Insights into Callisto’s interior: shallow craters and broad domes as a test for JUICE. 

Introduction: The differentiation state of a planetary body affects evolution and thermal structure, but the differentiation state of Callisto remains unclear. Our understanding of Callisto relies on data retrieved from NASA’s Galileo mission, which measured the moment of inertia (MOI) under an assumption of hydostaticity [1]. Models of Callisto’s internal structure with this inferred MOI were used to suggest a partially differentiated interior without full ice/rock separation [1]. Other works, however, argued that an undifferentiated Callisto is improbable because partial differentiation would lead to a runaway differentiation process [2].Interior structures that could explain the inferred high MOI value include: (1) a partially differentiated interior, (2) nonhydrostatic components on a differentiated Callisto, and (3) an undifferentiated outer shell overlying a fully differentiated interior, which we focus on here. An undifferentiated shell could result from a differentiated interior with full ice/rock separation but a melting front that does not reach the shallow subsurface (Fig. 1) and has been proposed for Uranian moons [3]. The undifferentiated shell would be a dense mixture of ice and rock, which would lead to a high MOI value with a more differentiated interior. Although this undifferentiated outer shell would have a negative density gradient, foundering of the shell is unlikely to occur [4].Here, we investigate the evolution of Callisto’s surface assuming an undifferentiated outer shell overlying a pure-ice mantle. The undifferentiated shell would behave as a high-density, high-viscosity layer, and the pure-ice mantle would act as a low-density, low-viscosity layer, analogous to ice tectonics on Ceres [5]. Topography and density differences would cause differential stresses that could drive upward deformation. Ice tectonic deformation would uplift the crater floor and may be another pathway to the shallow, anomalous craters observed by [6] that has been suggested to be the result of viscous relaxation [7].Methods: We simulate the evolution of topography on Callisto assuming an undifferentiated shell using the finite element method (FEM). We simulate an undifferentiated ice-rock layer (50% ice, 50% silicates [2]) on top of a pure ice layer. The interfaces between layers are flat, and the initial topography of the crater is taken from [7], which extrapolated depth-diameter ratios of smaller complex craters (