© L. Calcada/ESOPluto has an "upside down" atmosphere, a March 2009 study revealed. Temperatures rise, rather than drop, with altitude on the dwarf planet—the reverse of Earth.
Astronomers recently made the most detailed measurements to date of the concentration of the greenhouse gas methane in Pluto's atmosphere using the European Southern Observatory's Very Large Telescope.
The measurements showed that methane is the second most abundant gas in Pluto's atmosphere, and that the gas is actually warmer at higher elevations than at the icy surface.
As a result, Pluto's upper atmosphere is about 90 degrees Fahrenheit (50 degrees Celsius) warmer than the planet's surface.
The team, led by Emmanuel Lellouch of France's Paris Observatory, speculates that there is a thin frozen layer, or frozen patches, of methane and other gases on Pluto's surface.
As Pluto orbits closer to the sun, the frozen gases vaporize. This process, called sublimation, cools Plutos surface while warming the planet's atmosphere.
Leslie Young is a deputy project scientist for NASA's New Horizons probe, which is scheduled to settle into orbit around Pluto in 2015.
Young said the new discovery is exciting, especially since it hints at future unexpected observations from New Horizons.
"We know that Pluto changes with time as it moves around the sun," Young said. So combining Earth-based observations with New Horizons data taken from Pluto's orbit will provide new clues about the dwarf planet's behavior.
Findings detailed in the journal Astronomy and Astrophysics Letters.
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