The Leaky Moon: Why Io’s Atmosphere is Always Trying to Escape

Table of Links Abstract Introduction 1.1. Io as the main source of mass for the magnetosphere 1.2. Stability and variability of the Io torus system 1.3. Hypothesized volcanic mass supply events 1.4. Objective of this review Review of the relevant components of the Io-Jupiter system 2.1. Volcanic activity: hot spots and plumes 2.2 Io’s bound atmosphere 2.3 Exosphere and atmospheric escape 2.4 Electrodynamic interaction, plasma-neutral collisions, and the related atmospheric loss processes 2.5. Neutrals from Io in Jupiter’s magnetosphere 2.6. Plasma torus and sheet, energetic particles 2.7 Jupiter’s aurora and connections to the Io torus 2.8 Dust from Io Summary: What we know and what we do not know and 3.1 Current understanding for normal (stable) conditions 3.2 Canonical number for mass supply 3.3 Transient events in the plasma torus, neutral clouds and nebula, and aurora 3.4 Gaps in understanding, contradictions, and inconsistencies Future observations and methods and 4.1 Spacecraft measurements 4.2 Remote Earth-based observations 4.3 Modeling efforts Appendix, Acknowledgements, and References 2.3 Exosphere and atmospheric escape Escape takes place mostly above a certain level in the upper atmosphere known as the exobase where the transition from a collisional gas to a collisionless gas occurs (Figure 10). Below the exobase the atmosphere can be treated as a fluid, because the average distance a molecule or atom travels before making a collision – the mean free path – is shorter than the smallest macroscopic length scale. The latter is usually defined by the pressure scale height H which characterizes the exponential decay of pressure with altitude. Above the exobase is a quasi-collisionless region known as the exosphere where the mean free path exceeds the atmospheric scale height. Collisions are sufficiently infrequent that neutral atoms and molecules execute dynamical trajectories that are influenced by mostly Io’s and Jupiter’s gravitational field. 2.3.1 Exobase 2.3.2 Plume exobase and escape The exobase is defined for the hydrostatic equilibrium state and, moreover, for a single species represented by a single temperature. A similar definition of a corresponding altitude in a dynamic plume with bulk flow velocity is not possible. McDoniel et al. (2017) showed that plume particles elevated above the exobase of a purely-sublimated atmosphere may bounce off the sublimated atmosphere near the exobase when falling back towards the surface (Figure 11, right). In large plumes, the level where upward-moving sufficiently fast particles could escape without collisions is likely above the canopy shock, which is expected to be higher than exobase altitudes found in simulations of the sublimated atmosphere (Summers and Strobel, 1996; McDoniel et al., 2017). We also note that the top of Io’s atmosphere is not in local thermodynamic equilibrium (LTE) and thus different classes of molecules or atoms may have different temperatures and thus exobase altitudes. Generally, ejected plume gases do not have sufficient velocities to escape Io’s gravity directly. Under ballistic (collisionless) conditions, to reach an altitude of 400 km as inferred for the highest plumes, an ejection velocity of 1.2 km/s is needed. This is still well below Io’s escape velocity of 2.56 km/s or the velocity to reach the distance of the Hill radius (the radius where Io’s gravity is equal to Jupiter’s, near 5.8 RIo) of 2.33 km/s. Assuming a Maxwellian velocity distribution with a high core temperature of 800 K around an upward bulk velocity of 1.2 km/s, only less than 10-5 of the intact SO2 molecules reach the escape velocity. Even with an optimistic SO2 plume gas source rate of 105 kg/s, this yields an escape rate of ~1 kg/s, three orders lower than the canonical number. Ejection velocity, gas temperatures and SO2 source rates commonly assumed for simulating large plumes like the Pele plume are lower than our assumptions here (Zhang et al., 2003; 2004; McDoniel et al., 2017) and our approximation likely overestimates the escaping fraction. In addition, simulations revealed that the ejected plume gas is effectively slowed by falling gases in the canopy shocks, likely further reducing the escaping fraction (Zhang et al., 2003). This situation is vastly different from the Enceladus plume, where the surface gravity is 6% of Io’s surface gravity and the fraction of escaping molecules is two orders of magnitude higher than those returning to the surface (e.g., Tian et al., 2007; Villanueava et al., 2023). We note, however, that there might be potential pathways for direct volcanic escape that have not yet been explored, such as the dynamical behavior of volatiles originating from hot surface lavas with temperatures of 1200 K or higher. 