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> The current scientific consensus is that, like Earth, Mars once had a magnetic field that protected its atmosphere. Roughly 4.2 billion years ago, this planet's magnetic field suddenly disappeared, which caused Mars' atmosphere to slowly be lost to space.

What kind of event could cause the loss of a planet's magnetic field?

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It once had a liquid core (like Earth) which generated the magnetic field. Being a smaller planet, it cooled faster and now the core is solid, hence no more magnetic field.

> What kind of event could cause the loss of a planet's magnetic field?

Planetary magnetic fields require a liquid ferrous mantle or core. As Mars cooled, its liquid ferrous material solidified, after which its magnetic field disappeared. The same thing will eventually happen here, and the reason our field still exists is because Earth is a larger planet than Mars, requiring more time to cool down.

More details here: http://arachnoid.com/restoring_mars

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