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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.
