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The Moon once had a magnetic field, and Mars shows evidence of having had one early in its history, but neither object currently possess a magnetic field. But Mercury, which is halfway in size between the Moon and Mars, has an active and relatively strong magnetic field. Planetary scientists think that planets require a liquid core to sustain such a field, but the smaller the planet, the faster it cools. Mercury, by all right, should have a core as cold and dead as the Moon`s, yet the magnetic field persists. This may result from the presence of elements like sulfur that, when mixed with iron, lower the melting point and keep the core molten.