Galaxy 'may cause ice ages'

It is the spiral arms that do it

By Dr David Whitehouse BBC News Online science editor

Ice ages may be caused by our Solar System's passage through our galaxy's spiral arms during our orbit around the centre of the galaxy.

Dr Nir Shaviv, of the University of Toronto, Canada, has put forward evidence for a correlation between changes in the flux of cosmic rays reaching the Earth and the timing of past ice ages.

He believes that the cosmic rays are from stars that explode as we leave a galactic spiral arm. The increased cosmic ray flux triggers ice ages on Earth.

If this is so, then, as our Solar System is emerging from a spiral arm, the Earth will not experience another ice age for tens of millions of years.

Density waves

Galactic spiral arms are not permanent, rigid features of a galaxy. They are transient, formed from so-called density waves that travel around the galaxy every 500 million years or so.

Meteorites held cosmic ray clues

Many short-lived massive stars are born in the wake of the density wave and within a million years or so explode as supernovae just behind the spiral arm.

These supernovae are, according to Dr Shaviv, a major source of cosmic rays. He believes that the Earth is exposed to more cosmic rays when it leaves a galactic spiral arm than at other times.

He deduced the Earth's exposure to cosmic rays by looking at the cosmic ray exposure of over 30 meteorites that were exposed to space for long periods during the past billion years before they fell to Earth.

Detailed analysis of cosmic ray tracks in the meteorites, coupled with an estimate of their age suggests that the intensity of cosmic rays striking our Solar System varies with a period of about 143 million years.

'Good agreement'

The discovery may explain why the dinosaurs were declining before they were wiped out by an asteroid impact 67 million years ago.

About 70 million years ago our Solar System entered the Sagittarius-Carina spiral arm. There is evidence that following this the Earth's global temperature was reduced by about eight degrees Celsius.

Because this was a particularly warm period it was not enough to start a new ice age. It was, however, enough to cause the dinosaurs to decline.

Dr Shaviv says that this timescale agrees with the frequency of major ice ages in the Earth's geological past. It also agrees with our Solar System's passages through the spiral arms.

"Statistically, it turns out to be a very good agreement if you take into account all the uncertainties," he told BBC News Online.

He says that our current position in the galaxy, in the minor Orion spiral arm, should lead to cosmic ray fluxes about half of what we would get after leaving a major spiral arm.

The research is published in Physical Review Letters.