**The lack of iron, provided the impact of the planetesimal happened relatively late in the earth's formation and after the Earth's iron had sunk towards its core. In that manner the Moon could pick up mantle from the Earth that has less iron. Most of the planetesimal itself would have sunk into the earth's core. | **The lack of iron, provided the impact of the planetesimal happened relatively late in the earth's formation and after the Earth's iron had sunk towards its core. In that manner the Moon could pick up mantle from the Earth that has less iron. Most of the planetesimal itself would have sunk into the earth's core. |