Mega-impact formation of the Mars hemispheric dichotomy

Cosmic cataclysms: An impact origin for the Mars 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.