The Moon has too little iron, Mercury too much. How can both of these anomalies be the result of giant impacts? Explain how the same process can yield such apparently contradictory results.

Giant impact has resulted into formation of celestial bodies such as moon.

Moon was formed years after the Earth was formed. When Earth collided with very big size bodies, there was vaporization of chunks of the planets in to the space. Gravity plays its role and bounded those particles together thus forming moon.

This formation explains Moon is made up of lighter elements and contains too little iron where as Mercury was formed with the rest of the planets and just like earth, its core is formed first which is of iron and then collected the lighter elements to form crust and mantle.

Though both Moon and Mercury were the result of giant impacts yet they have different elements because mercury was made earlier and its is closer to moon hence it collected heavy elements such as iron to for the formation where as moon is the result of collision of Earth and other Mars - sized body hence its constituent particles are different that of mercury.