Mega-impact formation of the Mars hemispheric dichotomy

A 'near Mars object' that got too near is a possible explanation for one of the most prominent features we see on Mars — the north-south dichotomy of the planet's surface appearance. The southern highlands on Mars cover about 60% of the planet and are heavily cratered, while the northern lowlands are lightly cratered, are geologically younger, and the underlying crust is significantly thinner than in the south. The two favoured explanations for this 'hemispheric dichotomy' are mantle convection and a giant impact, but there is little available evidence to distinguish between the theories. Three Letters in this issue provide support for the giant impact model. Marinova et al. present the results of dynamical simulations of dichotomy-forming impacts that demonstrate the feasibility of a giant impact origin. Andrews-Hanna et al. use the gravity and topography of Mars to map the dichotomy boundary beneath the large Tharsis volcanic province, and find the boundary to be elliptical, consistent with an oblique giant impact origin. This 'Borealis basin' would be the largest impact scar in the Solar System. Nimmo et al. use numerical modelling to simulate the effects of vertical impacts. They find that the impact model, as well as excavating a crustal cavity of the correct size, can explain the observed crustal disruption and the formation of the northern lowlands crust by impact-generated melt. The Martian 'hemispheric dichotomy' is expressed as a dramatic difference in elevation and crustal thickness between the southern highlands and northern lowlands. A set of single impact initial conditions by which a large impactor can produce features that are consistent with the observed dichotomy's crustal structure have been found. Using 3D hydrodynamic simulations, the models produce large variations in post-impact states depending on impact energy, velocity and, importantly, impact angle. The Mars hemispheric dichotomy is expressed as a dramatic difference in elevation, crustal thickness and crater density between the southern highlands and northern lowlands (which cover ∼42% of the surface)1,2. Despite the prominence of the dichotomy, its origin has remained enigmatic and models for its formation largely untested3,4,5. Endogenic degree-1 convection models with north–south asymmetry are incomplete in that they are restricted to simulating only mantle dynamics and they neglect crustal evolution, whereas exogenic multiple impact events are statistically unlikely to concentrate in one hemisphere6. A single mega-impact of the requisite size has not previously been modelled. However, it has been hypothesized that such an event could obliterate the evidence of its occurrence by completely covering the surface with melt7 or catastrophically disrupting the planet3,8. Here we present a set of single-impact initial conditions by which a large impactor can produce features consistent with the observed dichotomy’s crustal structure and persistence. Using three-dimensional hydrodynamic simulations, large variations are predicted in post-impact states depending on impact energy, velocity and, importantly, impact angle, with trends more pronounced or unseen in commonly studied smaller impacts9. For impact energies of ∼(3–6) × 1029 J, at low impact velocities (6–10 km s-1) and oblique impact angles (30–60°), the resulting crustal removal boundary is similar in size and ellipticity to the observed characteristics of the lowlands basin. Under these conditions, the melt distribution is largely contained within the area of impact and thus does not erase the evidence of the impact’s occurrence. The antiquity of the dichotomy10 is consistent with the contemporaneous presence of impactors of diameter 1,600–2,700 km in Mars-crossing orbits3, and the impact angle is consistent with the expected distribution11.