LOS ANGELES - Computer-processed images of Pluto taken by the Hubble Space Telescope show that it is not simply a ball of ice and rock, but a dynamic world that undergoes dramatic atmospheric changes produced by its seasons, NASA said Thursday.

The images show an icy and dark molasses-colored world that is highly mottled and whose northern hemisphere is now getting brighter.

The images show that the body - once considered the ninth and most distant planet but now reduced to the status of dwarf planet - also turned noticeably redder in the two years after the turn of the millennium for reasons that are not clear, and that its equator features a large bright spot whose origin remains a mystery.

"Around the solar system, the only things whose surfaces change by a notable amount are Earth and Mars," said astronomer Mike Brown of the California Institute of Technology, a member of the team that made the findings. "Pluto has even more dramatic changes than anything else."

The new images will play a key role in targeting the New Horizons planetary probe, which was launched in 2006 and will swing by Pluto on July 14, 2015.

The probe will pass the planet so quickly that it will be able to image only one hemisphere at a limited number of sites. "These (images) have been used already to help plan the encounter," said astronomer Marc Buie of the Southwest Research Institute in Boulder, Colo., who led the team that reported the new findings. They will be published in the March issue of Astronomical Journal.

More than 350 images of Pluto were taken by Hubble in 2002 and 2003. But the object is so distant that its image in each of the pictures is only a few pixels across. To get better resolution, Buie cobbled together a network of 20 homemade computers that churned continuously for four years to produce the (relatively) high-resolution pictures. Working on a limited budget, Buie was able to build the computers for about $450 apiece.

"This must be one of the cheapest supercomputers you will ever encounter," he said at a news briefing for the National Aeronautics and Space Administration.

The images, he said, "are comparable to what you might see by looking at the moon with the naked eye. You can't see craters and geology, but it is the first step in understanding."

Pluto is in a highly eccentric orbit whose year lasts 248 Earth years. It reached its closest approach to the sun in 1989, becoming warm enough to begin melting its nitrogen and carbon monoxide ice, but still remaining a frigid -396 to -378 degrees Fahrenheit.

Comparison of the new images to those taken by Hubble in 1994 show that its atmosphere doubled during the period. Still, its atmospheric pressure is only about a millionth that of Earth. An astronaut standing in the Plutonian wind would likely not feel anything, Buie said, but "it is not so insubstantial as to give you something that can't do anything on the surface. It is extremely effective at moving frosts around on the surface."

In particular, the researchers see "a massive conveyor belt" that moves nitrogen frost evaporated from the pole in sunlight to the dark pole, where it refreezes.

After remaining constant "to a small fraction of a percent" for the 50 years that astronomers have observed Pluto, its surface color got about 20 percent to 30 percent redder over a two-year period at the beginning of the decade, Buie said. The change is not an artifact of the telescope, he added, because one of Pluto's moons is in the same image and its color has not changed. The reason for the color change "is still a mystery to be worked out."

One possible explanation for the red tinge is carbon compounds on the surface. The primary gases in the atmosphere are methane and hydrogen. When exposed to sunlight, hydrogen atoms break off and the carbon atoms combine, producing complex chemicals that are reddish. Eventually, when all the hydrogens have broken off, Buie said, the material should turn black like graphite.

"The mystery is why everything out there isn't completely and totally black," he added.

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