Magnetic Dynamo Theory-Mercury is a bit surprising because it has a weak magnetic field. Mercury is the smallest of the terrestrial planets, so its interior should have cooled off long ago. Also, Mercury spins slowly---once every 58.8 days. Mercury's high density tells us that it has a proportionally large iron-nickel core. Its magnetic field implies that Mercury's interior is probably partially molten. In mid-2007 astronomers announced independent evidence in favor of a molten core for Mercury. Using very careful observations of Mercury's rotation, they found that Mercury's core could not be solid (see the next section for moreon this technique). Mercury's situation was a major challenge to the magnetic dynamo theory. In true scientific fashion, the theory made a testable prediction: Mercury should have no magnetic field or one even less than Mars' one because its core should be solid. Observation, the final judge of scientific truth, contradicted the prediction. Should we have thrown out the magnetic dynamo theory then? Astronomers were reluctant to totally
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