Late accretion to Mercury: Global cratering, crust erosion, and accretion of exogenic materials

Origin and dynamical evolution of Mercury during the early stage of planet formation are still poorly understood (e.g., Ebel and Stewart, 2018, and references therein). Regardless of a scenario of Mercury’s formation, leftover planetesimals, asteroids, and comets would inevitably bombard Mercury’s surface after the completion of the primary accretion, including core-mantle separation. Such late impact bombardment is often called late accretion. The timing, duration and the dominant source (impactors’ origin) of late accretion to a terrestrial planet vary from model to model of planet formation (e.g., Abramov et al. 2013; Morbidelli et al. 2012; Morbidelli et al. 2018; Mojzsis et al. 2018; Brasser et al. 2020).

During late accretion, numerous small bodies impact on the surface of terrestrial planets – we define such impacts as cratering impacts. Late accretion would be a key process to characterize surface geomorphic and geochemical features of Mercury at the very last stage of its formation through global cratering, crust erosion, and accretion of exogenic impactors’ materials. Existing dynamical models indicate that Mercury experienced an impact bombardment with a total mass ranging from M_imp,tot ~ 0.08 × 10²⁰ to 8 × 10²⁰ kg (e.g., Abramov et al. 2013; Mojzsis et al. 2018; Brasser et al. 2020) and with impact velocity from v_imp ~ 30 to 40 km/s (e.g., Le Feuvre and Wieczorek, 2008). However, the fate of the impactors and their effects on Mercury’s surface through the bombardment are still not clear.

Outcomes of cratering, surface erosion, and accretion of impactor’s material strongly depend on impact conditions (impact velocity, impact angle, and size of impactor). Recently, Hyodo & Genda (2020) derived scaling laws of erosion mass of target material and accretion mass of impactor material upon cratering impacts as a function of impact velocity and impact angle in a unit of impactor’s mass. In this work, by using these scaling laws (Hyodo & Genda 2020) as well as those predict the size of craters upon cratering impacts (e.g., Melosh and Vickery, 1989), we aim to understand the degree of global cratering, crustal erosion, and accretion mass of impactor materials during late accretion to Mercury.

Here, we attempt not to prioritize any specific scenario for late accretion (i.e., planet formation models). Instead, using Monte-Carlo and semi-analytical approaches combined with the scaling laws, we aim to generalize the outcomes of late accretion as a function of the total mass of impactors and typical impact velocity to Mercury (Hyodo et al. 2020, under review).

Within the range of parameters of late accretion discussed above as reference values, we report that late accretion could remove, on average, 50 m to 10 km of the primitive crust of Mercury. The cratering process during late accretion is extensive for erasing the older geological surface records on Mercury (see also, Mojzsis et al. 2018). Our Monte-Carlo simulations indicate that about 40 to 50wt.% of the impactor’s exogenic materials – about 0.3 × 10¹⁹ to 1.6 × 10¹⁹ kg in total including a consideration that accreted impactors’ material could be re-ejected during successive cratering impacts – are heterogeneously implanted on Mercury’s surface during late accretion. About half of the accreted impactor’s materials would be vaporized and the rest would be completely melted. We expect that our results would be useful to bridge predictions of different planet formation models and the detailed surface observations of Mercury, including the BepiColombo mission (Hyodo, Genda & Brasser 2020, under review).