2.3.3 Thermally-driven escape In a gravitationally-bound atmosphere with an exosphere, the key non-dimensional parameter governing escape is the Jeans parameter λ, which is defined as 2.3.4 Non-thermal escape This paper is available on arxiv under CC BY-NC-SA 4.0 DEED license. Authors: (1) L. Roth, KTH Royal Institute of Technology, Space and Plasma Physics, Stockholm, Sweden and a Corresponding author; (2) A. Blöcker, KTH Royal Institute of Technology, Space and Plasma Physics, Stockholm, Sweden and Department of Earth and Environmental Sciences, Ludwig Maximilian University of Munich, Munich, Germany; (3) K. de Kleer, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 USA; (4) D. Goldstein, Dept. Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin, Austin, TX USA; (5) E. Lellouch, Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, Meudon, France; (6) J. Saur, Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany; (7) C. Schmidt, Center for Space Physics, Boston University, Boston, MA, USA; (8) D.F. Strobel, Departments of Earth & Planetary Science and Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA; (9) C. Tao, National Institute of Information and Communications Technology, Koganei, Japan; (10) F. Tsuchiya, Graduate School of Science, Tohoku University, Sendai, Japan; (11) V. Dols, Institute for Space Astrophysics and Planetology, National Institute for Astrophysics, Italy; (12) H. Huybrighs, School of Cosmic Physics, DIAS Dunsink Observatory, Dublin Institute for Advanced Studies, Dublin 15, Ireland, Space and Planetary Science Center, Khalifa University, Abu Dhabi, UAE and Department of Mathematics, Khalifa University, Abu Dhabi, UAE; (13) A. Mura, XX; (14) J. R. Szalay, Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA; (15) S. V. Badman, Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK; (16) I. de Pater, Department of Astronomy and Department of Earth & Planetary Science, University of California, Berkeley, CA 94720, USA; (17) A.-C. Dott, Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany; (18) M. Kagitani, Graduate School of Science, Tohoku University, Sendai, Japan; (19) L. Klaiber, Physics Institute, University of Bern, 3012 Bern, Switzerland; (20) R. Koga, Department of Earth and Planetary Sciences, Nagoya University, Nagoya, Aichi 464-8601, Japan; (21) A. McEwen, Department of Astronomy and Department of Earth & Planetary Science, University of California, Berkeley, CA 94720, USA; (22) Z. Milby, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 USA; (23) K.D. Retherford, Southwest Research Institute, San Antonio, TX, USA and University of Texas at San Antonio, San Antonio, Texas, USA; (24) S. Schlegel, Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany; (25) N. Thomas, Physics Institute, University of Bern, 3012 Bern, Switzerland; (26) W.L. Tseng, Department of Earth Sciences, National Taiwan Normal University, Taiwan; (27) A. Vorburger, Physics Institute, University of Bern, 3012 Bern, Switzerland. Table of Links Abstract Abstract Introduction 1.1. Io as the main source of mass for the magnetosphere 1.2. Stability and variability of the Io torus system 1.3. Hypothesized volcanic mass supply events 1.4. Objective of this review Review of the relevant components of the Io-Jupiter system 2.1. Volcanic activity: hot spots and plumes 2.2 Io’s bound atmosphere 2.3 Exosphere and atmospheric escape 2.4 Electrodynamic interaction, plasma-neutral collisions, and the related atmospheric loss processes 2.5. Neutrals from Io in Jupiter’s magnetosphere 2.6. Plasma torus and sheet, energetic particles 2.7 Jupiter’s aurora and connections to the Io torus 2.8 Dust from Io Summary: What we know and what we do not know and 3.1 Current understanding for normal (stable) conditions 3.2 