The composition of Solar system asteroids and Earth/Mars moons, and the Earth–Moon composition similarity

In the typical giant-impact scenario for the formation of the Moon, most of the Moon's material originates from the impactor. Any Earth-impactor composition difference should, therefore, correspond to a comparable Earth–Moon composition difference. Analysis of Moon rocks shows a close Earth–Moon composition similarity, posing a challenge for the giant-impact scenario, given that impactors were thought to significantly differ in composition from the planets they impact. However, our recent analysis of 40 planet formation simulations has shown that the oxygen isotope composition difference in Earth–Moon-like systems is consistent with observations for a significant fraction of the cases, thereby potentially resolving the composition similarity challenge. Here, we use a larger set of 140 simulations and improved statistical analysis to further explore this issue. We find that in 4.9–18.2 per cent (1.9–6.7 per cent) of the cases, the resulting composition of the Moon is in agreement with the observed value of Δ17O < 15 ppm (Δ17O < 6 ppm). These findings reaffirm our original results at higher statistical level and suggest that the Earth–Moon composition similarity could be resolved to arise from the primordial Earth-impactor composition similarity. We also explore the composition of giant-impact formed Mars’ moons as well as Vesta-like asteroids. We predict that the Mars-moon composition difference should be large, but smaller than expected if the moons are captured asteroids. Finally, we find that the leftover planetesimals (‘asteroids’) in our simulations are frequently scattered far away from their initial positions, thus potentially explaining the mismatch between the current position and composition of the Vesta asteroid.