Pluto Is Colder Than It Should Be

http://cfa-www.harvard.edu/press/pr0601.html

Harvard-Smithsonian Center for Astrophysics
Press Release

Release No.: 06-01
For Immediate Release: Tuesday, January 3, 2006

Note to editors: High-resolution artwork to accompany this release is
online at http://www.cfa.harvard.edu/press/pr0601image.html

A Planet Colder Than It Should Be

Cambridge, MA - Mercury is boiling. Mars is freezing. The Earth is just
right. When it comes to the temperatures of the planets, it makes sense
that they should get colder the farther away they are from the Sun. But
then there is Pluto. It has been suspected that this remote world might
be even colder than it should be. Smithsonian scientists now have shown
this to be true.

Scientists continue to discuss whether Pluto is a planet or should be
considered a refugee from the Kuiper belt. Whatever its classification,
Pluto and its moon Charon are certain to harbor secrets about the early
history of planet formation. Charon is roughly half the diameter of the
planet itself, and they form a unique pair in our solar system. How
they
came to be together remains a mystery.

Located thirty times farther away from the Sun than the Earth, sunlight
reaching the surface of Pluto is feeble at best, with daytime
resembling
dark twilight here at home. Pluto's temperature varies widely during
the
course of its orbit since Pluto can be as close to the sun as 30
astronomical units (AU) and as far away as 50 AU. (An AU is the average
Earth-Sun distance of 93 million miles.) As Pluto moves away from the
Sun, its thin atmosphere is expected to freeze and fall to the surface
as ice.

Reflected sunlight gathered with instruments such as the Keck telescope
in Hawaii and the Hubble Space Telescope suggested the surface of Pluto
might be colder than it should be, unlike Charon's. However, no
telescope capable of directly measuring their thermal emission (their
heat) was able to peer finely enough to distinguish the two bodies.
Their close proximity presented a formidable challenge since they are
never farther apart than 0.9 arcseconds, about the length of a pencil
seen from 30 miles away.

Now, for the first time, Smithsonian astronomers using the
Submillimeter
Array (SMA) on Mauna Kea in Hawaii have taken direct measurements of
thermal heat from both worlds and found that Pluto is indeed colder
than
expected, colder even than Charon.

"We all know about Venus and its runaway greenhouse effect," said Mark
Gurwell of the Harvard-Smithsonian Center for Astrophysics (CfA),
co-author on this study along with Bryan Butler of the National Radio
Astronomy Observatory. "Pluto is a dynamic example of what we might
call
an anti-greenhouse effect. Nature likes to leave us with mysteries -
and
this was a big one."

During the observations, the SMA utilized its most extended
configuration to obtain high-resolution interferometric data, allowing
separate "thermometer" readings for Pluto and Charon. It found that the
temperature of the ice-covered surface of Pluto was about 43 K (-382
degrees F) instead of the expected 53 K (-364 degrees F), as on nearby
Charon. This fits the current model that the low temperature of Pluto
is
caused by equilibrium between the surface ice and its thin nitrogen
atmosphere, not just with the incoming solar radiation. Sunlight
(energy) reaching the surface of Pluto is used to convert some of the
nitrogen ice to gas, rather than heat the surface. This is similar to
the way evaporation of a liquid can cool a surface, such as sweat
cooling your skin.

"These results are really exciting and fun as well," said Gurwell.
"Imagine taking something's temperature from almost three billion miles
away without making a house call!"

This research will be presented at the 207th meeting of the American
Astronomical Society in Washington, DC.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462


Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016