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Pluto's Warming Atmosphere.

DISTANT PLUTO HAS BEEN MOVING AWAY from the Sun since its 1989 perihelion, and astronomers worry that the diminishing sunlight will cause what little atmosphere Pluto has to "collapse" as frost onto its surface before a flyby mission can be mounted to this last unexplored planet. But for now, at least, Pluto seems to be holding its own. Results from two recent stellar occultations suggest that Pluto's atmospheric pressure has nearly tripled during the past 15 years and that the surface temperature has risen some 2[degrees]C during that time.

"It's very obvious that there's been a fundamental change," observes Marc W. Buie (Lowell Observatory), who was among those presenting Pluto results at last October's meeting of the American Astronomical Society's Division for Planetary Sciences.

The cause of the warming remains unclear. There's some evidence that Pluto's icy surface has become darker in recent years, in which case it would absorb more of the weak sunlight and drive more gas into the atmosphere. Alternatively, a frost layer near the north pole (now experiencing its spring season) may be sublimating more vigorously. Or maybe the planet's surface is reaching its peak warmth more than a decade after perihelion simply because of thermal inertia. "The hottest part of the day on Earth is usually around 2 or 3 p.m., rather than noon," explains David J. Tholen (University of Hawaii).

The only other record of Pluto occulting a star, in June 1988, revealed that the diminutive planet had a thin nitrogen atmosphere (with a surface pressure only 50 millionths that at sea level on Earth) laced with traces of methane and carbon dioxide. Astronomers waited patiently for another such opportunity, and by chance the distant planet passed over two stars just five weeks apart last July and August (S&T: July 2002, page 93). The path of the first event, on July 20th Universal Time, crossed central South America. Unfortunately, it skirted the giant telescopes arrayed in the Andes and was recorded only by two portable telescopes near the Peru-Chile border. However, on August 20th Pluto's star-shadow passed directly over the observatories atop Mauna Kea and Haleakala in Hawaii. The event was also recorded at Lick and Palomar Observatories in California and at Lowell in Arizona.

Two large research teams, led by James L. Elliot (MIT) and Bruno Sicardy (Paris Observatory), mounted observing campaigns for the recent events. They agree that the atmospheric pressure has jumped but remain divided on whether the wispy air mass has the same basic structure seen 14 years ago or whether new features have appeared.

Elliot notes that Pluto's case bears similarities to the situation on like-size Triton, Neptune's largest moon, whose thin atmosphere has also warmed in recent years. But months of tedious analysis remain before the light curves' subtle bumps and squiggles can be fully understood.

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Author:Beatty, J. Kelly
Publication:Sky & Telescope
Date:Jan 1, 2003
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