The Impact of Europa&#8217;s Deep Interior on the Electromagnetic Induction Signal from Europa&#8217;s Ocean
Authors: Khurana Krishan , Cao Hao , Stixrude Lars 
Published: March 2024

Under low pressure and temperature, most rocks have extremely low electrical conductivities (< 10-9 S/m) which rise dramatically at temperatures encountered in deep interiors of solid bodies. The presence of ferric iron (Fe3+), whose abundance is related to the oxygen fugacity of the rock, lowers the activation energy of conduction and enhances rock conductivity further. Most models of the interior of Europa are consistent with the presence of a highly-conducting metallic iron core with a radius between 200 and 700 km (e.g., Kuskov and Kronrod, 2005). Thus, the mantle and the core of Europa are likely highly conducting and are expected to create measurable induction response. Accounting for this deep conductivity would not only improve the modeling of the physical parameters of the ocean but help further constrain the properties of Europa&#8217;s deeper interior.Since the discovery of induction response from Europa&#8217;s ocean (Khurana et al., 1998), numerous studies have reexamined the electromagnetic induction signatures obtained by the Galileo spacecraft using increasingly sophisticated techniques to model the induction field and the moon/plasma interaction field (see e.g. Zimmer et al. 2000; Schilling et al. 2007; Vance et al. 2021). However, these studies have ignored the effect of induction from the deeper interior. Also, no studies have been performed for lower conductivities of the ocean and for longer period waves (such as the orbital period of Europa = 85.2 h) which can probe deeper even through a highly conducting ocean.To address this problem, we have used the recursive method of Srivastava (1966) for a multiple layer model of Europa&#8217;s deep interior. The results from this preliminary exploration show that for a range of core radii and mantle conductivities, the deeper interior modifies the signal from the ocean by several nT at the two primary frequencies. The 85.2 h period penetrates through the ocean and elicits increasing response from the deep mantle as its conductivity is increased (from generation of stronger eddy currents). The 11.1 h period on the other hand produces a weaker response &#160;with increasing mantle conductivity because the eddy currents are already at their highest level (100% induction from the ocean) but begin to follow a deeper path through the mantle and thus their magnetic response at the surface appears weaker.Khurana, K.K., M.G. Kivelson, D. J. Stevenson, and others (1998) Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto, Nature, 395, 777-780.Kuskov O.L. and V.A. Kronrod (2005) Internal structure of Europa and Callisto, Icarus, 177, 550-569.Srivastava, S.P. (1966) Theory of the magnetotelluric method for a spherical conductor, Geophys. J. Roy. Astro. Soc. 11, 373-387.Vance, S. D., Styczinski, M. J., Bills, B. G., Cochrane, C. J., Soderlund, K. M., Gomez-Perez, N., &amp; Paty, C. (2021). Magnetic induction responses of Jupiter's ocean moons including effects from adiabatic convection. J. Geophys. Res. : Planets, 126, e2020JE006418.Zimmer, C., K.K. Khurana, and M.G. Kivelson (2000) Subsurface oceans on Europa and Callisto:&#160; Constraints from Galileo magnetometer observations, Icarus, 147, 329.