Haze particles make Pluto colder than expected

Pluto is enshrouded with haze particles that make the icy dwarf planet much colder than predicted, a study has found.

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.

Scientists from the University of California, Santa Cruz in the US have found a novel cooling mechanism controlled by haze particles to account for Pluto's frigid atmosphere.

"It's been a mystery since we first got the temperature data from New Horizons," said Xi Zhang, assistant professor at UC Santa Cruz.

"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," said Zhang, lead author of the study published in the journal Nature.

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.

The result is an atmospheric temperature of about minus 203 degrees Celsius, instead of the predicted minus 173 Celsius.

According to Zhang, 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.

Extensive layers of atmospheric haze can be seen in images of Pluto taken by New Horizons.

The haze results from chemical reactions in the upper atmosphere, where ultraviolet radiation from the sun ionises nitrogen and methane, which react to form tiny hydrocarbon particles tens of nanometres 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," Zhang said.

The researchers are interested in studying the effects of haze particles on the atmospheric energy balance of other planetary bodies, such as Neptune's moon Triton and Saturn's moon Titan.

The findings may also be relevant to investigations of exoplanets with hazy atmospheres.