ScienceWeek

ScienceWeek

070905a

Solar System: Asteroid Collisions and Dinosaurs

Nature 449, 30-31 (6 September 2007) | doi:10.1038/449030a; Published online 5 September 2007

Solar System: Lethal billiards

Philippe Claeys & Steven Goderis

A huge collision in the asteroid belt 160 million years ago sent fragments bagatelling around the inner Solar System. One piece might have caused the mass extinction that wiped out the dinosaurs 65 million years ago.

What are the chances of the sky falling on our heads? If it's an asteroid hitting Earth that we're talking about, we can't be too sure. Not only do estimates of the current terrestrial meteorite impact rate differ by a factor of five to ten, depending on the approximations used1, but we don't know for certain whether that rate has remained constant or has varied throughout geological time.

A current theory proposes that the impact rate has increased during the past 100 million years or so. Bottke et al. present an intriguing explanation for why this might be, invoking errant fragments from a powerful ancient collision in the asteroid belt between Mars and Jupiter.

Clusters of impact craters and layers of material ejected in meteorite impacts, as well as higher levels of extraterrestrial material in some sedimentary rocks, seem to indicate that, during several glacial periods, the Earth–Moon system has suffered abnormally high rates of bombardment. The late Miocene epoch around 8 million years ago, for example, was marked by an increased flux of interplanetary dust particles between 1 microm and 1 mm across, which might have been produced by a collision within the asteroid belt. An asteroid or comet shower has similarly been put forward to explain the higher dust-particle flux in the late Eocene around 35 million years ago, an event that seems to be coupled with an unusually high concentration of impact craters. These include the two largest craters in recent geological history, Popigai in Siberia (100 km in diameter) and Chesapeake Bay off the Maryland coast (around 85 km in diameter).

And we can go even farther back in recording periods of heavy bombardment. The abundant micrometeorites in the 480-million-year-old Ordovician limestones of southern Sweden most probably reached Earth after a significant disruption had occurred in the asteroid belt. Several impact craters also seem to cluster around this age, although here the geological record is rather poor. Farther back still, recognized ejecta layers are concentrated in two time windows between 2.65 billion and 2.5 billion years ago and 3.47 billion and 3.24 billion years ago. Finally, the most dramatic series of events is undoubtedly the Late Heavy Bombardment of 3.8 billion years ago, the occurrence of which is inferred from the lunar cratering record. Although its traces have been erased by geological activity on Earth, extrapolation of the lunar data indicates the formation of up to 22,000 terrestrial craters with a diameter of more than 20 km. This catastrophic bombardment probably resulted from colliding asteroids disturbed by changes in the orbits of the giant gas planets.

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