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Friday, 10 March, 2000, 15:57 GMT
Lunar rock reveals life's clues
Nasa
Apollo 16 astronauts collect samples
By BBC News Online science editor Dr David Whitehouse

Measurements made of a speck of lunar dust may tell us how life blossomed on Earth, say researchers.

The scientists have analysed the history of impact cratering on the Moon and say they have found a surprising increase in the frequency of meteor impacts over the past 400 million years.

The clear inference is that the Earth also would have experienced a similar frequency of impacts over the same period, they write in the 10 March issue of the journal Science.

"From our point of view, that's in the recent past," says Timothy Culler, of the University of California, Berkeley.

The analysis suggests that the impact cratering rate had dropped steadily after a period of heavy bombardment when the Solar System was young, but then increased rapidly about 400 million years ago.

Gradual cleansing

Richard Muller, professor of physics at UC Berkeley, said: "It is not difficult to understand the slow decrease. It corresponds to a gradual cleansing of the young Solar System by Jupiter, the Sun, and passing stars. But it is difficult to conceive of a mechanism that could trigger an increase, particularly one that lasted 400 million years."

Tim Culler/UC Berkeley
A spherule from the Moon: Remnant of an ancient impact
The astronomers point out that the sudden increase coincides with the so-called "Cambrian explosion", a period in which life on Earth went through a dramatic increase in the number and diversity of species.

"Although most people assume that impacts cause death and destruction, it is possible that the additional stress of the impacts forced life to become more diverse and flexible," says Muller.

"Just as we stress trees, through pruning, to make them give more fruit, the stress cause by catastrophic impacts may have forced evolution into new directions."

The Berkeley team looked at a gram of lunar soil brought back by the Apollo 16 astronauts in 1971. In it they found 155 glassy spherules. These are formed when droplets of molten basalt are splashed out of a crater by the force and heat of an impact.

Goddess of retribution

Analysis showed that the spherules came from approximately 146 different craters. The age of the spherule, determined by isotope measurements, gives a record of lunar bombardment.

Nasa
Impacts may have spurred evolution
"Even though we don't know which crater was the source of each spherule, the distribution of the ages of the spherules from a single lunar site should reflect the age distribution of craters on the Moon," Muller says.

Controversially, Muller suggests that the sudden increase in cratering 400 million years ago offers indirect evidence for a distant companion star to the Sun that orbits it every 26 million years. When the existence of such a star was postulated in 1984, Muller suggested that it be called Nemesis after the Greek goddess of retribution.

"The increase in impacts could be due to a sudden change in the orbit of Nemesis," Muller says. "If a passing star perturbed Nemesis into a more eccentric orbit, that would account for the increase in impacts.

"This work opens up a new field that tells us something about the history of our Solar System that was totally unanticipated. Until now we did not realise how peculiar the past 500 million years has been."

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See also:

12 Nov 99 |  Sci/Tech
Moon mission targets mystery
26 Nov 99 |  Sci/Tech
Lunar link to volcanic past
22 Jul 99 |  Sci/Tech
Close-up on the Moon
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