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Mercury (Planet): Magnetic Field
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Mercury is the only rocky planet other than Earth to have a global magnetic field, which is about 1 percent as strong as Earth’s. However, scientists are puzzled as to why Mercury’s magnetic field is relatively weak. Theory predicts that it should be about 30 times stronger if it is generated in the same way proposed for Earth’s magnetic field. The presence of the field and its global extent suggest that the core of the planet is largely liquid iron, which produces a magnetic field as it moves. Scientists believe that Mercury’s crust insulates the planet’s outer core, allowing the planet to retain heat from radioactive decay and keeping the core liquid despite the very cold temperatures on the dark side of the planet.
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While Pluto is the smallest and Venus is the hottest, Mercury is the densest planet, with its interior dominated by a huge iron core. Evidently the core is partly molten, since it must act like a dynamo to generate Mercury's weak, but Earthlike, magnetic field. It is surprising that the core's heat has not all leaked into space through Mercury's thin mantle.
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Despite its slow rotation, Mercury has a relatively strong magnetosphere; with 1% of the magnetic field strength generated by Earth. It is possible that this magnetic field is generated in a manner similar to Earth's, by a dynamo of circulating liquid core material; current estimates suggest that Mercury's core is not hot enough to liquefy nickel-iron, but it is possible that materials with a lower melting point such as sulfur may be responsible. It is ... possible that Mercury's magnetic field is a remnant of an eariler dynamo effect that has now ceased, the magnetic field becoming "frozen" in solidified magnetic materials.
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Mercury is particularly dense for its size, and has a large iron core that extends to three quarters of its radius. It has a weak magnetic field, which suggests that a part of this core may still be liquid. The apparent absence of a thick mantle has prompted the suggestion that, early in its history, the planet suffered a major impact or impacts, which blasted away most of the mantle layer.
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