Pluto is colder than scientists expected due to atmospheric haze

Haze particles rather than atmospheric gases are responsible for atmospheric cooling mechanism.
By Laurel Kornfeld | Nov 17, 2017
Hydrocarbon haze particles in Pluto's atmosphere are the reason NASA's New Horizons spacecraft found that atmosphere to be significantly colder than scientists had expected, according to a new study based on mission data.

Prior to the July 2015 New Horizons flyby, scientists estimated the temperature of Pluto's atmosphere to be about 100 degrees Kelvin, which is equal to minus 280 degrees Fahrenheit or minus 173 degrees Celsius.

Gases in a planet's atmosphere trap heat from the Sun. Scientists' predictions for Pluto's atmospheric temperature before the flyby were based on their knowledge of the atmosphere's composition.

In the new, NASA-funded study, researchers propose hydrocarbon haze particles in Pluto's atmosphere drive a cooling mechanism that explains the lower than expected atmospheric temperature.

In the upper levels of Pluto's many-layered atmospheric haze, also discovered by New Horizons, ultraviolet radiation from the Sun interacts with haze particles of methane and nitrogen, ionizing them and emitting infrared radiation.

These chemical reactions radiate heat back into space, cooling the atmosphere, which New Horizons found to have a temperature of 70 degrees Kelvin, which equals minus 333 degrees Fahrenheit or minus 203 degrees Celsius.

"It's been a mystery since we first got the temperature data from New Horizons," explained Xi Zhang, assistant professor of Earth and planetary sciences at UC Santa Cruz and lead author of a study on the findings published in the journal Nature.

"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."

Once ionized, the haze particles fall through the atmosphere, where they stick together and grow larger before landing on the surface.

"We believe these hydrocarbon particles are related to the reddish and brownish stuff seen in images of Pluto's surface," Zhang said.

Pluto may not be alone in having an atmosphere whose temperature is driven by haze particles rather than by gases. The research team suggest studying the atmospheres of Saturn's moon Titan and Neptune's moon Triton as well as the atmospheres of hazy exoplanets to search for evidence of this phenomenon.

They also believe NASA's James Webb Space Telescope, scheduled to launch in 2019, will be able to detect the infrared radiation produced by Pluto's atmospheric haze particles.

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