BioEd Online

Most scientists think that the age of the dinosaurs was doomed when a killer asteroid struck Earth 65 million years ago. But new findings suggest that the giant reptiles may have been destined for death some 100 million years earlier, with the fateful collision of two faraway asteroids.

A crash in the Solar System's main asteroid belt set into motion a chain of events that was to eventually end the Cretaceous period, according to William Bottke, an astronomer at the Southwest Research Institute in Boulder, Colorado, and co-author of the study. His team's findings, published this week in Nature¹, also finger the killer — an ancient, 170-kilometre-wide rock.

Bottke's team stumbled across the finding while searching through the massive Sloan Digital Sky Survey database. The team was examining a group of fragmented rocks called the Baptistina asteroid family, which lies in the main asteroid belt between Mars and Jupiter, when they noticed a spot of sky that seemed to be empty. "We literally saw a hole," Bottke says.

He and his colleagues quickly realized that the hole was the result of the gravity from nearby Jupiter and Mars, which together had conspired to push and pull, over millions of years, some of these fragments further into the centre of the Solar System. Computer models run by the team show many of these fragments gradually slipping into orbits that cross Earth's, he says. "It was almost inevitable that a piece of it would hit the Earth."

Modelling the movements of the present-day fragments, they were able to project backwards in time to see what event started this family of asteroids. It was probably a collision of a 170-kilometre-wide rock with one 60 kilometres wide about 160 million years ago, their model shows, with the smaller asteroid pulverized by the impact.

One collision to the next

To link the Baptistina family to the scene of the crime, the group also looked at the spectra of two present-day fragments, to determine their chemical composition. This showed that they are made primarily of carbonaceous chrondite, a silicate material from the primordial Solar System.

Geologists have also found carbonaceous chondrite in terrestrial sediment and a meteorite recovered from the Pacific Ocean — both dating from the time of a giant impact in Chicxulub, near the Yucatán Peninsula in Mexico, 65 million years ago.

Bottke says he is "90% certain" that this ancient impact was caused by a chunk of asteroid spat out from the massive collision in his model — the same collision that made the present-day Baptistina family of asteroids.

Comet versus asteroid

The findings go a long way towards resolving whether an asteroid or comet was behind the dinosaur extinction event, according to Philippe Claeys, a geologist at Vrije University in Brussels. Some had previously thought that the presence of carbonaceous chrondite in geological records of this time suggested a comet, because this mineral is found only in the Solar System's oldest bodies

But the chemical link to the Baptistina family is now strong enough to suggest that some part of it was the killer, he says. "The evidence is weighing heavily towards the asteroid," says Claeys.

This isn't the first time that Bottke and his colleagues have linked a geological event on Earth to the asteroid belt. In January of last year, the team linked a shower of dust and meteors that fell around 8 million years ago to the disruption of an asteroid named Veritas².

This type of work serves as an important reminder that humans need to be mindful of Earth's surroundings, says Claeys. "We have to be aware of things that happen in the Solar System," he says. "Especially in the asteroid belt."