Abstract | Impacts are a fundamental process by which planets grow and are modified. Stochastic giant impacts on the terrestrial bodies mechanically and thermally affect a large portion of the planet's surface (Benz et al., 1988; Nakajima and Stevenson, 2015; Chau et al. 2018). Contrarily, small impacts, namely the cratering impacts, affect only a small area of the planet's surface and are much more frequent (Melosh 2011).
Here, using analytical and Monte Carlo approaches combined with the updated scaling laws (Hyodo and Genda 2020, 2021) for the escape mass of the target material and the accretion mass of the impactor material during the cratering impacts, we studied (1) whether late accretion significantly erodes Mercury, and (2) the fate of the impactors to Mercury during late accretion.
Existing dynamical models of late accretion indicate that Mercury experienced an intense impact bombardment after 4.5 Ga (a total mass of $\sim 8 \times 10^{18} - 8 \times 10^{20}$ kg with a typical impact velocity of $30 - 40$ km s$^{-1}$, depending on dynamical models). For this parameter range, we found that late accretion could remove 50 m to 10 km of the early (post-formation) crust of Mercury, but the change to its core-to-mantle ratio is negligible. Although the cratering is notable for erasing the older geological surface records on Mercury, we showed that $\sim 40 - 50$wt.% of the impactor's exogenic materials, including the volatile-bearing materials, can be heterogeneously implanted on Mercury's surface as a late veneer (at least $\sim 3 \times 10^{18} - 1.6 \times 10^{19}$ kg in total). About half of the accreted impactor’s materials are vaporized, and the rest is completely melted upon the impact.
In short, late accretion seems an inevitable dynamical process at the very last stage of the planet formation, and it affects Mercury's surface in both mechanical, chemical, and thermal aspects (Hyodo et al. 2021; see also Mojzsis et al. 2018).
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