Europa’s structural conditions for the existence of subsurface ocean and the absence of metallic core-driven magnetic field

Abstract

During the Galileo spacecraft’s flyby of Europa, magnetic field measurements detected an inductive signal due to the response of Europa’s interior conductors to temporal fluctuations in the Jovian magnetic field. In contrast, no signatures of intrinsic magnetic field originating from the dynamo motion in the metallic core were acquired. These measurements suggest that a global sub-surface ocean containing electrolytes exists beneath the solid ice shell and that the metallic core lacks convection. Europa’s interior is expected to be divided into the metallic core, rocky mantle and hydrosphere based on the moment of inertia factor estimated from gravity field measurements. Specifically, the thickness of the outermost water layer is 120 – 170 km, and the radius of the metallic core is 0.12 – 0.43 times the surface radius. No systematic investigation of Europa’s internal evolution has been conducted to estimate the current state of the subsurface ocean and to explain the absence of a core dynamo field within such uncertainty for internal structure and material properties (especially ice properties). Herein, I performed a numerical simulation of the long-term thermal evolution of Europa’s interior and investigated the temporal changes in the ocean thickness as well as the temperature and heat flow of the metallic core. If the ice reference viscosity is greater than 5 × 10¹⁴ Pa s, the sub-surface ocean can persist even in the absence of tidal heating. In the case of a tidal heating of 10 mW/m² and 20 mW/m², the ice shell thickness is $\le$  90 km if the ice reference viscosity is $\ge$  1 × 10¹⁵ and 1 × 10¹⁴ Pa s, respectively. Regardless of the ice reference viscosity, if the tidal heating is $\ge$  50 mW/m², the shell thickness will be $\le$  40 km. The thermal history of the metallic core is determined by the hydrosphere thickness and the metallic core density, and is unaffected by variations in the ice shell (ocean) thickness. Preferred conditions for the absence of the core dynamo include CI chondritic abundance for the long-lived radioactive isotopes, lower initial core–mantle boundary (CMB) temperature and thicker hydrosphere. The core may be molten without convection if the composition is near the eutectic in a Fe–FeS alloy, or not molten (without convection) if the composition is near the Fe or FeS endmember. Specifically, if the rocky mantle has a CI chondritic radioisotope abundance, any core composition and hydrosphere thickness allow the absence of the core dynamo if the initial temperature at the CMB is lower than 1,250 K. If the rocky mantle has the ordinary chondritic radioisotope abundance, or a higher initial temperature ($\sim$1,500 K) at the CMB, the core density lower than 6,000 kg/m³ is preferred for the absence of the core dynamo. In the case of the core composition near the eutectic one, a hydrosphere thicker than 150 km is required for the lacking core dynamo. The lower pressure of Europa’s rocky mantle due to its thinner hydrosphere compared with that of Ganymede may facilitate heat transfer in the mantle, lowering its temperature and making dynamo motion more challenging.