Evidence of an atmosphere on Pluto.

Evidence of an atmosphere on Pluto

Not much is known about tiny, distant Pluto beyond the existence of its single known moon, Charon, and the fact that methane in some form has been detected there. On June 9, however, at least eight separate groups of astronomers aiming their telescopes at the planet detected clear signs that it has an atmosphere. In fact, says James L. Elliot of the Massachusetts Institute of Technology, who co-led a team flying over the South Pacific aboard NASA's Kuiper Airborne Observatory, "Pluto has an atmosphere -- no question."

The occasion was Pluto's passing between the earth and a star, an event called an occultation, in which astronomers can learn a great deal from the way the star's light is shut off by the object getting in the way. The rings of Uranus, for example, were discovered that way in 1977, when the starlight blinked out not only behind the planet but also several times on either side of it.

Pluto's motion is imperfectly known, however, and until last week there had been no confirmed stellar occultations by Pluto on record, so astronomers gathered to take a look not only from the NASA aircraft but also from observatories in New Zealand, Australia and Tasmania. The starlight, they found, did not simply blink off and on as Pluto moved in front of it but got gradually fainter and brighter. The light, it seemed, had not merely been chopped off by the sharp edge of the planet's disk but was weakened and strengthened as it passed through a medium that looked increasingly dense with proximity to the disk -- like an atmosphere.

This week, the astronomers and their colleagues were analyzing their data, with answers in the offing to several fundamental questions. What is the atmosphere made of? Most gases would simply freeze onto the surface at Pluto's temperature, estimated to be within as little as 40[deg.]C of absolute zero. Even if past methane detections could not distinguish ice from gas, says Alan Stern of the University of Colorado in Boulder, there must be some gas above it. Other candidates include carbon monoxide and molecular nitrogen, and Stern notes that the "long shots" could include argon, resulting from the decay of radioactive potassium. The atmosphere, Elliot suspects, may turn out to be as much as a few hundred kilometers deep above a planet less than 2,400 km in diameter.