Balmy Days in the Outer Solar System

Global warming isn't happening only on Earth: A moon of Neptune, some 4.4 billion kilometers further from the sun, also is seeing the mercury rise. New observations show that the atmosphere of Neptune's largest moon, Triton, is much thicker today than it was a decade ago, according to a report in today's Nature. Astronomers suspect some of the nitrogen frost coating the moon's frigid surface has vaporized, thanks to a decades-long change of "seasons" on Triton as Neptune orbits the sun.

Voyager 2 detected an atmosphere around Triton during its flyby of Neptune in 1989. The air is exceedingly thin, some 50,000 times less dense than Earth's. Nevertheless, astronomers hoped to watch how Triton's wreath of nitrogen gas--which exists in delicate balance with the frost on its surface--changes over time. To do that, researchers monitor the flutterings of light from distant stars as Triton drifts in front of them, events called occultations. The Hubble Space Telescope watched one such event last November as Triton eclipsed a faint star in the constellation Sagittarius.

Hubble's data revealed that Triton's atmosphere is two to three times denser now than in 1989, says astronomer James Elliot of the Massachusetts Institute of Technology in Cambridge. Models predict that such pressure changes would result from a rise of about 2 degrees Celsius at the surface, from -236 to a pleasant -234 degrees. Elliot's team explains this rise by noting that Triton is nearing the peak of "summer" in its Southern Hemisphere. As a result, the sun's rays are striking a polar cap of nitrogen ice more directly, sublimating some of the ice into the atmosphere. "This process may happen to a much larger extent on Pluto," says co-author Heidi Hammel of MIT. The relevance to global warming on Earth is remote, Hammel notes, although Triton offers a chance to understand the physics of a far simpler atmosphere.

The Hubble measurements of the occultation are "beautiful," says planetary scientist William Hubbard of the University of Arizona, Tucson. However, he cautions that Hubble probed the atmosphere in only two places by tracing the star's path behind Triton along a single line. Data from a different eclipse last year recorded at several ground-based observatories, he says, should reveal the atmosphere's thickness at many points around the moon once analysis is complete.