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Modeling Ganymede's Surface Charging in Preparation for the JUICE Mission.

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Title: Modeling Ganymede's Surface Charging in Preparation for the JUICE Mission.
Authors: Tsai, Betty Pei‐Chun1,2 (AUTHOR) tsai@mps.mpg.de, Roussos, Elias1 (AUTHOR), Modolo, Ronan3 (AUTHOR), Holmberg, Mika K. G.4 (AUTHOR), Jia, Xianzhe5 (AUTHOR), Nordheim, Tom6 (AUTHOR), Reddy, Sachin Alexander7 (AUTHOR), Krupp, Norbert1 (AUTHOR), Fränz, Markus1 (AUTHOR), Narita, Yasuhito1,2 (AUTHOR), Plainaki, Christina1,8 (AUTHOR)
Source: Journal of Geophysical Research. Space Physics. Apr2026, Vol. 131 Issue 4, p1-27. 27p.
Subject Terms: Surface charging, Electric potential, Plasma interactions, Natural satellites, Magnetospheric physics, Electron backscattering, Plasma dynamics
Company/Entity: European Space Agency
Abstract: The European Space Agency's (ESA) JUICE mission (JUpiter ICy moons Explorer) is en route to the Jovian system to characterize Ganymede's subsurface ocean. Determining the ocean's conductivity and depth requires precise measurements of its induced magnetic field at the position of JUICE. Electron reflectometry provides additional constraints for the surface magnetic field, but this would require knowledge of Ganymede's surface electric potential. Here, we model the global electrostatic surface potential distribution on Ganymede using a semi‐analytical charging formulation, by feeding it with surface plasma environment parameters derived from different magnetospheric interaction simulations. We further contrast these estimates against self‐consistent charging simulations using the Spacecraft Plasma Interaction Software (SPIS). Our results indicate that surface potentials should be mostly negative, ranging down to about −100 V. All model predictions exhibit a consistent morphological trend: closed field line regions are more negatively charged than open field line ones. Depending on the input used, the driver behind this trend can either be the spatial variations of plasma density alone or combined density and electron temperature patterns, especially in the ionosphere; the electron current is the dominant driver of surface potential, with the relative influence of secondary electron, ion, and photoelectron currents varying depending on the location at Ganymede. While estimates of absolute surface potential are consistent with aspects of Juno flyby data, they are heavily dependent on how simulated plasma parameters are extrapolated to Ganymede's surface. Consequently, advanced modeling is required to achieve more reliable constraints of Ganymede's near‐surface environment description. Plain Language Summary: ESA's JUICE mission is traveling to Jupiter to study Ganymede, which likely hides a subsurface liquid water ocean. To characterize this ocean, the mission analyzes magnetic signals, but electrical charges accumulating on Ganymede's surface can interfere with these measurements. Therefore, understanding this "surface charging" is essential. In this study, we used computer simulations to map the electrical potential across Ganymede's surface. We found that the surface generally holds a negative charge, ranging from 0 to −100 V. A consistent pattern emerged: regions where magnetic field lines loop back to the surface are more negatively charged than regions connecting out to Jupiter, primarily because there is more plasma in those areas. This charging is mainly driven by electrons hitting the surface, though sunlight and other particles also play a role depending on the local conditions. These findings help scientists interpret magnetic field data more accurately, which is crucial for revealing the secrets of Ganymede's hidden ocean. Future measurements from the JUICE spacecraft will help refine these models further. Key Points: Ganymede's surface electrostatic potential is modeled using two plasma simulations and validated against a self‐consistent charging codeWe predict mostly negative surface potentials down to about −100 V, varying based on input plasma parameters and the simulation methodVariable plasma densities or temperatures cause closed field line regions to be more negatively charged than open field line regions [ABSTRACT FROM AUTHOR]
Copyright of Journal of Geophysical Research. Space Physics is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Modeling Ganymede's Surface Charging in Preparation for the JUICE Mission.
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  Data: <searchLink fieldCode="AR" term="%22Tsai%2C+Betty+Pei‐Chun%22">Tsai, Betty Pei‐Chun</searchLink><relatesTo>1,2</relatesTo> (AUTHOR)<i> tsai@mps.mpg.de</i><br /><searchLink fieldCode="AR" term="%22Roussos%2C+Elias%22">Roussos, Elias</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Modolo%2C+Ronan%22">Modolo, Ronan</searchLink><relatesTo>3</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Holmberg%2C+Mika+K%2E+G%2E%22">Holmberg, Mika K. G.</searchLink><relatesTo>4</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Jia%2C+Xianzhe%22">Jia, Xianzhe</searchLink><relatesTo>5</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Nordheim%2C+Tom%22">Nordheim, Tom</searchLink><relatesTo>6</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Reddy%2C+Sachin+Alexander%22">Reddy, Sachin Alexander</searchLink><relatesTo>7</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Krupp%2C+Norbert%22">Krupp, Norbert</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Fränz%2C+Markus%22">Fränz, Markus</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Narita%2C+Yasuhito%22">Narita, Yasuhito</searchLink><relatesTo>1,2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Plainaki%2C+Christina%22">Plainaki, Christina</searchLink><relatesTo>1,8</relatesTo> (AUTHOR)
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  Data: <searchLink fieldCode="JN" term="%22Journal+of+Geophysical+Research%2E+Space+Physics%22">Journal of Geophysical Research. Space Physics</searchLink>. Apr2026, Vol. 131 Issue 4, p1-27. 27p.
