The thermal and dynamic state of Europa's ice shell: Revealed by global-scale convection models

Abstract

Europa’s hydro- and cryosphere is of primary interest in the quest for habitable environments in the solar system (e.g., [1]). The ice shell, which connects the potential subsurface ocean to the surface, may itself provide niches for life if liquid brine pockets can form and exist for extended periods of time. It is thus crucial to understand the thermal and dynamic state of the ice shell in order to characterize the existence and transport of liquid brines within the ice shell. Recent work by [2] and [3] investigated the effects of temperature dependent thermal conductivity (k) as well as heat capacity (cp) and a complex composite rheology on convection in the ice shell. In this work, we build upon these previous efforts by combining the influence of both - varying thermodynamic parameters and complex rheology - in geodynamic simulations performed with the convection code GAIA [4]. Instead of a temperature-dependent heat capacity, we investigate the effect of a temperature- and depth-dependent thermal expansivity (α), which is a crucial term in determining the buoyancy induced by temperature differences. We study the dynamic state (Nu-Ra scaling), the mechanical state (elastic thickness, brittle-to-ductile transition, deformation maps), and the thermal state (bottom and top boundary heat flux, occurrence of brines) of the ice shell for various setups (using both constant and variable α and k) and input parameters (ice shell thickness and grain size). For selected models, i.e. distinct thermal and dynamic states, we calculate the local two-way attenuation based on [5], [6]. The resulting two-way attenuation patterns will offer initial insights into the radar's ability to penetrate to the ice-ocean interface. If attenuation proves excessive due to the presence of hot thermal plumes, making the sampling of the ice-ocean interface unlikely, the patterns can still provide valuable insights into the dynamic state of Europa's ice shell. This includes parameters such as the thickness of the conductive layer (the so-called stagnant lid) that forms in the top part of the ice shell or the wavelength of convective structures deeper in the ice shell.