Canonical number for mass supply 3.3 Transient events in the plasma torus, neutral clouds and nebula, and aurora 3.4 Gaps in understanding, contradictions, and inconsistencies Future observations and methods and 4.1 Spacecraft measurements 4.2 Remote Earth-based observations 4.3 Modeling efforts Introduction 1.1. Io as the main source of mass for the magnetosphere 1.2. Stability and variability of the Io torus system 1.3. Hypothesized volcanic mass supply events 1.4. Objective of this review Introduction 1.1. Io as the main source of mass for the magnetosphere 1.1. Io as the main source of mass for the magnetosphere 1.2. Stability and variability of the Io torus system 1.2. Stability and variability of the Io torus system 1.3. Hypothesized volcanic mass supply events 1.3. Hypothesized volcanic mass supply events 1.4. Objective of this review 1.4. Objective of this review Review of the relevant components of the Io-Jupiter system 2.1. Volcanic activity: hot spots and plumes 2.2 Io’s bound atmosphere 2.3 Exosphere and atmospheric escape 2.4 Electrodynamic interaction, plasma-neutral collisions, and the related atmospheric loss processes 2.5. Neutrals from Io in Jupiter’s magnetosphere 2.6. Plasma torus and sheet, energetic particles 2.7 Jupiter’s aurora and connections to the Io torus 2.8 Dust from Io Review of the relevant components of the Io-Jupiter system 2.1. Volcanic activity: hot spots and plumes 2.1. Volcanic activity: hot spots and plumes 2.2 Io’s bound atmosphere 2.2 Io’s bound atmosphere 2.3 Exosphere and atmospheric escape 2.3 Exosphere and atmospheric escape 2.4 Electrodynamic interaction, plasma-neutral collisions, and the related atmospheric loss processes 2.4 Electrodynamic interaction, plasma-neutral collisions, and the related atmospheric loss processes 2.5. Neutrals from Io in Jupiter’s magnetosphere 2.5. Neutrals from Io in Jupiter’s magnetosphere 2.6. Plasma torus and sheet, energetic particles 2.6. Plasma torus and sheet, energetic particles 2.7 Jupiter’s aurora and connections to the Io torus 2.7 Jupiter’s aurora and connections to the Io torus 2.8 Dust from Io 2.8 Dust from Io Summary: What we know and what we do not know and 3.1 Current understanding for normal (stable) conditions 3.2 Canonical number for mass supply 3.3 Transient events in the plasma torus, neutral clouds and nebula, and aurora 3.4 Gaps in understanding, contradictions, and inconsistencies Summary: What we know and what we do not know and 3.1 Current understanding for normal (stable) conditions Summary: What we know and what we do not know and 3.1 Current understanding for normal (stable) conditions 3.2 Canonical number for mass supply 3.2 Canonical number for mass supply 3.3 Transient events in the plasma torus, neutral clouds and nebula, and aurora 3.3 Transient events in the plasma torus, neutral clouds and nebula, and aurora 3.4 Gaps in understanding, contradictions, and inconsistencies 3.4 Gaps in understanding, contradictions, and inconsistencies Future observations and methods and 4.1 Spacecraft measurements 4.2 Remote Earth-based observations 4.3 Modeling efforts Future observations and methods and 4.1 Spacecraft measurements Future observations and methods and 4.1 Spacecraft measurements 4.2 Remote Earth-based observations 4.2 Remote Earth-based observations 4.3 Modeling efforts 4.3 Modeling efforts Appendix, Acknowledgements, and References Appendix, Acknowledgements, and References 2.3 Exosphere and atmospheric escape Escape takes place mostly above a certain level in the upper atmosphere known as the exobase where the transition from a collisional gas to a collisionless gas occurs (Figure 10). Below the exobase the atmosphere can be treated as a fluid, because the average distance a molecule or atom travels before making a collision – the mean free path – is shorter than the smallest macroscopic length scale. The latter is usually defined by the pressure scale height H which characterizes the exponential decay of pressure with altitude. Above the exobase is a quasi-collisionless region known as the exosphere where the mean free path exceeds the atmospheric scale height. Collisions are sufficiently infrequent that neutral atoms and molecules execute dynamical trajectories that are influenced by mostly Io’s and Jupiter’s gravitational field. 