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  Data: <searchLink fieldCode="DE" term="%22Surface+charging%22">Surface charging</searchLink><br /><searchLink fieldCode="DE" term="%22Electric+potential%22">Electric potential</searchLink><br /><searchLink fieldCode="DE" term="%22Plasma+interactions%22">Plasma interactions</searchLink><br /><searchLink fieldCode="DE" term="%22Natural+satellites%22">Natural satellites</searchLink><br /><searchLink fieldCode="DE" term="%22Magnetospheric+physics%22">Magnetospheric physics</searchLink><br /><searchLink fieldCode="DE" term="%22Electron+backscattering%22">Electron backscattering</searchLink><br /><searchLink fieldCode="DE" term="%22Plasma+dynamics%22">Plasma dynamics</searchLink>
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  Data: <searchLink fieldCode="DE" term="%22European+Space+Agency%22">European Space Agency</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: The European Space Agency's (ESA) JUICE mission (JUpiter ICy moons Explorer) is en route to the Jovian system to characterize Ganymede's subsurface ocean. Determining the ocean's conductivity and depth requires precise measurements of its induced magnetic field at the position of JUICE. Electron reflectometry provides additional constraints for the surface magnetic field, but this would require knowledge of Ganymede's surface electric potential. Here, we model the global electrostatic surface potential distribution on Ganymede using a semi‐analytical charging formulation, by feeding it with surface plasma environment parameters derived from different magnetospheric interaction simulations. We further contrast these estimates against self‐consistent charging simulations using the Spacecraft Plasma Interaction Software (SPIS). Our results indicate that surface potentials should be mostly negative, ranging down to about −100 V. All model predictions exhibit a consistent morphological trend: closed field line regions are more negatively charged than open field line ones. Depending on the input used, the driver behind this trend can either be the spatial variations of plasma density alone or combined density and electron temperature patterns, especially in the ionosphere; the electron current is the dominant driver of surface potential, with the relative influence of secondary electron, ion, and photoelectron currents varying depending on the location at Ganymede. While estimates of absolute surface potential are consistent with aspects of Juno flyby data, they are heavily dependent on how simulated plasma parameters are extrapolated to Ganymede's surface. Consequently, advanced modeling is required to achieve more reliable constraints of Ganymede's near‐surface environment description. Plain Language Summary: ESA's JUICE mission is traveling to Jupiter to study Ganymede, which likely hides a subsurface liquid water ocean. To characterize this ocean, the mission analyzes magnetic signals, but electrical charges accumulating on Ganymede's surface can interfere with these measurements. Therefore, understanding this "surface charging" is essential. In this study, we used computer simulations to map the electrical potential across Ganymede's surface. We found that the surface generally holds a negative charge, ranging from 0 to −100 V. A consistent pattern emerged: regions where magnetic field lines loop back to the surface are more negatively charged than regions connecting out to Jupiter, primarily because there is more plasma in those areas. This charging is mainly driven by electrons hitting the surface, though sunlight and other particles also play a role depending on the local conditions. These findings help scientists interpret magnetic field data more accurately, which is crucial for revealing the secrets of Ganymede's hidden ocean. Future measurements from the JUICE spacecraft will help refine these models further. Key Points: Ganymede's surface electrostatic potential is modeled using two plasma simulations and validated against a self‐consistent charging codeWe predict mostly negative surface potentials down to about −100 V, varying based on input plasma parameters and the simulation methodVariable plasma densities or temperatures cause closed field line regions to be more negatively charged than open field line regions [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Journal of Geophysical Research. Space Physics is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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        Value: 10.1029/2025JA035016
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      – Code: eng
        Text: English
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        PageCount: 27
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    Subjects:
      – SubjectFull: Surface charging
        Type: general
      – SubjectFull: Electric potential
        Type: general
      – SubjectFull: Plasma interactions
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      – SubjectFull: Natural satellites
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      – SubjectFull: Magnetospheric physics
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      – SubjectFull: Electron backscattering
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      – SubjectFull: Plasma dynamics
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      – SubjectFull: European Space Agency
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              Text: Apr2026
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