The latest set of pictures from NASA's Hubble Space Telescope provide the most detailed images yet of Pluto.
They show a dark, icy mottled world that is undergoing seasonal changes in its surface color and brightness.
Pluto has become much redder, while its illuminated northern hemisphere is getting brighter. These changes are probably caused by surface ice sublimating on the sunlit pole and then refreezing on the other as the dwarf planet heads into the next phase of its 248-year-long seasonal cycle.
The dramatic change in color apparently took place in a two-year period, from 2000 to 2002.
Overall, Pluto shows a white, dark-orange and charcoal-black terrain. The color is believed to be a result of ultraviolet radiation from the sun breaking up methane on Pluto's surface and leaving behind a dark and red carbon-rich residue.
Particularly noticeable is a bright spot that has been found to be unusually rich in carbon monoxide frost. It is a prime target for the New Horizons probe, which will fly by in 2015.
"Everybody is puzzled by this feature," says principal investigator Marc Buie of the Southwest Research Institute in Boulder.
The Hubble images are only a few pixels wide. But through a technique called dithering, multiple, slightly offset pictures can be combined through computer-image processing to synthesize a higher-resolution view than could be seen in a single exposure.
"This has taken four years and 20 computers operating continuously and simultaneously to accomplish," says Buie, who developed special algorithms to sharpen the Hubble data.