Universität zu Köln

Winkenstern, Jason ORCID: 0009-0001-5868-966X (2025). Exploration of Europa's Interior Using Electromagnetic Induction. PhD thesis, Universität zu Köln.

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Abstract

Jupiter's icy satellite Europa is expected to host a global subsurface ocean underneath its icy crust. This notion was supported by magnetic field measurements, in which the electromagnetic response of Europa to Jupiter's magnetic field has been discovered. Since then, magnetic field measurements have been used to probe Europa's interior, as its induction response is a function of the ocean's depth, thickness, and electrical conductivity. While Europa's subsurface ocean is commonly modelled as a radially symmetric layer, it is expected to be asymmetric in nature, both on a global scale due to tidal deformation, but also on local scales due to fractures and partial water melt in the icy crust. In this work, we investigate the detectability of water melt entrapped inside Europa's icy crust with magnetic sounding. For that, we first construct an analytical, iterative approach to solve the coupled induction between the global ocean and a local water reservoir. We find that the reservoir is strongly coupled to the ocean, i.e., its induction response to the ocean's induced dipole must be considered to accurately describe the overall induction response of the ocean-reservoir system. The ocean is weakly coupled to the reservoir, i.e., we can neglect its induction response to the reservoir's dipole within our prescribed precision. In this study, two scenarios are considered, a hypothetical flyby at 25 km altitude above Europa's surface and measurements directly at the surface. At 25 km altitude, reservoirs are not expected to be detectable, as their small induction signature falls off rapidly and could be obscured by small-scale fluctuations arising from plasma interactions in Europa's vicinity. At the surface, reservoirs could be detected by employing a network of at least two magnetometers, where one is placed directly above the region of interest, and a second right outside that region to resolve the spatial variability of the reservoir's induction response. Assuming a detectability limit of approximately 2 nT, derived from the strength of the small-scale fluctuations in magnetic field measurements, reservoirs with a radius larger than 8 km can be resolved, assuming a conductivity of 30 S/m. At larger radii, the necessary conductivity decreases, with 5 S/m required for a 20 km reservoir. Since the measurements would be taken at a fixed position, measuring over a long time period could allow us to better resolve periodic signals such as the reservoir's induction response, potentially enabling the detection of smaller and less conductive reservoirs. The characterization of Europa's subsurface ocean with electromagnetic induction results in a fairly unconstrained parameter space. This is partially due to the non-uniqueness of the induction method itself, but also due to the complexity of Europa's magnetic field environment, which is additionally perturbed by plasma interactions between the Jovian magnetosphere and Europa's atmosphere. In addition, the magnetospheric field of Jupiter along the spacecraft's trajectory is not exactly known, resulting in an unknown 'background' that is perturbed by the ocean's induction response and the plasma interactions. The second part of this thesis characterizes the uncertainties originating from the individual models for the inducing field, the plasma interaction, and the Jovian background field. These uncertainties propagate into the ocean's properties, restricting our ability to constrain the parameter space span by the ocean's depth, thickness, and electrical conductivity. We perform a chi-squared analysis, in which the squared deviation between the modelled magnetic field and the observed magnetic field is weighted against the model uncertainty. From this approach, uncertainties of the ocean properties are derived in the form of a range, i.e., we provide upper and lower limits for the depth, thickness, and conductivity. As this method cannot provide a lower limit on the ocean's depth, additional constraints from crater simulations are taken into account, with a minimum depth of 20 km. Here, we find a minimum conductivity of 0.45 S/m and a minimum thickness of 3.5 km. No upper limit of the conductivity or thickness could be resolved with the induction method, as the induction amplitude eventually reaches saturation. For the depth, our analysis yields an upper limit of approximately 90 km, above which the induction response generated within the ocean does not appropriately reproduce the observations. In addition, the robustness of the method is tested. For that, we apply small changes to individual model parameters and compare the resulting limits against the reference model. Here, we find that the interval length used to calculate the polynomial fit to the Jovian background field has the most noticeable effect on the resulting limits on the ocean properties. This notion emphasizes the relevance of the background fit in the exploration of Europa's subsurface ocean with electromagnetic induction.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Winkenstern, Jason jason.winkenstern@outlook.com orcid.org/0009-0001-5868-966X UNSPECIFIED
URN:	urn:nbn:de:hbz:38-752270
Date:	2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute for Geophysics and Meteorology
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Europa UNSPECIFIED Electromagnetic Induction UNSPECIFIED Space Physics UNSPECIFIED
Date of oral exam:	7 February 2025
Referee:	Name Academic Title Saur, Joachim Prof. Dr. Tezkan, Bülent Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/75227