It turns out solid-phase haze particles help cool the atmosphere of the dwarf planet Pluto. That’s according to new research by planetary scientists from the University of California, Santa Cruz, and elsewhere.
According to Zhang et al, Pluto’s atmosphere is unique among Solar System planetary atmospheres, as its equilibrium temperature is controlled primarily by solid-phase haze particles instead of gas molecules. This artist’s impression shows a view of the moon Charon through Pluto’s atmospheric haze layers above the mountain landscape of bedrock water ice covered partially with deposition of dark, reddish haze particles. Image credit: X. Liu.
The gas composition of a planet’s atmosphere generally determines how much heat gets trapped in the atmosphere.
For Pluto, however, the predicted temperature was much higher than actual measurements taken by NASA’s New Horizons spacecraft in 2015.
Dr. Xi Zhang from the University of California, Santa Cruz, and colleagues propose a novel cooling mechanism controlled by haze particles to account for the dwarf planet’s frigid atmosphere.
“The cooling mechanism involves the absorption of heat by the haze particles, which then emit infrared radiation, cooling the atmosphere by radiating energy into space,” they said.
“The result is an atmospheric temperature of minus 333 degrees Fahrenheit (minus 203 degrees Celsius), instead of the predicted minus 280 degrees Fahrenheit (minus 173 degrees Celsius).”
According to the team, the excess infrared radiation from haze particles in Pluto’s atmosphere should be detectable by NASA’s future James Webb Space Telescope, allowing confirmation of the hypothesis after the telescope’s planned launch in 2019.
“Extensive layers of atmospheric haze can be seen in images of Pluto taken by New Horizons,” the scientists said.
“The haze results from chemical reactions in the upper atmosphere, where ultraviolet radiation from the Sun ionizes nitrogen and methane, which react to form tiny hydrocarbon particles tens of nanometers in diameter.”
“As these tiny particles sink down through the atmosphere, they stick together to form aggregates that grow larger as they descend, eventually settling onto the surface.”
“We believe these hydrocarbon particles are related to the reddish and brownish stuff seen in images of Pluto’s surface,” Dr. Zhang said.
The research is published in the journal Nature.
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Xi Zhang et al. 2017. Haze heats Pluto’s atmosphere yet explains its cold temperature. Nature 551: 352-355; doi: 10.1038/nature24465
