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Solar radiation would kill us if we didn’t have earth‘s magnetic field. Did you know Mars’ field has faded? Earth‘s magnetic field is produced by liquid metal flows (convection) in its outer core: hot fluid rising and cooler fluid sinking. It’s like a giant dynamo.

I hadn’t realized the flow could be stopped like on Mars. It’s not clear why it happened. Hypotheses include that Mars is 11% of earth’s mass and smaller planets lose internal heat faster.