Pluto and its moon Charon are shown with a thin haze of organic particles covering Pluto’s sunlit side. The haze both cools Pluto’s upper atmosphere by radiating heat into space and absorbs ultraviolet light that helps propel methane molecules to escape. This explains why Pluto’s mesosphere is colder than expected and why methane is leaking and even coating Charon’s poles red. The effect was predicted by Xi Zhang, and new JWST/MIRI observations confirm it. The results have implications for understanding Titan’s haze and Earth’s early atmosphere.
A Haze that Cools and Warms Pluto
According to a new study, using JWST’s mid-infrared observations, a team led by Tanguy Bertrand detected thermal emission from this haze layer. The tiny aerosol particles are thought to be complex hydrocarbons (“tholins”) and ices. These particles absorb the Sun’s ultraviolet light, heating the upper atmosphere and giving methane molecules extra energy. The haze then re-radiates that energy as infrared light, cooling the middle layers.
In fact, Zhang’s models show Pluto’s gases alone would overheat the mesosphere, so the haze must supply net cooling to balance the energy budget. Together, these effects mean the haze largely controls Pluto’s atmospheric energy balance. How much net warming versus cooling occurs depends on particle size and composition.
Haze Drives Escape and Paints Charon Red
Pluto’s atmosphere is so thin that any nudge can send molecules into space. Planetary scientist Will Grundy estimated Pluto loses about 1.3 kg/s of methane, with roughly 2.5% intercepted by Charon. The haze layer provides that nudge: its particles absorb solar UV light, heating molecules until they can escape Pluto’s gravity. The escaping methane then deposits on Charon’s poles, where radiation transforms it into complex, reddish tholin compounds.
This process effectively lets Pluto “paint” Charon’s poles with organic red stain—a phenomenon not seen elsewhere in the Solar System. By linking Pluto’s climate and Charon’s surface chemistry, the haze-driven escape provides a rare example of atmospheric exchange on icy worlds.
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