I've been fascinated by dinosaurs ever since I was a young schoolboy. Later, after I'd fallen in love with astronomy, I was thrilled to learn that the giant reptiles were actually related to the stars — through the cosmic impact that rendered them extinct 65 million years ago. Little wonder then that I'm excited about a paper in this week's Nature, in which a trio of dynamicists argue that the deadly impact resulted from a titanic collision out beyond Mars.
Here's their story in a nutshell. Some 160 million years ago, an asteroid roughly 110 miles (170 km) across was blown to smithereens by a collision with a smaller body. The result: hundreds of thousands of mountain-size fragments, a few hundred of which were at least 6 miles (10 km) in diameter. The largest remaining lump, now known as 298 Baptistina, is 30 miles across and today occupies a stable orbit in the inner asteroid belt. But after the smashup some of the collisional shards leaked into the inner solar system through orbital resonances with Mars and Jupiter, causing a prolonged asteroid shower that peaked between 100 and 50 million years ago. Exit dinosaurs.
To make their case, William F. Bottke, David Vokrouhlický, and David Nesvorný (Southwest Research Institute) ran computer simulations that tracked the Baptistina "family" (about 3,000 asteroids with similar orbits) back through time. They discovered that subtle orbit-shifting processes known as the Yarkovsky and YORP effects eventually forced many of the fragments into a nearby orbital resonance with Mars, which in turn ejected them from the main belt.
What's more, members of the Baptistina family appear to be compositional matches to carbonaceous chondrite meteorites, which are uncommon. Chromium concentrations in 65-million-year-old sediment layers on Earth suggest that the dinosaur-killing impactor belonged to the same rare group. "Everything fell into place," says Bottke. The team claims there's at least a 90% probability that the mass extinction at the end of the Cretaceous period was caused by the Baptistina asteroid shower.
Geologist Jan Smit (Free University of Amsterdam, the Netherlands), an expert on the 110-mile-wide Chicxulub crater that led to the dinosaurs' demise, is excited about the Nature paper. "We've always wondered why we haven't found evidence for other 10-km impacts in the past 500 million years," he says. "This is a very elegant explanation." Indeed, Bottke and his colleagues argue, if there were no catastrophic asteroid collisions, strikes from Chicxulub-size objects across would on average occur only once every 350 million years or so.
According to Richard P. Binzel (Massachusetts Institute of Technology), the new result addresses "the essence of the problem" of getting asteroids into Earth-crossing orbits. "We're interested in the mechanisms that can deliver bodies to the vicinity of the Earth," Binzel explains. "So we need to know what role sudden collisions play as compared to slow orbital drift." Bottke estimates that fully 20% of the current population of near-Earth asteroids are leftovers from the Baptistina collision. "We're still in the tail of the asteroid shower," he says.
What about other impact targets in the inner solar system? The prominent lunar crater Tycho, only 109 million years old, might also have been gouged out by a Baptistina renegade. But Bottke warns that we know nothing about the composition of the Tycho impactor. On Venus, the large craters Mead, Isabella, Meitner, and Klenova are possible suspects — at least one of these was probably caused by the asteroid shower — but exact ages for these impact structures are not known.
Anyway, if Bottke, Vokrouhlický, and Nesvorný are right, the cosmic collision that produced Baptistina and its siblings also paved the way for mammals and, ultimately, humans. It's a fascinating thought — makes me feel like that young schoolboy again.