Accounting for the High Density of Planet Mercury
Venus, Earth, and Mars are all thought to have formed from similar materials. Their iron cores take up similar fractions of the planets, and their silicate mantles- -the mantle is the layer of material between a planet’s central core and its outer crust- -melt to produce similar kinds of volcanic rocks. By contrast, Mercury, the smallest and closest to the Sun of the four inner, or terrestrial, planets, may have been formed from a different bulk composition, or it may have formed under different conditions than the other terrestrial planets. The planet has a huge iron core, indicating either that the planet contains at least twice the iron content of the other terrestrial planets or that virtually all the iron from the bulk composition was pulled into its core, leaving its mantle almost iron-free. At the same time, the planet shows no recent volcanic activity, and so, unlike Venus, Earth, and Mars, its interior can be assumed to be stationary. The lack of volcanic activity is usually taken to mean that the planet has largely finished losing its internal heat. Mercury, however, has a small magnetic field. Magnetic fields are generally accepted to be caused by fluid movements in the mantle caused by heat loss. Somehow Mercury has created a balance that allows the generation of a magnetic field in the absence of mantle movements.
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