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

Abstract

[abridged] In the typical giant-impact scenario for the Moon formation 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. Here we use a large set of 140 simulations to show that the composition of impactors could be very similar to that of the planets they impact; in $4.9\%$-$18.2\%$ ($1.9\%$-$6.7\%$) of the cases the resulting composition of the Moon is consistent with the observations of $\Delta^{17}O<15$ ($\Delta^{17}O<6$ ppm). These findings suggest that the Earth-Moon composition similarity could be resolved as to arise from the primordial Earth-impactor composition similarity. Note that although we find the likelihood for the suggested competing model of very high mass-ratio impacts (producing significant Earth-impactor composition mixing) is comparable ($<6.7\%$), this scenario also requires additional fine-tuned requirements of a very fast spinning Earth. Using the same simulations we also explore the composition of giant-impact formed Mars-moons as well as Vesta-like asteroids. We find that the Mars-moon composition difference should be large, but smaller than expected if the moons are captured asteroids. Finally, we find that the left-over 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.