Bottke, William - Moon-Forming Impact Ejecta as the Source of the Earliest Lunar Bombardment

The earliest phase of lunar bombardment, defined by pre-Nectarian (pN) craters and basins on the Moon, has long been a mystery. Many argue pN impact events were derived from a long-lived leftover planetesimal population residing in the terrestrial planet region (Neukum and Ivavov 2001; Morbidelli et al. 2013). Problems with this model, however, have recently emerged. Analyses of ancient pN cratered terrains, as well as hydrocode models of South Pole Aitken (SPA) basin formation, suggest pN projectiles struck the Moon at ~ 10 km/s, 1.5-2 times lower than expectations from existing dynamical models of leftover planetesimals (Walsh et al. 2011; Marchi et al. 2012; Potter et al. 2013). Our own collisional and dynamical evolution simulations of leftover planetesimals have also had difficulty reproducing the characteristic signatures of pN craters/basins. 

Here we argue for an unexplored bombardment scenario that fits within the framework of planet and lunar formation models. We postulate that most pN impacts were produced by the relatively late return of ejecta from the giant impact (GI) that created the Moon. The GI was probably the biggest youngest impact to ever take place in the terrestrial planet region, and simulations indicate that several percent of an Earth mass was ejected out of cis-lunar space by this event (e.g., Jackson and Wyatt 2012; Canup 2012). Tracking this material using a suite of collisional and dynamical models, we find GI ejecta returns in some abundance to strike the Moon at ~10 km/s over an interval of many tens of Myr.  En route to the Moon, the population undergoes extensive collisional evolution, enough to reproduce the wavy shape of the observed pN crater size frequency distributions upon impact.

Our model results predict that the oldest pN- and SPA-cratered terrains formed ~8 and 15 Myr after the GI, respectively.  SPA basin may have even formed earlier than these times, which would explain the absence of SPA-produced secondary craters on nearby pN terrains (Bottke et al. 2013). This would require processes on SPA to erase 20 < D < 100 km craters for ~7 Myr after SPA formed, a plausible scenario considering the nature of the early lunar crust and lunar magma ocean (Elkins-Tanton et al. 2011).   

We also find that considerable GI ejecta hit the Moon prior to the oldest pN terrains, with the projectiles presumably slamming into a thin hot mushy lunar crust. The consequences of such impact events are unknown, but we suspect they would leave behind features similar to the flat palimpsest-like basins on Callisto. Such outcomes could explain why several prominent pN basins discussed by Wilhelms (1987), such as Procellarum, Australe, and Tranquillitatis, lack the topographic and gravity signatures of younger basins defined by GRAIL data.

Accordingly, our results provide key constraints on the time-varying nature of the earliest lunar crust, the evolution of the lunar magma ocean, the size frequency distributions of both GI ejecta and leftover planetesimals, and planet formation itself.