Final answer:
The assertion that the Moon is made almost entirely of iron due to its magnetic field is false. Magnetic fields can result from various factors, and the Moon's weak magnetization does not indicate a high iron content. Different outcomes from giant impacts account for the varying iron contents on the Moon and Mercury.
Step-by-step explanation:
The statement that the Moon must be made almost entirely of iron because it has a magnetic field is false. The Moon does have some magnetized regions, but it lacks a global magnetic field similar to Earth's. Instead, it's believed that these magnetic patches are remnants of an ancient magnetic field, which could have been produced by a molten metallic core generating electric currents in the past. The presence of a magnetic field doesn't necessarily mean an object is composed mainly of iron. For example, the Earth has a magnetic field due to its molten iron core, but its surface is largely composed of non-magnetic materials like rock.
Regarding the Moon's and Mercury's iron content being the result of giant impacts, this can be explained by considering that the same type of event can have different outcomes depending on the specific circumstances, such as the size, speed, and angle of the impacting body, as well as the composition of the targeted celestial body. An impact could strip away outer layers of a body, leaving it with a relatively higher core-to-mantle ratio, as might be the case with Mercury, or it could add lighter material and dilute the iron content, as might have happened with the Moon.