Extreme Exospheric Dynamics at Charon: Implications for the Red Spot

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. Plain Language Summary Charon's thin methane atmosphere undergoes “explosive” pulsations owing to the Pluto‐Charon's systems' near sideways tilt to the Sun, according to new computer simulations that we present here. Spring sunrise may drive polar methane frozen during the centuries long winter night back into Charon's atmosphere, causing the whole atmosphere to briefly and drastically surge in pressure by a factor of almost 1000 every equinox. During these exceedingly brief episodes, taking place just a few years out of the Pluto‐Charon system's 248 year orbit around the Sun, polar caps of methane frost tens of microns thick may be suddenly swapped between north and south, evaporated and then re‐frozen from the spring to the autumnal polar zones. Charon's polar red spot, seen by the New Horizons spacecraft, is suspected to be material synthesized from frozen methane by backscattered solar ultraviolet light. However we find that Charon's polar caps are frozen too fast and thick for synthesis of much material more complex than ethane. Nevertheless ethane, being less volatile than methane, stays frozen to Charon's surface for decades after spring sunrise, and may under exposure to solar wind be converted to permanent red‐colored surface deposits that contribute to the origins of the red spot. Key Points Surges in Charon's exosphere, driven by spring‐sunrise sublimation of polar methane, produce “flash frozen” CH4 frost at the autumn pole Polar frost grows