Did an Asteroid Kill Mars' Magnetic Field?

Nov. 19, 2008 -- Deep in Mars' past , an asteroid struck the planet with such titanic force that it could've killed off the planet's entire magnetic field, according to a new study.

When the Red Planet formed, it is thought to have been much like a young Earth -- hot, full of water, and roaring with a molten, churning core and mantle. The liquid rock and metal formed a magnetic dynamo that helped protect its surface and thick atmosphere from cosmic radiation.

Then, beginning around 4.2 billion years ago, it was suddenly pummeled with at least 20 asteroids between 200 and 500 kilometers (124 and 311 miles) in diameter, each leaving a crater. By contrast, the object thought to have killed of the dinosaurs on Earth is estimated to have been eight to 10 kilometers (five to six miles) wide.

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"These things were enormous," James Roberts of the University of California, Santa Cruz, said. "You would not want to be around when one of these hit."

One of the last giant meteors blew a hole 3,000 kilometers (1,864 miles) wide in the planet, creating Utopia basin in the planet's northern hemisphere. At about 4.1 billion years old, Utopia is the oldest crust on the planet that doesn't show signs of magnetism, meaning the rocks must have cooled at a time when there was no magnetic field.