Editor's Summary
26 June 2008
Cosmic cataclysms
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.
Editorial: The unlikely matters
The study of cosmic impacts and the effects they have offers two lessons for students of science.
doi:10.1038/4531143a
Commentary: What Spaceguard did
A survey of large objects near Earth has shown that there is little risk of a cataclysmic impact in the next century. Alan Harris asks if such cataloguing efforts should continue.
doi:10.1038/4531178a
News and Views: 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.
doi:10.1038/4531191a
News Feature: Gallery feature: All craters great and small
From a 5-millimetre dent on a satellite to a 3-kilometre pit in the surface of Mars, the scars of impact events can be seen at every scale. We present a gallery of some particularly appealing ones from Earth and beyond.
doi:10.1038/4531170a
News Feature: Planetary science: The hole at the bottom of the Moon
Eric Hand
A giant crater on the lunar farside holds the key to a catastrophic bombardment that reshaped the Moon, Earth and other planets. Eric Hand reports.
doi:10.1038/4531160a
News Feature: Planetary science: The burger bar that saved the world
David Chandler
Fewer people are searching for near-Earth asteroids, astronomer David Morrison said in the 1990s, than work a shift in a small McDonalds. But that group — a little larger now — has over the past two decades discovered a host of happily harmless rocks, and in doing so reduced the risk of an unknown asteroid blighting civilization. David Chandler puts together the story in the words of those who watched, and those who watched the watchers.
doi:10.1038/4531164a
News Feature: Planetary science: Tunguska at 100
Duncan Steel
The most dramatic cosmic impact in recent history has gathered up almost as many weird explanations as it knocked down trees, writes Duncan Steel.
doi:10.1038/4531157a
