Pluto, the dwarf planet on the outer edge of our solar system, has a dramatically ruddier hue than it did just a few years ago, NASA scientists said Thursday, after examining photos taken by the Hubble Space Telescope.

They said the distant orb appears mottled and molasses-colored in recent pictures, with a markedly redder tone that most likely is the result of surface ice melting on Pluto's sunlit pole and then refreezing on the other pole.

The remarkable color shift, which apparently took place between 2000 and 2002, confirms that Pluto is a dynamic world undergoing dramatic atmospheric changes and not simply a ball of ice and rock, according to scientists at the US space agency.

They said they will compare Hubble pictures taken in 1994 with some from 2002 and 2003, as they search for more signs of seasonal change, including evidence that Pluto's northern polar region has gotten brighter, while the southern hemisphere has darkened.

"The Hubble observations are the key to... showing how it all makes sense by providing a context based on weather and seasonal changes, which opens other new lines of investigation," said the leader of the study, principal investigator Marc Buie of the Southwest Research Institute in the western US city of Boulder, Colorado.

Pluto -- declassified as a full-fledged planet in August 2006 -- has a 248-year orbit and an axial tilt which, unlike Earth, alone drives the seasons. The icy orb's seasons are asymmetrical because of its elliptical orbit.

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, NASA said.

Scientists are hoping to collect additional riveting snapshots of Pluto when NASA's next space probe, dubbed New Horizons, flies by the dwarf planet in 2015.

Hubble underwent repair during a space shuttle mission last year that left it with a new camera and spectrograph, as well as spruced up scientific instruments.

The repair job marked the end of NASA's human missions to the beloved Hubble. Launched in 1990, the telescope was repaired and upgraded in 1993, 1997, 1999, 2002 and 2008.

Last year's final upgrade extended the life of Hubble another five years.