Planetary science: Forming the martian great divide.

Early in its history, Mars suffered a convulsion that left a lasting geological and topographical scar. The latest work adds to evidence that the cause was external — a massive impact. 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. A snapshot of a simulation with favoured impact conditions is shown on the cover. The snapshot is about 30 min after impact and the colour codes for internal energy (Image: S. Lombeyda, Caltech Center for Advanced Computing Research; M. Marinova and O. Aharonson, California Institute of Technology). 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. In an accompanying News & Views, Walter Kiefer sums up the evidence for the impact model. Elsewhere in this issue, the results of a smaller but still enormous impact event are remembered. The Tunguska event laid waste to a large swath of forest to the north of Lake Baikal in Central Siberia on the morning of 30 June 1908. In a News Feature, Duncan Steel goes back to basics to sift through the contemporary evidence collected in the days after the blast. That, combined with modern studies of the area and theoretical work, points to a fragment of comet Encke as the likely impactor. A small group of astronomers began to look for near-Earth objects (NEOs) capable of causing Tunguska-like events in the 1960s and 1970s, and with the realization that an asteroid impact can cause events as drastic as mass extinctions, the Spaceguard project was begun in the 1998 to search more methodically for NEOs. As Alan Harris explains in a Commentary, the results of Spaceguard are encouraging. It looks like we are 'safe' for the foreseeable future, but NEO watchers say that we need to keep up our guard. Eric Hand reports on the giant lunar crater that holds the key to a catastrophic bombardment, and a Gallery Feature celebrates the beauty of impact craters from across the Solar System.