Why is it so cold on Pluto?

It’s cold on Pluto. Very cold. During the New Horizons flyby of Pluto in 2015 we learned that the temperature is colder than previously thought, -203 ºC. Now a new paper in Nature, co-authored by SETI Institute scientist Hiroshi Imanaki, lead author, Xi Zhang, University of Santa Cruz, explains that haze is the cause of Pluto’s cold temperature.

Pluto
Pluto's receding crescent from NASA's New Horizons spacecraft, taken when the spacecraft was 120,000 miles (200,000 kilometers) away from Pluto. Download full size Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

The research team found that Pluto’s atmosphere is different from others in our solar system in that hydrocarbon haze particles control the cold temperature, as opposed to gas molecules.

Hiroshi explains:

Pluto turns out to be a unique in that haze layers in the atmosphere control both heating and cooling. Saturn’s moon, Titan, has a similar nitrogen atmosphere and trace amounts of methane, but doesn’t behave in the same way as Pluto’s. The major difference is the total atmospheric pressure. Titan has the surface pressure of 1.5 bar, but Pluto has only at ~ 10 micro bar.  Both bodies are shrouded by the global haze layers, which are generated through photochemistry of methane high in the atmospheres. While the cooling by gas species works effectively in the upper atmosphere of Titan, the haze particles almost entirely control the atmospheric radiation balance in Pluto’s thin atmosphere.

The work here combines the two big discoveries by the New Horizon Mission into a synergic solution: Pluto’s atmospheric hazes maintain the very cold temperature in the upper atmosphere. This is important for affecting atmospheric escape from such a small body, i.e. the colder upper atmosphere can maintain more atmosphere.

This dominant role of haze particles in the thermal balance ultimately comes from the unusual high amount of atmospheric methane even though Pluto’s atmosphere is so thin and cold. The sources and supply of atmospheric methane in Pluto’s atmosphere remains an open question.

The Nature article, Haze heats Pluto’s atmosphere yet explains its cold temperature is available online now.