Understanding Jupiter’s deep interior: the effect of a dilute core

Abstract

Context. The Juno spacecraft has significantly improved the accuracy of low-order even gravitational harmonics. It has been demonstrated that a dilute core is helpful to interpret Juno’s gravity measurements. However, introducing a dilute core adds a new degree of freedom to Jupiter’s interior models in addition to the uncertainties in the equations of state for hydrogen and helium. Aims. We present four-layer structure models for Jupiter where a dilute core region is added above a central compact core of rocks. The effect of the dilute core on the structure and composition of Jupiter is investigated in detail. Combined with current knowledge of Jupiter’s composition and thermal state, we aim to obtain information on the dilute core. Also, we investigate the effect of equations of state for hydrogen and helium on the predictions of the core mass and heavy element abundance. Methods. In the four-layer structure model, the heavy element abundances in the outer two envelopes and the mass of the compact core were adjusted to reproduce Jupiter’s equatorial radius as well as Juno’s gravity observations. Different dilute core configurations were constructed in terms of its size and composition and different equations of state for hydrogen and helium were used in interior structure calculations. Optimized calculations were then performed to investigate the effect of dilute cores and equations of state on Jupiter’s internal structure and composition. Results. It is found that the absolute values of J6 and J8 tend to decrease as helium becomes more depleted in the dilute core region. Most interior structure calculations seem to prefer an inward decrease of the helium mass fraction from the metallic envelope to the dilute core region. We also show that the core mass and heavy element abundance in Jupiter are dependent upon the rock-to-ice ratio in the dilute core region, the temperature jump from the molecular to metallic envelope, and the equations of state for hydrogen and helium. The resulting heavy-element mass in the core is generally larger than the three-layer structure models owing to the heavy elements dissolved in the dilute core region, and the global heavy-element abundance is in good agreement with the available dilute-core predictions.