Pluto Events Perplex Astronomers
New Horizons mission, which could be launched toward the distant planet as soon as 2006. And during the past five weeks telescopes have captured the passage of Pluto in front of not one, but two faint stars. Although analysis of the stars' brief disappearances has only begun, it is already clear that observing teams from the United States and Europe have come to very different conclusions about the state of Pluto's atmosphere.
During the first event, on July 20th (Universal Time), Pluto's shadow crossed South America and, unfortunately, barely missed passing over a string of major observatories in the Andes. The only "hits" came from astronomers with portable setups: Marc Buie and Oscar Saa used a 14-inch Celestron and a CCD camera near the small Chilean town of Mamiña, while Francois Colas had a CCD-equipped Meade LX200 12-inch telescope a little farther north near Arica. According to Bruno Sicardy, who coordinated the European effort, seven other ground teams in Chile, Ecuador, Peru, and Venezuela were either clouded out or experienced technical difficulties. Fortunes improved for the August 21st occultation, as the broad path passed over observatories in Hawaii and the Far West.
In planning for this year's events, astronomers believed they could end years of speculation about these disparate models, as well as whether Pluto's wisps of nitrogen, methane, and carbon monoxide are freezing out onto the surface as the planet slips farther from the Sun. (Pluto passed the perihelion point of its 248-year-long orbit in 1989.) "We've seen that the atmosphere is really changing," says James L. Elliot, who coordinated the U.S. observing effort. "It's not like it was in 1988 that's firm." He notes that at the highest levels the gas is now 10° to 30° colder than the 110° Kelvin found previously, and the light curve's distinctive kink is gone.
But all this talk of "dramatic changes" in the atmosphere is being met with skepticism by Sicardy. "We have no firm evidence for a cooldown of the upper atmosphere," he counters. Moreover, the diagnostic kink can be seen in his team's light curves from both occultations, though not as conspicuously as it was 14 years ago. "We don't see much change, though there are some differences."
Both teams agree that this confusing situation would be much clearer if only they knew Pluto's true diameter, which is near 2,300 km but still uncertain by several tens of kilometers. There's a chance that one of the tracings from August 21st will probe all the way to the surface, which, Buie notes, would provide a "real breakthrough." Also, since Elliot's team recorded the event in many visible and infrared wavelengths, it should be possible to distinguish between the "haze-layer" and "inversion-layer" models. "Stay tuned," Buie says: another promising occultation may be observable on November 7th.