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Photochemical Modeling of Pluto’s Atmosphere

Presentation #308.04 in the session Pluto System (Oral Presentation)

Published onOct 23, 2023
Photochemical Modeling of Pluto’s Atmosphere

In 1976, methane was detected on Pluto (Cruikshank et al., 1976). The New Horizons mission mapped the vertical profiles of N2, CH4, C2H2, C2H4, C2H6, and haze from a UV solar occultation in 2015 (Young, 2017). Hydrocarbon and methane processing by UV photons and energetic particles produces other hydrocarbons and haze precursors. A bimodal haze distribution is necessary to explain the specific scattering patterns observed by New Horizons (Fan et al., 2022, Gao et al., 2017). Pluto’s low temperature (30° C lower than predicted) is explained by the presence of haze, which acts as a cooling agent. (Zhang et al., 2017). Previous occultations suggest a seasonal variation of haze in the atmosphere, which would in turn affect Pluto’s temperature profile. We use a 1-dimensional photochemical and transport model, KINETICS, to simulate how Pluto’s hazes respond to a change in solar UV flux (comparable to the UV flux from a solar flare) and the timeline on which this response occurs. This model was modified to include ion chemistry (both anions and cations), which we showed to have a substantial effect (roughly 108.5 vs 1010.5 seconds at the bottom of the atmosphere and 106 vs 108 seconds at the top of the atmosphere), in delaying the enhancement in formation of C2Hx hydrocarbon species. We also run a base case with only neutrals and subsets of ions (both anions and cations, only anions, only cations, and neither anions nor cations) to compare with other published modeling efforts and to analyze the effect of ions on the photochemistry of haze precursors. Our result shows that ions, specifically cations, have a substantial effect on haze production and should not be ignored in future efforts. Regardless of the configuration of the model run, C8H2 has the highest increase in mixing ratio in the upper atmosphere after the change in flux. Notably, if the model excludes cations, production of C2H4 in the lower atmosphere begins substantially earlier than production of other hydrocarbon species. Most haze precursors, 3x104 years after the change in flux, increase by 20-40% compared to their steady state pre-flare mixing ratio. The sole notable exception to this is HCN, which slightly decreased at all altitudes in all models.

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