Non-thermal atmospheric escape of sulfur and oxygen on Io driven by photochemistry and atmospheric sputtering
Authors: Huang Xu , Gu Hao , Cui Jun 
Published: July 2024

IntroductionIo, the innermost of Jupiter&#8217;s four Galilean satellites, processes a unique SO2-dominated atmosphere comprising sulfides, oxides (such as SO, S, O2, and O), and other minor alkali and chlorine compounds [1]. In general, three mechanisms are responsible for the generation of an atmosphere on Io, including active volcanism, frost sublimation, and surface sputtering, of which the former two are more important [2]. Both the active volcanism arising from Jupiter's powerful tidal forces and the SO2 frost sublimation from Io's surface release large amounts of gases to replenish the tenuous atmosphere of Io, triggering a rich and complicated photochemical network, which may be a significant source of photochemical escape on Io [3-4]. Meanwhile, Io suffers from intense ion bombardment from Jupiter&#8217;s magnetosphere [5]. This constant atmospheric erosion by energetic ion precipitation, referred to as atmospheric sputtering, also serves as an important mechanism of Io&#8217;s atmospheric escape.AimsWith the aid of constantly accumulated understandings of Io&#8217;s space environment and atmospheric photochemistry, as well as the updated laboratory measurements [6], we evaluate the non-thermal escape of sulfur and oxygen on Io driven by both photochemistry and atmospheric sputtering [7]. A comprehensive review of the atmospheric escape process on Io is also provided.MethodsThe sputtering yield and escape probability are introduced to evaluate the escape intensity driven by the above two mechanisms. A one-dimensional Test Particle Monte Carlo (TPMC) Monte Carlo model is constructed to track the energy degradation of incident energetic ions and atmospheric recoils from which the sputtering yields and escape probabilities of different atmospheric species are determined. Different plasma populations (S++ and O+) and atmospheric conditions are compared, including high-density volcanic and low-density quiet atmospheric states, in which various chemical channels (photodissociation, neutral-neutral, ion-neutral, and dissociative recombination reactions) are considered. The background atmosphere and ionosphere are adapted from previous photochemical models of [3] and [4].Results and ConclusionsOur calculations suggest a total escape rate of 3&#215;1029 atom s&#8722;1 driven by atmospheric sputtering on Io, and SO2 is the dominant sputtered species. The photochemical escape rates are (1.1&#8722;2.0) &#215; 1027 s&#8722;1 for total O and (1.5&#8722;6.7) &#215; 1026 s&#8722;1 for total S, occurring mainly in the atomic form. Further investigations reveal that (1) S++ is the most efficient species for atmospheric sputtering on Io and sputtering yields increase substantially with increasing incident ion mass, energy, and incidence angle; (2) The photochemical escape rates vary extensively with the atmospheric conditions, especially in terms of the intensity of volcanic eruption, resulting in the chemical escape rate increases by up to a factor of five. Photochemistry is the most chemical escape channel. (3) By comparing multiple escape mechanisms including thermal escape (Jeans escape) and non-thermal escape, we conclude that atmospheric sputtering is the dominant mechanism driving atmosphere escape at Io. Photochemical escape outweighs Jeans escape for both atomic O and S for the quiet atmosphere scenario, while for the volcanic scenario, it is likely important for atomic S only.&#160;Reference:[1] Giono, G., &amp; Roth, L. (2021). Io's SO2 atmosphere from HST Lyman-&#945; images: 1997 to 2018. Icarus, 359, 114212.[2] de Pater, I., Goldstein, D., &amp; Lellouch, E. (2023). The Plumes and Atmosphere of Io. Io: A New View of Jupiter&#8217;s Moon, 233-290.[3] Summers, M. E., &amp; Strobel, D. F. (1996). Photochemistry and vertical transport in Io's atmosphere and ionosphere. Icarus, 120(2), 290-316.[4] Moses, J. I., Zolotov, M. Y., &amp; Fegley Jr, B. (2002). Photochemistry of a volcanically driven atmosphere on Io: Sulfur and oxygen species from a Pele-type eruption. Icarus, 156(1), 76-106.[5] Crary, F. J., &amp; Bagenal, F. (1997). Coupling the plasma interaction at Io to Jupiter. Geophysical Research Letters, 24(17), 2135-2138.[6] Bagenal, F., &amp; Dols, V. (2020). The space environment of Io and Europa. Journal of Geophysical Research: Space Physics, 125(5), e2019JA027485.[7] Huang, X., Gu, H., Cui, J., Sun, M., &amp; Ni, Y. (2023). Non-Thermal Escape of Sulfur and Oxygen on Io Driven by Photochemistry. Journal of Geophysical Research: Planets, 128(9), e2023JE007811.