Dynamo Simulations of Jupiter's Magnetic Field: The Role of Stable Stratification and a Dilute Core. Issue 11 (15th November 2022)

• Record Type: Journal Article
• Authors: Moore, K. M.; Barik, A.; Stanley, S.; Stevenson, D. J.; Nettelmann, N.; Helled, R.; Guillot, T.; Militzer, B.; Bolton, S.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 11(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-11-15
• Subjects: Jupiter -- magnetic field -- dynamo -- dilute core -- stably stratified layer -- interior; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2022JE007479
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: Understanding Jupiter's present‐day interior structure and dynamics is key to constraining planetary accretion models. In particular, the extent of stable stratification (i.e., non‐convective regions) in the planet strongly influences long‐term cooling processes, and may record primordial heavy element gradients from early in a planet's formation. Because the Galileo entry probe measured a subsolar helium abundance, Jupiter interior models often invoke an outer stably stratified region due to helium rain. Additionally, Juno gravity data suggest a deeper, potentially stratified dilute core extending halfway through the planet. However, fits to Jupiter's gravitational data are non‐unique, and outstanding uncertainty over the equations of state for hydrogen and helium remain. Here, we use high‐resolution numerical magnetohydrodynamic simulations of Jupiter's magnetic field to place constraints on the extent of stable stratification within the planet. We find that compared to traditional interior models, an upper stably stratified layer between 0.9 and 0.95 Jupiter radii ( R J ) helps to explain both Jupiter's dipolar magnetic field and zonal winds. In contrast, an extended dilute core that is entirely stably stratified (no convective layers) yields significantly worse fits to both. However, our models with extended deep stratification still generate dipolar magnetic fields if an upper stratified region is also present. Overall, we find that a planet with a dilute core i.e., strongly stably stratified is increasingly challenging to reconcile with Jupiter's magnetic field and winds. Thus if a dilute core is present, alternative modalities such as a fully convective dilute core, a complex multilayered interior structure, or double diffusive convection may be required. Plain Language Summary: Giant planets like Jupiter provide important clues to how the solar system formed. Jupiter is often thought of as a dynamic and well‐mixed ball of hydrogen and helium gas that has been convectively cooling for billions of years. However, data from the NASA Juno Mission spacecraft suggests large regions of the planet may actually be "stably stratified, " that is, not convective. Understanding the extent of this process is important since it would affect how Jupiter's temperature cools over time, and may challenge our ideas of how giant planets are formed. In this paper, we simulate the fluid dynamics of Jupiter's interior to investigate two potential stable regions: (a) an upper region (potentially due to helium rain, where helium and hydrogen separate like oil and water) and (b) a lower stratified region corresponding to the so‐called "dilute core, " which may extend halfway through the planet. We find that the upper stable layer helps explain both Jupiter's magnetic field and winds. In contrast, the lower stratified region does not explain either unless the upper stable layer is also present. Our results have implications for the present‐day interior dynamics of giant planets, as well as their past formation and evolution. Key Points: Numerical dynamo models provide important constrains on Jupiter's interior An upper stably stratified layer (SSL) (e.g., helium rain) helps explain Jupiter's magnetic field and winds A fully stratified dilute core is inconsistent with Jupiter's field and winds, unless there is also an upper SSL