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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.

Abstract:

Charon's exosphere may exhibit extreme seasonal dynamics, with centuries of quiescence punctuated by short lived (∼4 earth years) exospheric surges near the equinoxes, as spring sunrise bi-annually drives frozen methane off the polar night zones. Charon's pole-centric red spot has been proposed to be the product of Ly-α photolysis of frozen methane into refractory hydrocarbon “tholins”, but the role of exospheric dynamics in the red material's formation has not been investigated. We show with exospheric modeling that methane “polar-swap”, in which exospheric CH4 sublimated from the spring polar zone is rapidly re-frozen onto the autumn hemisphere, deposits ∼30 μm polar frosts too thick for Ly-α light to penetrate. Ethane, the primary methane photoproduct under these conditions, may unlike methane remain frozen decades after polar sunrise under solar wind exposure. Solar wind radiolysis of polar ethane frost synthesizes higher-order refractories that may contribute to the coloration of Charon's polar zones.