2.3.1 Exobase 2.3.2 Plume exobase and escape The exobase is defined for the hydrostatic equilibrium state and, moreover, for a single species represented by a single temperature. A similar definition of a corresponding altitude in a dynamic plume with bulk flow velocity is not possible. McDoniel et al. (2017) showed that plume particles elevated above the exobase of a purely-sublimated atmosphere may bounce off the sublimated atmosphere near the exobase when falling back towards the surface (Figure 11, right). In large plumes, the level where upward-moving sufficiently fast particles could escape without collisions is likely above the canopy shock, which is expected to be higher than exobase altitudes found in simulations of the sublimated atmosphere (Summers and Strobel, 1996; McDoniel et al., 2017). We also note that the top of Io’s atmosphere is not in local thermodynamic equilibrium (LTE) and thus different classes of molecules or atoms may have different temperatures and thus exobase altitudes. Generally, ejected plume gases do not have sufficient velocities to escape Io’s gravity directly. Under ballistic (collisionless) conditions, to reach an altitude of 400 km as inferred for the highest plumes, an ejection velocity of 1.2 km/s is needed. This is still well below Io’s escape velocity of 2.56 km/s or the velocity to reach the distance of the Hill radius (the radius where Io’s gravity is equal to Jupiter’s, near 5.8 RIo) of 2.33 km/s. Assuming a Maxwellian velocity distribution with a high core temperature of 800 K around an upward bulk velocity of 1.2 km/s, only less than 10-5 of the intact SO2 molecules reach the escape velocity. Even with an optimistic SO2 plume gas source rate of 105 kg/s, this yields an escape rate of ~1 kg/s, three orders lower than the canonical number. Ejection velocity, gas temperatures and SO2 source rates commonly assumed for simulating large plumes like the Pele plume are lower than our assumptions here (Zhang et al., 2003; 2004; McDoniel et al., 2017) and our approximation likely overestimates the escaping fraction. In addition, simulations revealed that the ejected plume gas is effectively slowed by falling gases in the canopy shocks, likely further reducing the escaping fraction (Zhang et al., 2003). This situation is vastly different from the Enceladus plume, where the surface gravity is 6% of Io’s surface gravity and the fraction of escaping molecules is two orders of magnitude higher than those returning to the surface (e.g., Tian et al., 2007; Villanueava et al., 2023). We note, however, that there might be potential pathways for direct volcanic escape that have not yet been explored, such as the dynamical behavior of volatiles originating from hot surface lavas with temperatures of 1200 K or higher. 2.3.3 Thermally-driven escape In a gravitationally-bound atmosphere with an exosphere, the key non-dimensional parameter governing escape is the Jeans parameter λ, which is defined as 2.3.4 Non-thermal escape This paper is available on arxiv under CC BY-NC-SA 4.0 DEED license. This paper is available on arxiv under CC BY-NC-SA 4.0 DEED license. available on arxiv Authors: (1) L. Roth, KTH Royal Institute of Technology, Space and Plasma Physics, Stockholm, Sweden and a Corresponding author; (2) A. Blöcker, KTH Royal Institute of Technology, Space and Plasma Physics, Stockholm, Sweden and Department of Earth and Environmental Sciences, Ludwig Maximilian University of Munich, Munich, Germany; (3) K. de Kleer, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 USA; (4) D. Goldstein, Dept. Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin, Austin, TX USA; (5) E. Lellouch, Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, Meudon, France; (6) J. Saur, Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany; (7) C. Schmidt, Center for Space Physics, Boston University, Boston, MA, USA; (8) D.F. Strobel, Departments of Earth & Planetary Science and Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA; (9) C. Tao, National Institute of Information and Communications Technology, Koganei, Japan; (10) F. Tsuchiya, Graduate School of Science, Tohoku University, Sendai, Japan; (11) V. Dols, Institute for Space Astrophysics and Planetology, National Institute for Astrophysics, Italy; (12) H. Huybrighs, School of Cosmic Physics, DIAS Dunsink Observatory, Dublin Institute for Advanced Studies, Dublin 15, Ireland, Space and Planetary Science Center, Khalifa University, Abu Dhabi, UAE and Department of Mathematics, Khalifa University, Abu Dhabi, UAE; (13) A. Mura, XX; (14) J. R. Szalay, Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA; (15) S. V. Badman, Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK; (16) I. de Pater, Department of Astronomy and Department of Earth & Planetary Science, University of California, Berkeley, CA 94720, USA; (17) A.