Mercury 2

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…is composed primarily of iron, and that it makes up about seventy-five per cent of the planet. Further support for this lies in the fact that Mercury was discovered to generate a magnetic field. Earth's magnetic field is formed by its liquid iron core, so scientists believe that this may generate Mercury's, too. Mariner 10 only photographed part of Mercury, but scientists believe that there is no reason to think that the remainder is any different.