A WOBBLE in Mercury鈥檚 orbit could have wiped out the dinosaurs.
Computer simulations show that the planet鈥檚 orbit wobbled 65 million years
ago. This could have been just the thing to send an asteroid hurtling towards
Earth, resulting in the huge impact that caused the mass extinction at the end
of the Cretaceous period, says Bruce Runnegar from the Center for Astrobiology
at the University of California, Los Angeles.
But other researchers disagree. Mark Bailey, director of the Armagh
Observatory in Northern Ireland, says that the theory relies on an unlikely
chain of events. 鈥淚 can鈥檛 believe that Mercury has an effect on anything in the
Solar System. It鈥檚 such a small planet.鈥�
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Runnegar and his colleagues used computer models to map out the Solar System
for the past 250 million years. In particular, they looked at the perihelion of
each planet鈥攖he point in its orbit when it is closest to the Sun.
The perihelion of Earth, for example, rotates around the Sun with a period of
hundreds of thousands of years. Because of subtle tugs and pulls between the
planets, this period changes slightly over time. Now Runnegar and his colleagues
have found that these changes in both the Earth鈥檚 and some outer planets鈥� orbits
had a knock-on effect on the inner planets, they told an Earth Systems Processes
meeting in Edinburgh this week.
One of these blips significantly changed Mercury鈥檚 orbit 65 million years
ago, their model suggests. This wobble would have pulled at the asteroid belt,
increasing the chances that asteroids in the Hungarias region would be knocked
out of place. Now the researchers are running a fresh set of models to see how
much the orbits of these asteroids changed.
It wouldn鈥檛 have been enough to send a shower of asteroids into the Earth,
but Runnegar says the wobble could have sent a single asteroid onto a collision
course with our planet. 鈥淚t鈥檚 quite plausible that some of the asteroids were
affected,鈥� he says.
Bailey says that the group鈥檚 modelling is admirable. But the link with the
dinosaurs is 鈥渢enuous to say the least鈥�, he says. 鈥淚t鈥檚 a great long chain of
缚辫辞蝉蝉颈产濒测蝉鈥�.鈥�
Runnegar accepts that it鈥檚 extremely hard to model the workings of the Solar
System millions of years ago, particularly since the planets seem to move more
chaotically the further back you go. 鈥淚t looks complicated, and it is,鈥� he
says.
But he and his team have tested the accuracy of their model by making tiny
changes to the initial positions of the planets and rerunning time. They found
the results were similar enough to give them the confidence that their numbers
are reliable up to 70 million years ago. 鈥淲e achieved significantly better
accuracy than previous work,鈥� says Runnegar.
Now he is planning to run his models forward in time, to see just when the
next potentially catastrophic planetary wobble will take place.