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  Geotimes - June 2007 - So, when did Earth become attractive?

So, when did Earth become attractive?

In Earth’s infancy, it did not have a magnetosphere, the magnetic field that shields our planet from solar radiation that would otherwise strip away our life-supporting atmosphere. Ever since Earth’s protective field was discovered in 1958, scientists have been wondering when did the planet grow up and become attractive?

David Dunlop, a geophysicist at the University of Toronto, had proposed that Earth’s magnetic field had developed by 2.7 billion years ago. Now, however, magnetic signatures found preserved inside ancient crystals push that date back by at least half a million years, reported John Tarduno, a geophysicist at the University of Rochester in New York, and colleagues in the April 5 issue of Nature. The finding has implications for understanding the evolution of Earth’s deep interior as well as for when life began.

Researchers have been able to gather magnetic information from certain types of rocks that help them look for clues about when changes in Earth’s magnetic field occurred. As magnetic minerals within a rock cool and harden during formation, they record the field’s strength and direction. Researchers can tease out these signatures, which provide a snapshot of the magnetic field at an exact moment in time.

Some chemical or geologic processes, like heating at extreme temperatures, however, can effectively erase the original magnetic signatures in many rocks more than 540 million years old. Tarduno and colleagues looked at rocks from a section of Earth’s crust in South Africa, which the researchers say have been relatively undisturbed for 3.2 billion years. Crystals within the rocks acted as a “jacket,” Tarduno says, shielding the tiny, micrometer-sized fragments of magnetite inside from such geologic interference.

Tarduno and colleagues probed the magnetite pieces and measured their tiny magnetic fields using a Superconducting Quantum Interference Device, or SQUID. When this device’s two electrodes are cooled to close to absolute zero (about negative 460 degrees Fahrenheit), their magnetic field is separated from the electrodes. Thus, passing the magnetite through the field, now between the electrodes, increases the magnetite’s magnetic field to the point it could be measured.

The team found that a magnetic field was indeed present 3.2 billion years ago. While that date is earlier than any previous measurement, it is well within the oldest age of 3.5 billion years that theoretical models have predicted. The researchers also found that the field’s strength at that time was within 50 percent of today’s. Even half as strong, Tarduno says, this field could protect the Earth’s atmosphere from the sun.

The next step is for the researchers to try and look even further back in time to better define when Earth’s magnetic field first developed. To do so, Tarduno says that he, Michael Watkeys and Axel Hofmann of KwaZulu-Natal University in South Africa plan to track down some even older, yet still preserved South African samples. This will be challenging, he says, given that the older a rock is, the more time it has been exposed to the geologic processes that could have contaminated it.

In the meantime, some researchers think that clues for when Earth’s magnetic field was born may be found on the moon. Atmospheric gases from Earth have been found within lunar dust grains that date to about 3.9 billion years ago, suggesting that at that time, no magnetic field strong enough to hold an atmosphere in place existed on Earth. If this is correct, then this data could support Tarduno’s work that suggests Earth’s magnetic field formed by at least 3.2 billion years ago. That’s still a long time to be attractive.

Kathryn Hansen

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