Ion Chemistry May Drive Responses in Haze Precursors at Pluto to Solar Variability

Photochemical hazes at Pluto were first confirmed by the New Horizons flyby, and recent observations demonstrate large changes (>10x) in haze opacity on short timescales of a few years. Model comparisons with the most recent measurements have constrained properties of the haze particles (including radius, fractal dimension, and vertical distribution), and previous works have examined the formation of these haze precursors with a photochemical model (e.g., Gladstone et al., 2020). In this work, we aim to use a photochemical model to explore the response of photochemically derived haze precursors at Pluto to solar variability, in an attempt to explain the observed opacity variations. We use the Flare Irradiance Spectral Model (FISM) to estimate daily averaged EUV spectral irradiance at Pluto for cases corresponding to solar rotation maximum/minimum and the 2003 solar flare event (as well as pre-flare conditions). We adapt those results to modify the model of Gladstone et al. (2020). Our first results demonstrate that neutral hydrocarbon chemistry responds by ~10s of percent but only on much longer timescales to the observations. We next hypothesized that ion chemistry may drive quicker variability in the chemistry than neutrals. Titan’s haze precursor chemistry (including ions) has been well examined in previous works (Willacy et al., 2022), and the two worlds may be quite similar: small icy worlds with an N2-CH4 atmosphere (although we acknowledge the different atmospheric pressures). In this presentation, we will discuss the ion chemistry which responds on timescales of ~days to years. However, we also find that the models with different treatments of condensation and have notable differences in the reaction network. We will also present a comparison of these two models and discuss plans to merge them to develop a more complete photochemical model of haze precursors at Pluto.