Air Apparent: Pluto's Eternal Atmosphere
A World With Atmosphere: Pluto is never airless. Image: NASA, ESA, and M. Buie (Southern Research Institute)
Although billions of kilometers from the sun, frigid Pluto has an Earthly air: an atmosphere made mostly of nitrogen, the same gas that constitutes 78 percent of the air we breathe. But Pluto pursues such an elliptical orbit around the sun that all of that gas might freeze onto its surface when farthest and coldest. On May 4, however, Pluto passed in front of a star in the constellation Sagittarius, allowing observers to watch the atmosphere block some of the star's light and deduce that the air is so substantial it never disappears.
That passage was key to understanding the atmosphere's future, says Catherine Olkin, a planetary scientist at the Southwest Research Institute in Boulder, Colo., whose team tracked the so-called occultation. In work submitted to Icarus she and her colleagues report that Pluto's atmosphere is now thicker than ever before seen.
Astronomers discovered the atmosphere in 1988, when Pluto occulted another star. An airless Pluto would have cut off the star's light abruptly, but instead the starlight faded gradually, revealing air with roughly one one-hundred-thousandth the surface pressure of our own—equivalent to the terrestrial atmosphere 80 kilometers high.
Pluto is so distant that completing a single orbit takes it 248 years. Pluto came closest to the sun in 1989 and has been receding from the star ever since. When Pluto ventures out to its most distant point, in 2113, it will be 3 billion kilometers farther, and sunlight on its surface will be 36 percent weaker, than in 1989. "Many scientists have predicted that Pluto's atmosphere would collapse as it traveled away from the sun," Olkin says. "Receiving less sunlight, the gas would condense onto the surface." Mars, whose orbit is also rather elliptical, temporarily loses a quarter of its air every time its southern hemisphere experiences winter, when Martian gas freezes onto the south polar cap.
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