There is a north-south divide on Mars. The southern highlands covering about 60% of the planet, are heavily cratered, while the northern lowlands are lightly cratered and geologically younger, with a thinner underlying crust. Favoured explanations for this 'hemispheric dichotomy' are mantle convection or a giant impact, but there is little evidence to distinguish between the theories. Three Letters in this issue provide support for the giant impact model. Marinova et al. present dynamical simulations of dichotomy-forming impacts that demonstrate the feasibility of a giant impact origin. A simulation with favoured impact conditions is shown on the cover: the snapshot is about 30 min after impact and the colours code for internal energy. (Cover image: S Lombeyda, Caltech Center for Advanced Computing Research/ M Marinova & O Aharonson, Caltech.) Andrews-Hanna et al. use gravity and topography data to map the dichotomy boundary beneath the Tharsis volcanic province, and find an elliptical boundary, consistent with an oblique giant impact origin. Nimmo et al. use numerical modelling to show that a vertical impact, as well as excavating a crustal cavity of the right size, can explain the observed crustal disruption and the formation of the northern lowlands crust. In an accompanying News & Views , Walter Kiefer sums up the evidence for the impact model.