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Carbon monoxide found in Pluto's air

P.A.S. Cruickshank

This artist's impression highlights Pluto's huge atmosphere of carbon monoxide. The source of the gas is erratic evaporation from the dwarf planet's mottled icy surface. The sun appears at the top, as seen in the ultraviolet radiation that is thought to force some of the dramatic atmospheric changes. Pluto's largest moon, Charon, is at lower right.

After nearly two decades of searching, astronomers have detected carbon monoxide in Pluto’s thin atmosphere, as they expected. But they didn’t expect to find so much of it. Pluto's dramatic seasonal changes serve as further evidence that the dwarf planet is one surprising little bugger.

"Everything about Pluto is surprising," Jane Greaves, an astronomer at the University of St. Andrews in Scotland, told me. Greaves presented the new results today at the Royal Astronomical Society's National Astronomy Meeting in Wales.

Five years ago, Pluto was at the center of a controversy over the definition of planethood — which resulted in the creation of the dwarf-planet category, a new class of celestial objects. More recent observations have pointed up still more peculiarities about Pluto. For example, scientists have found that the faraway world's surface features are changing, that its atmosphere contains clouds, and that it might even harbor a pool of liquid beneath its icy shell.

Pluto's thin atmosphere, which was previously known to contain nitrogen and methane, is thought to freeze out and rise up as the world traces its eccentric orbit around the sun. Traces of frozen carbon monoxide have been detected on Pluto's surface, which led astronomers to assume that carbon monoxide gas should be found in the atmosphere as well.

Greaves and her colleagues detected the presence of carbon monoxide in a big way. Previous observations suggested that Pluto's atmosphere extended out to a distance of more than 60 miles (100 kilometers). The new results, obtained by the 15-meter James Clerk Maxwell Telescope in Hawaii, indicate that the atmosphere goes out much farther: more than 1,875 miles (3,000 kilometers), or a quarter of the way out to Charon, the largest of Pluto's three moons.

"Carbon monoxide has been searched for but never before detected," Alan Stern, principal investigator for NASA's New Horizons mission to Pluto, told me in an email. "If confirmed, this is a significant technical achievement, and there will be some interesting scientific implications as well."

Why has the atmosphere grown that much? Greaves speculated that sunlight has been hitting bright patches of frozen material on Pluto's surface, warming up the ice and causing liberal amounts of carbon monoxide and other gases to boil off. "This is now for some reason boiling off more than it was 10 years ago," when scientists in Spain made an extensive study of Pluto's atmosphere, Greaves said.

That may seem counterintuitive, considering that Pluto was at its closest point to the sun back in 1989 and is now moving farther away. But Greaves suggested that Pluto was experiencing thermal inertia — the same phenomenon that explains why the hottest time of year comes in late summer rather than midsummer. Pluto is currently in the late summertime of its full orbital cycle, which lasts for 248 Earth years.

Mark Sykes, director of the Arizona-based Planetary Science Institute, said Pluto's larger atmosphere "is not necessarily a surprise."

"My thought is that we have the sun pushing for higher latitudes on Pluto, and that the net increase in gas production as those polar ices increasingly sublimate is dominating over the freeze-out on the dark pole," he told me in an email.

Carbon monoxide tends to act as a coolant — unlike carbon dioxide or methane. which are greenhouse gases linked to global warming on Earth. The balance between the trace amounts of carbon monoxide and methane is probably a critical factor controlling the ups and downs of Pluto's nitrogen-dominated atmosphere.

"Seeing such an example of extraterrestrial climate change is fascinating," Greaves said in a news release. "This cold simple atmosphere that is strongly driven by the heat from the sun could give us important clues to how some of the basic physics works, and act as a contrasting test bed to help us better understand the earth's atmosphere."

Greaves and other scientists will be tracking what happens to the carbon monoxide and other constituents of Pluto's atmosphere for years to come, but the biggest revelations are expected to come when the New Horizons probe flies past Pluto in 2015.

In the years since New Horizons was launched in 2006, Pluto has been removed from its niche as the "ninth planet" of the solar system — and is now seen instead as one of potentially scores or hundreds of dwarf planets. But Greaves said the icy world has retained its peculiar appeal, no matter what you call it.

"I don't think the name you classify it by is that big a deal," she told me.

More about Pluto:

In addition to Greaves, co-authors of the research paper titled "Discovery of Carbon Monoxide in the Upper Atmosphere of Pluto" include Christiane Helling and Per Friberg. The paper has been accepted for publication in the Monthly Notices of the Royal Astronomical Society Letters.

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