Asteroid girl did dinos in

Scientists find proof of minor planet Baptistina's daughter, which smashed into Earth.

It was once suggested, to illustrate the chaotic and unpredictable way in which natural systems behave, that the beat of a butterfly's wing in China could eventually trigger a hurricane in the Atlantic. A bit of an exaggeration, perhaps, but the point was that even in the theoretically deterministic world of Newtonian mechanics, only a small amount of complexity is needed to make practical prediction well nigh impossible.

So, perhaps, it is not as far fetched as it sounds to suggest that the collision 160 million years ago of two space rocks resulted in the stormy death almost 100 million years later of the dinosaurs.

Rocky issues

That such a collision was responsible for the demise of the dinosaurs is a suggestion made in a recent issue of Nature by William Bottke and his colleagues at the Southwest Research Institute in Boulder, Colorado, US.

These rocky minor planets, most of which lie between Mars and Jupiter, come in families.

The assumption is that such families are the daughters of single, larger bodies that have been hit by other asteroids and smashed to pieces. Dr Bottke thinks he has traced one such family back to its original collision.

The largest member of the family in question is called Baptistina and is some 40km across. Apart from her, the team identified more than 2,000 smaller objects from her family.

One of the chaotic things that happen to the paths taken by asteroids is caused by sunlight, and is known as the Yarkovsky effect. Like orbiting planets spin on their axes, asteroids, too, rotate. When part of an asteroid's surface passes from night to day, it starts to warm up.

Since the heat the asteroid radiates back into space depends on its temperature, the afternoon side gives off more heat than the morning side, creating a reaction on one side (afternoon). Thus, a small force pushing on the afternoon side gradually distorts the asteroid's orbit.

Destructive family

The result is that the daughters of Baptistina have become scattered from their mother.

The date of the original collision can be calculated from the pattern of this scattering.

That turns out to be 160 million years ago, give or take 20 million years.

That the dinosaurs were killed 65 million years ago by an asteroid impact is now widely accepted. Dr Bottke's contention is that the deed was done by one of Baptistina's daughters. His evidence is twofold. First, the surviving debris from the impact has the same composition as Baptistina and her offspring. The second is the "case of the missing daughters".

The cluster of Baptistina's young has a gap in it. This corresponds to a place where the gravitational pull of Mars and Jupiter have changed the orbits of asteroids much faster than the Yarkovsky effect could manage alone. Such asteroids would have adopted orbits that cut across those of the inner planets, including Earth. Collisions were inevitable.

Taking into account the circumstances existing 65 million years ago, Dr Bottke reckons it more than 90 per cent probable that the dinosaur killer was one of Baptistina's daughters.

The hole made by that collision, in the Yucatan Peninsula in southern Mexico, has been buried in layers of sediment under the sea.

You would not have wanted to witness, at close quarters, its creation.

The story: Looking back

Researchers — William Bottke and David Nesvorny of Southwest Research Institute in Colorado and David Vokrouhlicky of Charles University in Prague, Czech — believe the collision created the Baptistina asteroid family.

As the fragments spread, about 2 per cent of the escapees went on to hit Earth, while others pummelled the moon, the researchers said.

One fragment created the 108-mile-wide Chicxulub crater on Mexico's Yucatan peninsula. Another is thought to have created the 50-mile-wide Tycho crater on the moon.

"We are in the tail end of this shower now," Dr Bottke said. "Our simulations suggest that about 20 per cent of the present-day, near-Earth asteroid population can be traced back to the Baptistina family."