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"Supergiant" Asteroid Shut Down Mars's Magnetic Field

Ker Than
for National Geographic News
May 11, 2009
 
A "supergiant" asteroid several times larger than the one that likely killed the dinosaurs struck Mars with such force that it shut down the planet's magnetic field, scientists say.

Based on the number of large craters present, scientists think very early Mars suffered 15 or so giant impacts within a span of about a hundred million years.

(Related: "Texas-Size Asteroid Slammed Early Mars, Studies Say.")

Now a new computer model suggests Mars's magnetic field may have been slowly weakened by four especially large impacts and then snuffed out completely by a fifth and final blow.

That impact created the 2,000-mile-wide (3,300-kilometer-wide) Utopia crater, which dates back roughly 4.1 billion years, said study team member James Roberts of the Johns Hopkins Applied Physics Lab in Maryland.

"It's possible that the four earlier impacts set everything up, and the Utopia crater was the straw that broke camel's back."

Bombardment

Earth has a magnetic field in part because of heat transfer between the planet's rotating molten core and the relatively cooler mantle layer above it.

This temperature difference helps create what's known as an electric dynamo, which keeps the magnetic field stable over time.

But when the solar system was first forming it went through a tumultuous time known as the Late Heavy Bombardment, in which several large asteroids—remnants of planetary formation—smashed into young Mars, Earth, Venus, and Mercury.

This is about the time the Utopia crater was formed, and roughly the same time that scientists think the Martian magnetic field shut down, said Roberts, whose research is detailed in a recent issue of the Journal of Geophysical Research.

According to the new model, the Utopia impact injected so much heat into the Martian mantle that it drastically reduced the temperature difference driving the dynamo.

"If the mantle becomes too hot, it's not able to cool the core as efficiently … and there is no magnetic field," Roberts told National Geographic News.

Without a magnetic field, Mars was exposed to the full brunt of the solar wind, the continuous stream of charged particles emitted by the sun.

The solar wind could then have slowly eroded away Mars's atmosphere until only a wispy envelope of gas remained.

Drastic climate change would have soon followed, helping to create the desiccated Mars we know today.

Earth Defense

While a similar impactor could conceivably shut down Earth's magnetic field, it would have to be much larger than the one that struck early Mars, Roberts said.

Earth's mantle is twice as thick as that of Mars, and its core churns much more vigorously, making it more difficult to shut down the dynamo effect that generates the field. (Learn more about Earth's dynamic processes.)

Venus also lacks a global magnetic field, but scientists think that is because its mantle is dryer and stiffer and thus less conducive to heat flow.

Sabine Stanley, a geophysicist at the University of Toronto who was not involved in the study, noted that the idea of magnetic field-killing asteroids is not new.

But the latest study, Stanley said, is the first to model the phenomena in detail and to finger a potential culprit crater on Mars.
 

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