The terrestrial planet formation paradox inferred from high-resolution N-body simulations

Recent improvements to GPU hardware and the symplectic N-body code GENGA allow for unprecedented resolution in simulations of planet formation. In this paper, we report results from high-resolution N-body simulations of terrestrial planet formation that are mostly direct continuation of our previous 10 Myr simulations (Woo et al., 2021a) until 150 Myr. By assuming that Jupiter and Saturn have always maintained their current eccentric orbits (EJS), we are able to achieve a reasonably good match to the current inner solar system architecture. However, due to the strong radial mixing that occurs in the EJS scenario, it has difficulties in explaining the known isotopic differences between bodies in the inner solar system, most notably between Earth and Mars. On the other hand, assuming initially circular orbits for Jupiter and Saturn (CJS) can reproduce the observed low degree of radial mixing in the inner solar system, while failing to reproduce the current architecture of the inner solar system. These outcomes suggest a possible paradox between dynamical structure and cosmochemical data for the terrestrial planets within the classical formation scenario. In addition, we use our high-resolution simulations to study the assembly of Earth in unprecedented detail, focusing on its collisional history. We find that more than 90% of giant impacts (GIs) on Earth occur within 80 Myr in the EJS simulations, matching the possible early timing of the Moon-forming GI based on the ages recorded by various meteoritic samples. However, the CJS and EJS scenarios both result in a leftover planetesimal mass that is 1-2 orders of magnitude greater than what currently exists in the solar system. Our overall results strongly suggest the need to consider alternative initial conditions for terrestrial planet formation in the solar system. For example, it may be necessary to assume a lower initial mass in the asteroid belt in order to lower the subsequent effect of radial mixing in the EJS scenario, as well as reduce the mass of leftover planetesimals. Similarly, in the CJS scenario, including an early giant planet instability within 80 Myr could make the scenario dynamically feasible.