6 September 2007

Asteroid that wiped out dinosaurs traced

Agençe France-Presse
The extinction of the dinosaurs 65 million years ago has been traced to a collision between two monster rocks in the asteroid belt nearly 100 million years earlier.
Asteroid that wiped out dinosaurs traced

Monster collision: An Asteroid Belt smash-up involving a rocky behemoth called Baptistina may have started a chain of events that led to the impact of a devastating asteroid on Earth 65 million years ago. Credit: Southwest Research Institute/Don Davis

PARIS: The extinction of the dinosaurs 65 million years ago has been traced to a collision between two monster rocks in the asteroid belt nearly 100 million years earlier.

The smash drove a giant sliver of rock into Earth’s path, eventually causing the climate-changing impact that ended the reign of the dinosaurs and enabled the rise of mammals. Other asteroid fragments smashed into the Moon, Venus and Mars, pocking their faces with mighty craters, U.S. and Czech researchers now believe.

Mixing skills in time travel, jigsaw-making and carbon chemistry, the scientists carried out a computer simulation of the jostling among orbital rubble left from the building of the Solar System. The sleuths were guided by an intriguing clue – a large asteroid called (298) Baptistina, which shares the same orbital track as a group of smaller rocks.

Turning the clock back

As they report today in the U.K. journal Nature, they turned the clock back on the simulation and found that the Baptistina bits not only fitted together, they were also remnants of a giant parent asteroid, around 170-kilometres-wide, that once cruised the innermost region of the asteroid belt.

Around 160 million years ago – the best bet in a range of 140 to 190 million years – another giant some 60-kilometres-wide whacked this behemoth. From this soundless collision was born a huge cluster of rocks, they said, including 300 bodies larger than 10 km and 140,000 bodies larger than one kilometre.

Over aeons, the fragments found new orbits with the help of something called the Yarkovsky effect, in which energy from the Sun gives a tiny yet inexorable push to orbiting rocks. As the family gradually split up, a large number of chunks – perhaps one in five of the bigger ones – crept their way out of the asteroid belt and became ensnared by the gravitational pull of the inner planets.

Around 65 million years ago, one 10-kilometre-wide piece crunched into Earth, unleashing a firestorm and kicking up clouds of dust that filtered out sunlight. In this enduring winter, much vegetation was wiped out and the species that depended on them also became extinct. Only those animals that could cope with the new challenge.

The trace of the great event, called the Cretaceous/Tertiary (or K-T) extinction event, can be seen today in the shape of a 180-kilometre-diameter impact crater at modern-day Chicxulub, in Mexico’s Yucatan peninsula.

Rock sampling

The trio of researchers – William Bottke and David Nesvorny of Southwest Research Institute in Colorado, U.S. and David Vokrouhlicky of the Czech Republic’s Charles University in Prague – took their theory a stage further and checked out sediment samples from the Chicxulub site. They found traces of a mineral called carbonaceous chondrite, which is only found in a tiny minority of meteorites, as the Earthly remains of plummeting asteroids are called. Most asteroids can be excluded from the Chicxulub event, but not Baptistina-era ones, they contend.

Putting simulation and chemical evidence together, the team rule out theories that a comet was to blame rather than an asteroid, and say there is a “more than 90 per cent” probability that the killer rock was a refugee from the Baptistina family.

The investigators also argue that there’s a 70 per cent chance that a four-kilometre-wide Baptistina asteroid hit the Moon some around 108 million years ago, forming the 85-kilometre crater Tycho.

The probability is lower than for Chicxulub because it is based only on a simulation. The peak of “Baptistina bombardment” was probably around 100 million years ago but is not over yet, the paper cautions.

Many of the asteroids that skim dangerously close to Earth today owe their orbits to that great collision in the deep past, according to the authors. “We are in the tail end of this shower now,” says Bottke. “Our simulations suggest that about 20 per cent of the present-day near-Earth asteroid population can be traced back to the Baptistina family.”

An important point raised by the study “is how severe the repercussions of cataclysmic collisions in the asteroid belt can be for the Earth–Moon system,” commented geologists Philippe Claeys and Steven Goderis of Vrije University in Brussels, Belgium, in an accompanying commentary also in Nature. “The terrestrial impact record needs to be scrutinized more closely to identify and understand these periods of more intense bombardment, and to link them to the huge and dangerous game of billiards continuously being played out between the orbits of Mars and Jupiter,” they said.


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