The formation of Mercury remains an unsolved mystery. The planet closest to the sun has a disproportionately large metallic core—accounting for about 70% of its mass—and a relatively small rocky mantle. Until now, the most widely accepted explanation was that Mercury lost much of its crust and mantle after colliding catastrophically with a large celestial body. However, dynamic simulations show that this type of impact involving bodies of very different masses is extremely rare.

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