Dynamical Avenues for Mercury’s Origin. I. The Lone Survivor of a Primordial Generation of Short-period Protoplanets
The absence of planets interior to Mercury continues to puzzle terrestrial-planet formation models, particularly when contrasted with the relatively high derived occurrence rates of short-period planets around Sun-like stars. Recent work proposed that the majority of systems hosting hot super-Earths attain their orbital architectures through an epoch of dynamical instability after forming in quasi-stable, tightly packed configurations. Isotopic evidence seems to suggest that the formation of objects in the super-Earth-mass regime is unlikely to have occurred in the solar system as the terrestrial-forming disk is thought to have been significantly mass deprived starting around 2 Myr after the formation of calcium-aluminum-rich inclusions—a consequence of either Jupiter’s growth or an intrinsic disk feature. Nevertheless, terrestrial-planet formation models and high-resolution investigations of planetesimal dynamics in the gas-disk phase occasionally find that quasi-stable protoplanets with mass comparable to that of Mars emerge in the vicinity of Mercury’s modern orbit. In this paper, we investigate whether it is possible for a primordial configuration of such objects to be cataclysmically destroyed in a manner that leaves Mercury behind as the sole survivor without disturbing the other terrestrial worlds. We use numerical simulations to show that this scenario is plausible. In many cases, the surviving Mercury analog experiences a series of erosive impacts, thereby boosting its Fe/Si ratio. A caveat of our proposed genesis scenario for Mercury is that Venus typically experiences at least one late giant impact.