Space Forum / Astronomy / June 2006

http://www.cnn.com/2006/TECH/space/01/03/pluto.temp/index.html

Astronomers: Pluto colder than expected
By Ker Than
SPACE.com

Tuesday, January 3, 2006; Posted: 3:06 p.m. EST (20:06 GMT)

(In October, astronomers announced the discovery of two tiny new moons
orbiting Pluto.  RELATED)

(SPACE.com) -- Earth-bound astronomers taking Pluto's temperature have
confirmed suspicions that the planet is colder than it should be. It's
thought that the planet's lower temperature is the result of interactions
between its icy surface and thin nitrogen atmosphere.

Using the Submillimeter Array, or SMA, a network of radio telescopes located
in Hawaii, astronomers found that Pluto's average surface temperature was
about 43 Kelvin (-382 degrees Fahrenheit) instead of the expected 53 Kelvin
(-364 degrees Fahrenheit), which is what the temperature of Pluto's largest
moon, Charon, is.

Unlike Pluto, Charon has no atmosphere, so its surface temperature was what
astronomers predicted based on its geological makeup and reflectivity.

Pluto is located thirty times farther away from the sun than Earth, about
three billion miles, and receives only about 1/1000th of the light that our
planet receives. Pluto's surface temperature varies widely because of its
erratic orbit, which can send it as close as 30 astronomical units (AU), or
as far away as 50 AU.

An AU is the average distance from the Earth to the sun, about 93 million
miles.

A popular planet

Pluto has been the focus of a lot of scientific attention lately. Its status
as a planet is currently being debated since the discovery in July of an
even larger object located even farther out in the solar system, in a region
of space known as the Kuiper Belt.

In October, astronomers announced the discovery of two tiny, new moons
orbiting the planet, each one only 30 to 100 miles in diameter.

Astronomers have suspected since the early 1990's that Pluto is colder than
it should be, but they were unable to confirm their suspicions until
recently because it was difficult to tease apart the heat emissions of Pluto
from nearby Charon.

Charon is unusually large for a moon and scientists are still not sure how
it and Pluto became paired. According to one hypothesis, Pluto was struck
long ago by another body that was nearly identical in size, spinning off
Charon as a result.

From Earth, Pluto and Charon appear about 0.9 arcseconds apart, and trying
to tell them apart is like trying to make out the length of a pencil from 30
miles away.

The SMA is the first telescope able to make the required measurement. Other
telescopes like the Very Large Array (VLA) in New Mexico have higher
resolution than the SMA, but are less sensitive to colder objects.

"We're the first to have the combination of resolution and sensitivity to be
able to do this experiment," said Mark Gurwell, an astronomer at the
Harvard-Smithsonian Center for Astrophysics and a co-author on the study.

Anti-greenhouse effect

Astronomers think Pluto's colder than expected temperature reading involves
interactions between nitrogen ice on the planet's surface and the nitrogen
gas that makes up its atmosphere.

The two forms of nitrogen are in a constant state of careful flux: as Pluto
moves away from the Sun, the nitrogen gas "condenses," freezing and falling
back to the surface as ice. The opposite happens when Pluto is closer to the
Sun.

Planets like Venus and Earth experience a natural greenhouse effect, where
sunlight energy striking the surface is absorbed and used to heat the
surface. On Pluto, the opposite happens.

"Pluto is a dynamic example of what we might call an anti-greenhouse
effect," Gurwell said.

Instead of being absorbed and warming the surface, sunlight striking Pluto
is used to convert nitrogen ice on its surface into gas. A similar process
happens when humans sweat: the evaporation of sweat from the skin has a
cooling effect because the evaporated water carries away the body's excess
heat.

The finding could apply to other planets in the solar system which have
condensable atmospheres like Mars, or even to extra sola- planets, Gurwell
told SPACE.com.

In the future, Gurwell hopes that astronomers will be able to make even more
precise measurements of Pluto using the Atacama Large Millimeter Array
(ALMA) currently being built in Chile. The ALMA is scheduled for completion
in 2012 but early scientific experiments could begin as soon as 2008.

Gurwell expects that the ALMA will allow astronomers to see where on Pluto's
surface nitrogen gas is being generated and whether the atmosphere is spread
out evenly across the entire planet or concentrated in hot spot like on
Saturn's moon, Enceladus.

"We're breaking new ground, laying the foundation for work that will happen
in the future," Gurwell said.

The finding will be presented at the 2005 meeting of the American
Astronomical Society that begins on January 8 in Washington, D.C.