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:
http://occult.mit.edu/research/occultations/Candidates/Predictions/P126.html.
In addition, Buie’s diary chronicling his observing experiences during
the July occultation can be viewed at
http://www.lowell.edu/users/buie/occ/p126/diary.html.

Support

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.