Analytical forward models for magnetic induction of asymmetric, icy ocean worlds with implications for spacecraft investigations

Abstract

Oscillating magnetic fields are screened within electrically conducting materials by induced currents, which generate secondary, induced magnetic fields. Induced fields are measurable outside the conducting body, and adopt a strength and spatial structure dependent on both the excitation field and the conductivity structure of the body. This dependence has been exploited to sound the interior and ionospheric structure of Earth and the Galilean moons of Jupiter, providing the strongest evidence yet for the presence of Europa's subsurface ocean. Forward models of magnetic induction are essential to magnetic investigations by Europa Clipper and JUICE, two missions in development that intend to characterize the oceans of Europa and Ganymede, respectively. These forward models will also be vital elements of future missions to explore the moons of the ice giants. Until the present work, all ocean induction models have adopted simplified structure for the conductivity, with a single, spherically symmetric, uniformly conducting layer for the ocean and sometimes additional layers for the ionosphere, mantle, and/or core. Such an approach neglects the altitude-dependent density of charge carriers in the ionosphere and depth-dependent pressure and temperature within the ocean, all of which contribute to radial dependence of conductivity. Also neglected are the effects of layer asymmetry on the induced magnetic fields, which result in modifications to the magnetic moments that can bias the inversion of measurements necessary to constrain interior structure. This dissertation details recent advances in magnetic induction forward models and their application to magnetic sounding of icy moons. The impacts of depth-dependent conductivity structure within ocean layers are examined in application to Europa, Ganymede, and Callisto, informed by self-consistent geophysical models generated with the open source framework PlanetProfile. Analytical forward models for asymmetric conducting layers are derived and applied to Europa, Callisto, the uranian moon Miranda, and the neptunian moon Triton based on descriptions of possible asymmetry found in the literature. Implications for magnetic investigations by future spacecraft missions are discussed. The products of the present work permit investigation of realistic depth-dependence and asymmetry within ocean worlds for the first time. Open source software is provided to facilitate application of the presented models.