spacetoday.net: Pluto's atmosphere expanding

Posted: Thu, Jul 10, 2003, 10:47 AM ET (1447 GMT)
New Horizons illustration (JHUAPL)

Pluto's tenuous atmosphere is expanding and warming even as the planet moves away from the Sun, scientists reported this week. In a paper published in Thursday’s issue of the journal Nature, a team of planetary astronomers reported that Pluto's atmosphere, as of August 2002, has expanded considerably since it passed perihelion in 1989. The atmosphere, composed primarily of nitrogen with a surface pressure of only a few microbars, has likely warmed by about one degree Celsius as well. The researchers attribute the delayed warming and expansion of the atmosphere to the same lag on Earth that causes the warmest part of the day to be in the afternoon, and not directly at noon. The observations were made by observing a stellar occultation, when Pluto passed directly between the Earth and a distant star; a similar occultation in 1988 allowed astronomers to discover the existence of the atmosphere itself. The results suggest that an imminent collapse, or "freeze-out", of Pluto's atmosphere, as some planetary scientists had feared, is unlikely. Instead, the atmosphere may only be beginning to collapse when NASA's New Horizons mission arrives at Pluto in the middle of the next decade.