No dilute core produced in simulations of giant impacts on to Jupiter

ABSTRACT A giant impact has been proposed as a possible formation mechanism for Jupiter’s dilute core – the planet’s inferred internal structure in which the transition between its core of heavy elements and its predominantly hydrogen–helium envelope is gradual rather than a discrete interface. A past simulation suggested that a head-on impact of a $10~\mathrm{ M}_{\rm{\oplus }}$ planet into an almost fully formed, differentiated Jupiter could lead to a post-impact planet with a smooth compositional gradient and a central heavy-element fraction as low as $Z\approx 0.5$. Here, we present simulations of giant impacts on to Jupiter using improved numerical methods to reassess the feasibility of this scenario. We use the REMIX smoothed particle hydrodynamics (SPH) formulation, which has been newly developed to improve the treatment of mixing in SPH simulations. We note that, as in previous works, chemical mixing is not included in these models and that incorporating such processes at sub-particle scales could improve numerical convergence. We perform giant impact simulations with varying speeds, angles, pre-impact planet structures, and equations of state. In all of our simulations, heavy elements re-settle over short time-scales to form a differentiated core, even in cases where the core is initially disrupted into a transiently mixed state. A dilute core is not produced in any of our simulations. These results, combined with recent observations that indicate Saturn also has a dilute core, suggest that such structures are produced as part of the extended formation and evolution of giant planets, rather than through extreme, low-likelihood giant impacts.