Haze keeps Pluto cool by kicking heat out to space

Blame Pluto’s haze for the dwarf planet’s unexpected chilliness. Clusters of hydrocarbons in the atmosphere radiate heat back into space, keeping the dwarf planet cool, a new study suggests. Pluto may be the only world in the solar system whose atmospheric temperature is controlled by solid particles, rather than gas, researchers report in the Nov. 16 Nature.

Most planets’ temperatures are set by the gas content of their atmospheres, since certain gases trap heat from the sun more efficiently than others (SN Online: 6/8/15). Based on the nitrogen, methane and carbon monoxide in Pluto’s atmosphere, scientists predicted the dwarf planet’s upper atmosphere would be a brisk –173° Celsius.

But when the New Horizons spacecraft flew past Pluto in 2015, the probe found temperatures closer to –203° — with no obvious explanation.

The spacecraft found another surprising feature: Pluto’s atmosphere is separated into about 20 distinct haze layers. Solid hydrocarbon particles tens of nanometers wide clump together in the atmosphere to form the haze before eventually settling onto Pluto’s surface (SN: 4/15/17, p. 14).

This haze could be what’s keeping the atmosphere cool, Xi Zhang of the University of California, Santa Cruz and his colleagues say. The team calculated that those hydrocarbon clumps could absorb heat from the sun as well as from gases in the atmosphere and radiate it back into space. Ultimately, the haze would cool the atmosphere instead of warming it.

That makes Pluto unique in the solar system, the researchers say. Other hazy worlds, like Saturn’s moon Titan, still have temperatures that are mostly determined by gas.

“On Pluto, almost the whole game is haze,” says planetary scientist Robert West of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who wrote a commentary on the new work, also published in the Nov. 16 Nature. “That was a real surprise.”

West thinks Zhang and colleagues’ argument “sounds convincing.” But he’s reserving judgement until the James Webb Space Telescope, due to launch in 2019, can see if Pluto glows brighter than expected in infrared wavelengths, a sign that the planet is wrapped in a radiating haze. “That will make it a closed case.”