Pluto's Atmosphere Is Expanding, Researchers Say

http://web.mit.edu/newsoffice/nr/2003/pluto.html

Pluto's atmosphere is expanding, researchers say
Massachusetts Institute of Technology
July 9, 2003

Contact:
Deborah Halber
MIT News Office
Phone: 617-258-9276
Email:

CAMBRIDGE, Mass. - Pluto's atmosphere is expanding even
as it continues on its long orbit away from the sun, a team of
astronomers from MIT, Boston University, Williams College,
Pomona College, Lowell Observatory and Cornell University
report in the July 10 issue of Nature.

The team, led by James Elliot, professor of planetary astronomy
at MIT and director of MIT's Wallace Observatory, made this
finding by watching the dimming of a star when Pluto passed in
front of it on Aug. 20, 2002. The team carried out observations
using eight telescopes at Mauna Kea Observatory, Haleakala,
Lick Observatory, Lowell Observatory and Palomar
Observatory.

Elliot said the new results seem counterintuitive, because
observers assumed Pluto's atmosphere would begin to collapse
as it cooled. In fact, the temperature of Pluto's mostly nitrogen
atmosphere has increased around 1 degree Celsius since it was
closest to the sun in 1989.

Elliot attributes the increase to the same lag effect that we
experience on Earth-even though the sun is most intense at its
highest point at noon, the hottest part of the day is around 3
p.m. Because Pluto's year is equal to 248 Earth years, 14 years
after Pluto's closest approach to the Sun is like 1:15 p.m. on
Earth. At the rate of Pluto's orbit, it may take another 10 years
to cool down and would just be beginning to cool when the
NASA New Horizons mission to Pluto, scheduled to be
launched in 2006, reaches it in 2015.

Pluto's predominantly nitrogen atmosphere is in vapor pressure
equilibrium with its surface ice, and can therefore undergo large
changes in pressure in response to small changes in surface ice
temperature. As its icy surface gets colder, it condenses into
fresh white frost that reflects more of the sun's heat and gets
colder still. As space dirt and objects collect on its surface, it
darkens and absorbs more heat, accelerating the warming
effect. Pluto has been darkening since 1954.

"The August 2002 data have allowed us to probe much more
deeply into Pluto's atmosphere and have given us a more
accurate picture of the changes that have occurred," Elliot said.

Pluto's orbit is much more elliptical than that of the other
planets, and its rotational axis is tipped by a large angle
relative to its orbit. Both factors could contribute to drastic
seasonal changes.

Since 1989, for example, the sun's position in Pluto's sky has
changed by more than the corresponding change on the Earth
that causes the difference between winter and spring. Pluto's
atmospheric temperature varies between around -235 and
-170 degrees Celsius, depending on the altitude above the
surface.

Pluto has nitrogen ice on its surface that can evaporate into the
atmosphere when it gets warmer, causing an increase in
surface pressure. If the observed increase in the atmosphere
also applies to the surface pressure-which is likely the
case-this means that the average surface temperature of the
nitrogen ice on Pluto has increased slightly more than 1 degree
Celsius over the past 14 years.

STUDYING ATMOSPHERES WITH SHADOWS

Researchers study faraway objects through
occultations-eclipse-like events in which a body (Pluto in this
case) passes in front of a star, blocking the star's light from
view. By recording the dimming of the starlight over time,
astronomers can calculate the density, pressure and
temperature of Pluto's atmosphere.

Observing two or more occultations at different times provides
researchers with information about changes in the planet's
atmosphere. The structure and temperature of Pluto's
atmosphere was first determined during an occultation in 1988.
Pluto's brief pass in front of a different star on July 19 led
researchers to believe that a drastic atmospheric change was
under way, but it was unclear whether the atmosphere was
warming or cooling.

The data resulting from this occultation, when Pluto passed in
front of a star known as P131.1, led to the current results. "This
is the first time that an occultation has allowed us to probe so
deeply into Pluto's atmosphere with a large telescope, which
gives a high spatial resolution of a few kilometers," Elliot said.
He hopes to use this method to study Pluto and the Kuiper Belt
objects more frequently in the future.

MISSION TO PLUTO

NASA recently authorized the New Horizons Pluto-Kuiper
Belt mission to start building spacecraft and ground systems.
The mission will be the first to Pluto and the Kuiper Belt.
Richard P. Binzel, professor of earth, atmospheric and planetary
sciences (EAPS) at MIT, is co-investigator.

The New Horizons spacecraft is scheduled to launch in January
2006, swing past Jupiter for a gravity boost and scientific
studies in 2007, and reach Pluto and its moon, Charon, as early
as summer 2015. Pluto is the only planet not yet observed at
close range. This mission will seek to answer questions about
the surfaces, atmospheres, interiors and space environments of
the solar system's outermost planet and its moon.

In the meantime, researchers hope to use SOFIA, a 2.5-meter
telescope mounted in an aircraft being built by NASA in
collaboration with the German space agency, starting in 2005.
SOFIA would be able to be sent to the right location around the
globe to best observe occultations, providing high-quality data
on a much more frequent basis than is possible using
ground-based telescopes alone.

In addition to Elliot, MIT co-authors are recent physics
graduate Kelly B. Clancy; graduate students Susan D. Kern and
Michael J. Person; recent MIT graduate Colette V. Salyk; and
aeronautics and astronautics senior Jing Jing Qu.

The Williams College collaborators included Jay M. Pasachoff,
professor of astronomy; Bryce A. Babcock, staff physicist;
Steven V. Souza, observatory supervisor; and undergraduate
David R. Ticehurst. They used the University of Hawaii's
telescope at the 13,800-foot altitude of the Hawaiian volcano
Mauna Kea and a Williams College electronic detector normally
part of eclipse expeditions.

Pomona College collaborators are Alper Ates and Ben
Penprase. The Boston University collaborator is Amanda Bosh.
Lowell Observatory collaborators are Marc Buie, Ted Dunham,
Stephen Eikenberry, Cathy Olkin, Brian W. Taylor, and
Lawrence Wasserman. Boeing collaborators are Doyle Hall and
Lewis Roberts.

The United Kingdom Infrared Telescope collaborator is Sandy
K. Leggett. U.S. Naval Observatory collaborators are Stephen
E. Levine and Ronald C. Stone. The Cornell collaborator is
Dae-Sik Moon. David Osip and Joanna E. Thomas-Osip were
at MIT and are now at the Carnegie Observatories. John T.
Rayner is at NASA's Infrared Telescope Facility. David Tholen
is at the University of Hawaii.

This work is funded by Research Corp., the Southwest
Research Institute, the National Science Foundation and
NASA.

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