The Borealis basin and the origin of the martian crustal 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 hemispheric dichotomy between the southern highlands and northern lowlands on Mars is characterized by a change in crustal thickness along an apparently irregular boundary, which can be traced around the planet, except where it is presumably buried beneath the Tharsis volcanic rise. The gravity and topography of Mars is used to constrain the location of the dichotomy boundary beneath Tharsis, and finds that the dichotomy boundary along its entire path around the planet is accurately fit by an ellipse measuring about 10,600 by 8,500 km. The most prominent feature on the surface of Mars is the near-hemispheric dichotomy between the southern highlands and northern lowlands. The root of this dichotomy is a change in crustal thickness along an apparently irregular boundary, which can be traced around the planet, except where it is presumably buried beneath the Tharsis volcanic rise1,2. The isostatic compensation of these distinct provinces2,3 and the ancient population of impact craters buried beneath the young lowlands surface4 suggest that the dichotomy is one of the most ancient features on the planet3. However, the origin of this dichotomy has remained uncertain, with little evidence to distinguish between the suggested causes: a giant impact5,6 or mantle convection/overturn7,8,9. Here we use the gravity10 and topography11 of Mars to constrain the location of the dichotomy boundary beneath Tharsis, taking advantage of the different modes of compensation for Tharsis and the dichotomy to separate their effects. We find that the dichotomy boundary along its entire path around the planet is accurately fitted by an ellipse measuring approximately 10,600 by 8,500 km, centred at 67° N, 208° E. We suggest that the elliptical nature of the crustal dichotomy is most simply explained by a giant impact, representing the largest such structure thus far identified in the Solar System.