes, which opens other new lines of investigation,” says principal investigator Marc Buie of the Southwest Research Institute in Boulder.

The Hubble images will help New Horizon mission specialists pick sites for surveying when the mission flys past Pluto in 2015. The spacecraft will pass by the planet so quickly, however, that only one hemisphere will be photographed in detail. A prime target for follow up observations is a puzzling bright spot that has been independently noted to be unusually rich in carbon monoxide frost, and has a sharp boundary alongside a region of dark black surface material. “Everybody is puzzled by this feature,” comments Buie.

The Hubble images also will help New Horizons scientists better calculate the exposure time required for each Pluto snapshot, which is important for taking the most detailed pictures possible since there will be no time for re-imaging. The Hubble images themselves are just a few pixels wide, but through a technique called dithering, multiple, slightly offset pictures are combined through computer-image processing to synthesize a higher-resolution view than can be seen in a single exposure. “This has taken four years and 20 computers operating continuously and simultaneously to accomplish,” says Buie, who developed the special algorithms to sharpen the Hubble data.

More Hubble imaging of Pluto is planned with the new Wide Field Camera 3 prior to the arrival of New Horizons.

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