New Horizons Observations of the Atmosphere of Pluto

Abstract

New Horizons data provide a snapshot of the current state of Pluto's atmosphere. Winds are slow and mostly controlled by sublimation of surface ices. Molecular nitrogen is the primary constituent below 1,800 km, while methane and carbon monoxide are important minor species. Photolysis of these gases leads to a thin haze that encompasses Pluto from the surface up to >500-km altitude and is important in heating and cooling the atmosphere. A cold (∼70 K) upper atmosphere curtails the escape of Pluto's molecular nitrogen to space, although there is substantial escape of methane (∼5 × 1025 molecules s−1), coincidentally about equal to its loss by photochemistry. It is unknown if the current atmosphere is representative of its long-term average state. From the inferred rapid rate of haze settling, it seems that Pluto's atmosphere must occasionally undergo collapse to allow time for radiation processing of the colorless haze material into the dark deposits found on the surface. ▪ This article outlines what has been gleaned about Pluto's atmosphere in the years since the New Horizons flyby. ▪ Pluto's atmosphere is most similar to Titan's—with the photochemistry of supervolatile nitrogen and hydrocarbons resulting in a kind of factory for cold haze production. ▪ Much has been learned about Pluto's atmosphere, but many new questions have arisen, and these will likely remain unanswered until there is a follow-up mission—no doubt a long time from now.