3D modeling of organic haze in Pluto's atmosphere

The New Horizons spacecraft, which flew by Pluto on July 14, 2015, revealed the presence of haze in Pluto's atmosphere that were formed by CH₄/N₂ photochemistry at high altitudes in Pluto's atmosphere, as on Titan and Triton. In order to help the analysis of the observations and further investigate the formation of organic haze and its evolution at global scales, we have implemented a simple parameterization of the formation of organic haze in our Pluto General Circulation Model. The production of haze in our model is based on the different steps of aerosol formation as understood on Titan and Triton: photolysis of CH₄ in the upper atmosphere by Lyman-α UV radiation, production of various gaseous species, and conversion into solid particles through accumulation and aggregation processes. The simulations use properties of aerosols similar to those observed in the detached haze layer on Titan. We compared two reference simulations ran with a particle radius of 50 nm: with, and without South Pole N₂ condensation. We discuss the impact of the particle radius and the lifetime of the precursors on the haze distribution. We simulate CH₄ photolysis and the haze formation up to 600 km above the surface. Results show that CH₄ photolysis in Pluto's atmosphere in 2015 occurred mostly in the sunlit summer hemisphere with a peak at an altitude of 250 km, though the interplanetary source of Lyman-α flux can induce some photolysis even in the Winter hemisphere. We obtained an extensive haze up to altitudes comparable with the observations, and with non-negligible densities up to 500 km altitude. In both reference simulations, the haze density is not strongly impacted by the meridional circulation. With No South Pole N₂ condensation, the maximum nadir opacity and haze extent is obtained at the North Pole. With South Pole N₂ condensation, the descending parcel of air above the South Pole leads to a latitudinally more homogeneous haze density with a slight density peak at the South Pole. The visible opacities obtained from the computed mass of haze, which is about 2–4×10⁻⁷ g cm−2 in the summer hemisphere, are similar for most of the simulation cases and in the range of 0.001-0.01, which is consistent with recent observations of Pluto and their interpretation.