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Pluto maps raise new questions

M. Buie / SwRI / NASA / ESA
Click for video: Hubble Space Telescope images, taken in 2002-2003, were
combined to produce these maps of Pluto. Click on the image to watch Pluto spin.

Pluto hasn't been getting much respect lately, but today the Hubble Space Telescope's team unveiled maps of the dwarf planet that are just a foretaste of the extreme close-up to come.

The maps spark fresh questions about the icy world that was discovered 80 years ago this month: Why has Pluto's northern hemisphere brightened so quickly over the course of just a few years? What's causing darker spots in the south? And why is Pluto getting redder all over?

"We think these changes are actually driven by seasonal changes," said Marc Buie, a planetary scientist at the Colorado-based Southwest Research Institute.

Huge amounts of methane and nitrogen ice appear to be moving from one part of the world to another through Pluto's wisp of an atmosphere. One particularly bright spot appears to be rich in frozen carbon monoxide.

So what's the precise mechanism for the shift? "That's a mystery," Buie said. The complete answers might well have to wait until 2015, when NASA's New Horizons probe swings past Pluto and its moons.

'A fascinating world'
The color variations seen in Hubble imagery captured between 1994 and 2003 represent "the biggest changes we've ever seen" in the solar system, said Caltech astronomer Mike Brown, the world's most accomplished dwarf-planet discoverer.

Brown noted that "this really is the time when we expected that exciting things were going to happen on Pluto." Buie said its northern hemisphere is getting a lot of springtime exposure to the sun's rays, while the other hemisphere is going through a dark autumn. At the same time, Pluto is getting significantly farther away from the sun, due to its highly elliptical 248-year orbit, which is expected to produce a global cooling effect.

Buie said Pluto's temperature ranges from 351 below zero Fahrenheit to minus-382 (60 to 43 Kelvin). If Earth's orbit were as extreme as Pluto's, our planet's temperatures would be swinging from the 60s in springtime to 90 degrees below zero F in the fall (16 to -68 degrees Celsius), Brown said. That illustrates how wild Pluto's weather can get.

"It's a ridiculously extreme place to be," Brown said.

One hypothesis is that methane frost is falling out of Pluto's atmosphere as it gets colder, creating the brighter whitish-orangish spots. Over time, the sun's ultraviolet radiation breaks down the methane, resulting in a residue of dark reddish-black carbon. Buie said another idea is that frozen nitrogen is being taken up into the atmosphere, leaving behind "fairy castle structures" that scatter light more efficiently.

"There are enough theories out there to basically describe every scenario you could imagine," Buie told me.

M. Buie / SwRI / NASA / ESA
A comparison of Pluto imagery from 1994 (top) and 2002-2003 (bottom) shows
significant changes in surface brightness.

The findings give the solar system's most controversial world a little extra mystique. It's been a rough few years for Pluto. When Brown found an object on the solar system's rim that was bigger than Pluto in 2005, that set off a controversy leading to Pluto's demotion from the nine-planet lineup a year later. Pluto's status remains controversial - as explained in my book, "The Case for Pluto."

Brown said observations from Pluto could be compared with studies of other dwarf planets, farther away in the icy Kuiper Belt, to help unravel the mysteries behind the surface variations.

During today's teleconference, Brown joked that someone sent him an e-mail claiming that "Pluto was mad at me and ... that's why it was getting red." More seriously, Brown said the newly released research showed that asking whether or not Pluto was an official planet was no longer "a terribly interesting question."

"Pluto is just a fascinating world, and it really doesn't care what we call it," he said. "It's a great place to study, and we'll be learning a lot more about it in years to come."

In a sentimental sidelight, Brown noted that today's report came on what would have been the 104th birthday of Pluto's discoverer, Clyde Tombaugh (1906-1997).

Four years of number-crunching
The data behind the new maps came from Hubble's Advanced Camera for Surveys in 2002 and 2003, before the camera went on the blink. The ACS was repaired last year during the final shuttle servicing mission to the space telescope.

In each of Hubble's pictures of Pluto, the dwarf planet's images was no more than a few pixels wide. But Buie wrote special software to process the scant data and create a sharper composite image that was roughly analogous to our naked-eye view of Earth's moon.

"This has taken four years and 20 computers operating continuously and simultaneously to accomplish," Buie said in today's news release from NASA.

At one point Buie had to interrupt his analysis of Pluto's surface to focus on two new moons that turned up by surprise in the Hubble imagery. He and his colleagues announced the discovery of those moons, now known as Nix and Hydra, in 2005.

A paper on the fresh findings is due to be published in March's issue of The Astronomical Journal. In addition to Buie, the authors include Lowell Observatory's William Grundy and Eliot Young, Leslie Young, and Alan Stern of Southwest Research Institute. Stern is also the principal investigator for the New Horizons mission.

Buie said future observations using Hubble's brand-new Wide Field Camera 3 may shed more light on Pluto's dark (and bright) mysteries. Definitive answers might not be available until New Horizons gives Pluto its extreme close-up in 2015, however.

The new findings are already being used to plan the detailed choreography for the probe's encounter - and the bright spot of frozen carbon monoxide is a prime target. "Everybody is puzzled by this feature," Buie said.

Update for 5:25 p.m. ET: I've repeatedly tweaked the text in an effort to characterize the nature of seasonal changes on Pluto more accurately, but I'm still puzzled. And maybe that's precisely the point.

Update for 2:25 a.m. ET Feb. 5: Watch Pluto spin in this video.

Update for 5 p.m. ET Feb. 5: I've tweaked the text still more to provide a betterĀ comparison of temperature variations on Pluto and Earth.

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