-C. Dott, Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany; (18) M. Kagitani, Graduate School of Science, Tohoku University, Sendai, Japan; (19) L. Klaiber, Physics Institute, University of Bern, 3012 Bern, Switzerland; (20) R. Koga, Department of Earth and Planetary Sciences, Nagoya University, Nagoya, Aichi 464-8601, Japan; (21) A. McEwen, Department of Astronomy and Department of Earth & Planetary Science, University of California, Berkeley, CA 94720, USA; (22) Z. Milby, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 USA; (23) K.D. Retherford, Southwest Research Institute, San Antonio, TX, USA and University of Texas at San Antonio, San Antonio, Texas, USA; (24) S. Schlegel, Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany; (25) N. Thomas, Physics Institute, University of Bern, 3012 Bern, Switzerland; (26) W.L. Tseng, Department of Earth Sciences, National Taiwan Normal University, Taiwan; (27) A. Vorburger, Physics Institute, University of Bern, 3012 Bern, Switzerland. Authors: Authors: (1) L. Roth, KTH Royal Institute of Technology, Space and Plasma Physics, Stockholm, Sweden and a Corresponding author; (2) A. Blöcker, KTH Royal Institute of Technology, Space and Plasma Physics, Stockholm, Sweden and Department of Earth and Environmental Sciences, Ludwig Maximilian University of Munich, Munich, Germany; (3) K. de Kleer, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 USA; (4) D. Goldstein, Dept. Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin, Austin, TX USA; (5) E. Lellouch, Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, Meudon, France; (6) J. Saur, Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany; (7) C. Schmidt, Center for Space Physics, Boston University, Boston, MA, USA; (8) D.F. Strobel, Departments of Earth & Planetary Science and Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA; (9) C. Tao, National Institute of Information and Communications Technology, Koganei, Japan; (10) F. Tsuchiya, Graduate School of Science, Tohoku University, Sendai, Japan; (11) V. Dols, Institute for Space Astrophysics and Planetology, National Institute for Astrophysics, Italy; (12) H. Huybrighs, School of Cosmic Physics, DIAS Dunsink Observatory, Dublin Institute for Advanced Studies, Dublin 15, Ireland, Space and Planetary Science Center, Khalifa University, Abu Dhabi, UAE and Department of Mathematics, Khalifa University, Abu Dhabi, UAE; (13) A. Mura, XX; (14) J. R. Szalay, Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA; (15) S. V. Badman, Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK; (16) I. de Pater, Department of Astronomy and Department of Earth & Planetary Science, University of California, Berkeley, CA 94720, USA; (17) A.-C. Dott, Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany; (18) M. Kagitani, Graduate School of Science, Tohoku University, Sendai, Japan; (19) L. Klaiber, Physics Institute, University of Bern, 3012 Bern, Switzerland; (20) R. Koga, Department of Earth and Planetary Sciences, Nagoya University, Nagoya, Aichi 464-8601, Japan; (21) A. McEwen, Department of Astronomy and Department of Earth & Planetary Science, University of California, Berkeley, CA 94720, USA; (22) Z. Milby, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 USA; (23) K.D. Retherford, Southwest Research Institute, San Antonio, TX, USA and University of Texas at San Antonio, San Antonio, Texas, USA; (24) S. Schlegel, Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany; (25) N. Thomas, Physics Institute, University of Bern, 3012 Bern, Switzerland; (26) W.L. Tseng, Department of Earth Sciences, National Taiwan Normal University, Taiwan; (27) A. Vorburger, Physics Institute, University of Bern, 3012 Bern, Switzerland.