Once upon a time, Mars had a magnetic field, just like Earth. Four billion years ago, it vanished, taking with it the planet's chances of evolving life as we know it. Now scientists have proposed a new explanation for its disappearance.__

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A model of asteroids striking the red planet suggests that, while no single impact would have short-circuited the dynamo that powered its magnetism,__ __a quick succession of 20 asteroid strikes could have done the job.

"Each one crippled a little bit," said geophysicist Jafar Arkani-Hamed of the University of Toronto, author of the new study. "We believe those were enough to cripple, cripple, cripple, cripple until it killed all of the dynamo forever."

Rocky planets like Earth, Mars, Mercury and even the moon get their magnetic fields from the movement of molten iron inside their cores, a process called convection. Packets of molten iron rise, cool and sink within the core, and generate an electric current. The planet's spinning turns that current into a magnetic field in a system known as a dynamo.

Magnetic fields can shield a planet from the constant rain of high-energy particles carried in the solar wind by deflecting charged particles away from the surface. Some studies have suggested that Earth's magnetic field could have protected early life forms from the sun's most harmful radiation, allowing more complex life to develop. But traces of magnetism in the Martian surface reveal that the red planet lost its magnetic field some four billion years ago, leaving its atmosphere to be dessicated by the harsh solar wind.

Previous studies suggested that a massive impact could have shut down Mars's dynamo by warming the mantle layer, disrupting the heat flow from the core to the mantle and shutting down convection. The fact that the crust of Mars's younger impact craters is not magnetized supports this idea. Earlier computer models by geophysicist James Roberts of Johns Hopkins University showed that the largest known impacts on Mars could turn the mantle to a warm blanket, bringing the dynamo to a standstill.

But Arkani-Hamed's new study in the Journal of Geophysical Research suggests that just one impact wouldn't suffice. The dynamo would recover in less than one hundred million years. "The magnetic field should come back again," he said.

To make his case, Arkani-Hamed modeled the heat that could have been produced when – according to some geophysicists – an asteroid the size of Texas hit Mars about 4.5 billion years ago, producing the biggest impact in our solar system's history. Called the Borealis impact, it may have flattened Mars's entire northern hemisphere.

This mega-impact would have flattened out the heat cycle inside the planet, too, snuffing out the dynamo within about 20,000 years.__ __Without the cold compress of the mantle to siphon heat away from the core, convection wouldn't have a chance.

But left alone, convection would have recovered in the outer parts of the core, and eventually penetrated deep and started the whole core churning again. The Borealis impact would have crippled the dynamo, but not killed it outright.

"If there were a dynamo at 4.5 billion years, it could cease, go away and regenerate after about 100 million years," he said.

But perhaps__ several impacts in a row could do the __job. The planet's crater record shows that Mars suffered 20 impacts in quick succession between 4.2 and 3.9 billion years ago. In work to be presented at the Lunar and Planetary Science Conference in The Woodlands, Texas this March, Arkani-Hamed teamed up with Roberts to show that just the five largest of these impacts could have shut down the magnetic field. The impacts came so rapidly that the dynamo had no time to recover before the next crippling blow arrived.

____"This research is important because it shows that this scenario is plausible. It could have physically happened," said Wesley Watters of Cornell University, who was not involved in the new research. "But to test this model versus another is enormously difficult to do."

To really figure out when and how Mars lost its magnetic field, we'd need to know the ages of lots of Martian rocks with the same kind of precision with which we know them on Earth.

"We just don't have that for Mars," he said.

Image: NASA

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