Final answer:
The belief that the Moon must be made almost entirely of iron because it has some regions with a magnetic field is false. The Moon's magnetization results from ancient periods of a potentially molten metallic core, and its composition cannot be inferred directly from its weak magnetic properties.
Step-by-step explanation:
The statement "The Moon has a magnetic field and thus must be made almost entirely of iron" is false. The Moon does have some regions of magnetization on its surface, but this is due to an ancient era where the Moon may have had a molten metallic core that could generate a magnetic field. Mercury and the Moon having different iron content as a result of giant impacts show how similar events can create varying outcomes. The impact on the Moon may have caused most of the iron to be drawn away with the debris, leaving the Moon with less iron, while Mercury retained a higher iron content due to its unique impact history.
Magnetization of the Moon's surface is likely due to the cooling of molten rock in the presence of a magnetic field, not because it's made almost entirely of iron. Furthermore, despite iron being a magnetic metal, a planet or a moon can have a magnetic field only if it has an active molten metallic core and conditions suitable for a geodynamo to operate. The Moon's mass and gravitational force are too weak to retain an atmosphere, further indicating that its internal composition and magnetic properties are not equivalent to a body made primarily of iron.