The Solar Wind Prevents Reaccretion of Debris after Mercury’s Giant Impact

Abstract

Abstract The planet Mercury possesses an anomalously large iron core, and a correspondingly high bulk density. Numerous hypotheses have been proposed in order to explain such a large iron content. A long-standing idea holds that Mercury once possessed a larger silicate mantle that was removed by a giant impact early in the the solar system’s history. A central problem with this idea has been that material ejected from Mercury is typically reaccreted onto the planet after a short (∼Myr) timescale. Here, we show that the primordial solar wind would have provided sufficient drag upon ejected debris to remove them from Mercury-crossing trajectories before reimpacting the planet’s surface. Specifically, the young Sun likely possessed a stronger wind, fast rotation, and strong magnetic field. Depending upon the time of the giant impact, the ram pressure associated with this wind would push particles outward into the solar system, or inward toward the Sun, on sub-Myr timescales, depending upon the size of ejected debris. Accordingly, the giant impact hypothesis remains a viable pathway toward the removal of planetary mantles, both on Mercury and extrasolar planets, particularly those close to young stars with strong winds.