Why Pluto Turned Out to be Colder Than Expected

A cooling mechanism controlled by haze particles could be responsible for keeping Pluto's atmosphere more frigid than scientists expected, a study says. The gas composition of a planet's atmosphere generally determines how much heat gets trapped in the atmosphere. For the dwarf planet Pluto, however, the predicted temperature based on the composition of its atmosphere was much higher than actual measurements taken by NASA's New Horizons spacecraft in 2015.

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The new study published in the journal Nature proposes a novel cooling mechanism that could solve the puzzle. "It's been a mystery since we first got the temperature data from New Horizons," said study first author Xi Zhang, Assistant Professor at University of California, Santa Cruz, US. "Pluto is the first planetary body we know of where the atmospheric energy budget is dominated by solid-phase haze particles instead of by gases," Zhang added.

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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, according to the study. The result is an atmospheric temperature of about minus 203 degrees Celsius, instead of the predicted minus 173 degrees Celsius. 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 nanometres in diameter.

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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," Zhang said. The excess infrared radiation from haze particles in Pluto's atmosphere should be detectable by the James Webb Space Telescope, allowing confirmation of his team's hypothesis after the telescope's planned launch in 2019, Zhang said.

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