VLT reveals details of

Pluto’s atmosphere

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 03 March, 2009

A thin envelope of nitrogen and methane cloaks the dwarf planet Pluto in an atmosphere that is 50 degrees warmer than the surface, according to observations made with ESO’s Very Large Telescope.

Pluto is a dwarf planet composed primarily of rock and ice, and is about 40 times further from the Sun than Earth. As a result, surface temperatures barely rise above minus 220 degrees Celsius. Scientists have long known that Pluto possesses a tenuous atmosphere, made up of nitrogen and traces of methane and possibly carbon monoxide.

But in a new study, a team of astronomers used the stellar occultation method - a phenomenon that occurs when a Solar System body blocks the light from a background star - to demonstrate that Pluto’s upper atmosphere was some 50 degrees warmer than the surface, or minus 170 degrees Celsius. They also found unexpectedly large amounts of methane in the atmosphere, indicating that methane is the second most common gas in Pluto’s atmosphere, representing half of one percent of the molecules.

Artist’s impression of how the surface of Pluto might look, according to one of the two models that a team of astronomers has developed to account for the observed properties of Pluto’s atmosphere, as studied with CRIRES. The image shows patches of pure methane on the surface. At the distance of Pluto, the Sun appears about 1000 times fainter than on Earth. Image: ESO.

“With lots of methane in the atmosphere, it becomes clear why Pluto’s atmosphere is so warm,” says Emmanuel Lellouch, lead author of the paper reporting the results. Following up with observations using the CRyogenic InfraRed Echelle Spectrograph (CRIRES), attached to ESO’s Very Large Telescope, the astronomers also revealed that the atmosphere as a whole, not just the upper atmosphere, has a mean temperature of minus 180 degrees Celsius, and so it is indeed significantly warmer than the surface.

“It is fascinating to think that with CRIRES we are able to precisely measure traces of a gas in an atmosphere 100,000 times more tenuous than the Earth’s, on an object five times smaller than our planet and located at the edge of the Solar System,” says co-author Hans Ulrich Kaufl. “The combination of CRIRES and the VLT is almost like having an advanced atmospheric research satellite orbiting Pluto.”

The observations indicate that Pluto’s atmosphere is undergoing a temperature inversion, increasing by between three and 15 degrees with every kilometre increase in height. In comparison, on Earth, the temperature decreases through the atmosphere by about six degrees per kilometre.

Astronomers used the CRIRES instrument on ESO's VLT to study Pluto's atmosphere. Image: ESO.

As Pluto moves away from the Sun during its 248 year long orbit, its atmosphere gradually freezes and falls to the ground. But when it is closer to the Sun, as it is now, the surface temperature increases, causing the ice to sublimate into gas. Just as sweat cools the body as it evaporates from the surface of the skin, this sublimation has a cooling effect on the surface of Pluto.

The composition of the atmosphere also influences the planet’s temperature inversion. “We were able to show that these quantities of methane play a crucial role in the heating processes in the atmosphere and can explain the elevated atmospheric temperature,” says Lellouch.

The scientists are considering two different models to explain the properties of Pluto’s atmosphere: either the surface is covered with a thin layer of methane which will inhibit the sublimation of the nitrogen frost, or, pure methane patches exist on the surface. “Discriminating between the two [models] will require further study of Pluto as it moves away from the Sun,” says Lellouch. “And of course, NASA’s New Horizons space probe will also provide us with more clues when it reaches the dwarf planet in 2015.”

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