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Pluto's Atmosphere is Changing
Pluto's Atmosphere is Changing
15 Aug 2002
(Source: Lowell Observatory)

New findings by astronomers from Lowell Observatory and the Massachusetts Institute of Technology (MIT) indicate that Pluto's atmosphere is undergoing a cooling trend and other global changes.

Using data from the most recent Pluto occultation, Dr. Marc Buie of Lowell Observatory (Flagstaff, Ariz.) and Dr. James Elliot of MIT (Cambridge, Mass.) discovered that Pluto's atmosphere has changed drastically since the last time Pluto occulted a star 14 years ago. Buie observed the occultation and Elliot compared Buie's findings with data from the 1988 Pluto occultation.

"In the last 14 years, one or more changes have occurred," Buie said. "Pluto's atmosphere is undergoing global cooling, while other data indicates that the surface seems to be getting slightly warmer. Some change is inevitable as Pluto moves away from the sun, but what we're seeing is more complex than expected."

Buie hopes these findings will give additional urgency to NASA's plans to send a spacecraft to Pluto, the only planet not yet observed up close. The Pluto-Kuiper Belt mission, planned to launch in 2006 and reach Pluto a decade later, seeks to answer questions about the surfaces, atmospheres, interiors and space environments of the solar system's outermost objects, including Pluto and its moon, Charon.

"We cannot fully explain what has caused these dramatic changes to Pluto's atmosphere," Buie said. "The Pluto-Kuiper Belt mission is our best hope for putting all the puzzle pieces together." Last month, the Senate Appropriations Committee included money in NASA's budget for the Pluto-Kuiper Belt mission. Buie said he is hopeful that the U.S. House of Representatives also will fund the project.

During a stellar occultation, an object passes in front of a star either partially or completely blocking the star's light from view. By recording how the dimming of the starlight changes over time, scientists can calculate the density, pressure and temperature of the object's atmosphere. Observing two or more occultations by the same body at different times lets astronomers determine whether the object's atmosphere has changed.

The structure and temperature of Pluto's atmosphere was first determined during an occultation in 1988. That occultation plus additional data revealed that Pluto has a tenuous, extended atmosphere composed of nitrogen with traces of methane and carbon monoxide. Results also showed that the light signature from the occulted star dimmed gradually then abruptly dropped off -- a puzzling phenomenon thought to be caused either by a smog layer or an abrupt decline in atmospheric temperature.

Assisted by Sr. Oscar Saa of the Cerro Tololo Inter-American Observatory, Buie used a 14-inch portable telescope in Northern Chile to record Pluto's brief pass in front of the distant star P126A on July 19. Buie and Elliot's findings are intriguing. This Pluto occultation revealed a noticeably different light signature than the 1988 event. The abrupt drop in starlight seen in the 1988 occultation is no longer present and Pluto's atmosphere has cooled by 20-55 degrees Fahrenheit. Both factors indicate that a dynamic atmospheric change is taking place.

"A 1997 Triton occultation revealed that the surface of Triton, Neptune's largest moon, had warmed since the Voyager spacecraft first explored the moon in 1989," Elliot said. "But the changes observed in Pluto's atmosphere are much more severe." Buie said he is eager to continue exploring our solar system's most distant planet, and is determined to unravel what these atmospheric changes mean and why they are happening. Astronomers will have another opportunity August 20 when Pluto passes in front of a star known as P131.1.

"Pluto has always been one of the most fascinating objects in the solar system to me," Buie said. "These drastic changes to its atmosphere, coupled with the possibility that Pluto's surface is getting warmer, make exploring the planet that much more compelling."

Observing Teams

Astronomers from around the globe attempted to observe the July 19 Pluto occultation using small/portable and large/stationary telescopes. Observation attempts were made by: Elliot and MIT student Michael Person using the twin 6.5-meter Magellan telescopes in Chile; Edward Dunham from Lowell Observatory and Kris Sellgren of Ohio State University using large telescopes at Cerro Tololo Inter-American Observatory in Chile; Jay Pasachoff, Steve Souza and David Ticehurst from Williams College; Brian Taylor and Cathy Olkin from Lowell Observatory; European teams led by Bruno Sicardy; English, Belgium and Spanish astronomers in the Canary Islands; and astronomers using the Gemini South telescope at Cerro Pachon.

Preparing for the Occultation

The main challenge of observing a stellar occultation for a small body like Pluto is predicting where the shadow's path will fall on Earth. Predicting a solar eclipse is much easier because the Sun and Moon are large, allowing solar eclipse paths to be accurately calculated years in advance. The P126A occultation (July 19) was identified several years ago with data from MIT's Wallace Astrophysical Observatory located in Westford Massachusetts. Late this spring, several hundred exposures of Pluto and the star were recorded at Lowell Observatory by Edward Dunham and students Joyance Meechai and Andy Morrison; other exposures were taken at the U.S. Naval Observatory's Flagstaff Station by astronomers Ron Stone and Steve Levine. These data were reduced by astronomers Amanda Bosh and Lawrence Wasserman at Lowell Observatory, and then passed to MIT, where students Michael Person, Katie Carbonari, Alison Klesman, Eric McEvoy, Shen Qu did further reductions. Elliot and MIT student Kelly Clancy carried out the final calculations. The result of the prediction can be seen on the Web site: In addition, Buie's diary chronicling his observing experiences during the July occultation can be viewed at


Lowell Observatory's occultation research is supported by the NASA Planetary Astronomy Program and the Friends of Lowell Observatory. Elliot's (MIT) research is partly supported by the National Science Foundation and by the NASA Planetary Astronomy Program; the Williams College expedition was supported by Research Corporation and New Horizons. Logistical support was provided by the National Optical Astronomy Observatories (NOAO), which operates the Cerro Tololo Inter-American Observatory for the National Science Foundation.

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