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

Abstract

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 study 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. 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$\times$10$^{14}$ Pa s, the ocean can persist even in the absence of tidal heating. In the case of a tidal heating of 10 and 20 mW/m$^{2}$, the ice shell thickness is $\le$90 km if the ice viscosity is $\ge$1$\times$10$^{15}$ and 1$\times$10$^{14}$ Pa s, respectively. Regardless of the ice viscosity, if the tidal heating is $\ge$50 mW/m$^{2}$, 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.