A post-accretionary lull in large impacts on early Mars

The early impact bombardment of Mars has been linked to the bombardment history of the inner Solar System as a whole. However, the timing and number of basin-forming impacts on Mars are poorly constrained. The Borealis basin - thought to be the largest and oldest known Martian impact basin - forms the crustal dichotomy between the northern lowlands and southern highlands. Four unambiguous large basins post-dating Borealis have been identified, but as many as 32 additional basins larger than 1,000 km in diameter have been proposed. Here we use gravity and topography analyses to show that the crustal dichotomy boundary was excavated by only one later impact basin (Isidis), which probabilistically indicates that fewer than 12 large basins across the globe could post-date the boundary and pre-date the established younger basins. Moreover, the relatively pristine topography and crustal thickness at the dichotomy boundary indicates that younger basins should be similarly well preserved. This suggests that the post-Borealis large basin population is limited to only the four known younger basins, with estimated ages between 3.8 and 4.1 Gyr ago (Ga). We present geochemical arguments that Borealis dates to near 4.5 Ga. Combined with Monte Carlo simulations, we argue that, instead of a gradually declining impactor flux, a lull in large basin-forming impacts occurred between about 4.1 and 4.4 Ga on Mars, separating the endgame of accretion from a putative late heavy bombardment similar to that proposed for the Moon and asteroid belt.