Solar system

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.

Abstract:

<jats:title>Abstract</jats:title> <jats:p>We present an analysis of all currently available ground-based imaging of Neptune in the mid-infrared. Dating between 2003 and 2020, the images reveal changes in Neptune’s mid-infrared (∼8–25 <jats:italic>μ</jats:italic>m) emission over time in the years surrounding Neptune’s 2005 southern summer solstice. Images sensitive to stratospheric ethane (∼12 <jats:italic>μ</jats:italic>m), methane (∼8 <jats:italic>μ</jats:italic>m), and CH<jats:sub>3</jats:sub>D (∼9 <jats:italic>μ</jats:italic>m) display significant subseasonal temporal variation on regional and global scales. Comparison with H<jats:sub>2</jats:sub> S(1) hydrogen quadrupole (∼17.035 <jats:italic>μ</jats:italic>m) spectra suggests that these changes are primarily related to stratospheric temperature changes. The stratosphere appears to have cooled between 2003 and 2009 across multiple filtered wavelengths, followed by a dramatic warming of the south pole between 2018 and 2020. Conversely, upper-tropospheric temperatures—inferred from ∼17 to 25 <jats:italic>μ</jats:italic>m imaging—appear invariant during this period, except for the south pole, which appeared warmest between 2003 and 2006. We discuss the observed variability in the context of seasonal forcing, tropospheric meteorology, and the solar cycle. Collectively, these data provide the strongest evidence to date that processes produce subseasonal variation on both global and regional scales in Neptune’s stratosphere.</jats:p>