Charon’s refractory factory

Abstract

We combine novel laboratory experiments and exospheric modeling to reveal that “dynamic” Ly-α photolysis of Plutonian methane generates a photolytic refractory distribution on Charon that increases with latitude, consistent with poleward darkening observed in the New Horizons images. The flux ratio of the condensing methane to the interplanetary medium Ly-α photons, φ, controls the distribution and composition of Charon’s photoproducts. Mid-latitude regions are likely to host complex refractories emerging from low-φ photolysis, while high-φ photolysis at the polar zones primarily generate ethane. However, ethane being colorless does not contribute to the reddish polar hue. Solar wind radiolysis of Ly-α–cooked polar frost past spring sunrise may synthesize increasingly complex, redder refractories responsible for the unique albedo on this enigmatic moon.

Extreme Exospheric Dynamics at Charon: Implications for the Red Spot

“…Ethane, according to experiment (Lo et al., 2015; Raut et al., 2022), constitutes a large (>∼60%) proportion of Charon's photolytic material owing to the dilution effect, as rapid methane accretion and burial below the optical depth hinders photo‐processing of new C 2 H 6 molecules into higher‐order hydrocarbons. Ethane will remain stuck to the surface for a decade or more after the spring methane sublimation, undergoing solar wind processing into higher‐order refractory hydrocarbons before itself being volatilized into the exosphere near Charon's ∼60 K maximum surface temperature.…”

“…We modeled Ly‐α photolysis using the photo‐conversion cross section σ d = 9 × 10 −22 m 2 (Raut et al., 2022) with the equation $$d\eta /dt={F}_{C{H}_{4}}-{\sigma }_{d}{F}_{\mathit{Ly}\alpha }{\eta }_{0}$$ for the competition of CH 4 accretion flux $${F}_{C{H}_{4}}$$ (net adsorption minus desorption) with conversion to photoproducts by Ly‐α photons (with flux F Lyα = 3.5 × 10 11 m −2 s −1 (Grundy et al., 2016)). Here η denotes methane column density frozen to the surface versus time t , while η 0 is the CH 4 column density in range of the Ly‐α light: We assume η 0 = ρd ∼ 7 × 10 20 m −2 (methane density times Ly‐α absorption depth) if η > ρd , and η 0 = η if η ≤ ρd .…”