Why is the Moon less dense than the Earth?

The Moon was probably formed when an object the size of Mars collided with Earth about 4.5 billion years ago.

The material that formed the Moon was mostly from Earth’s mantle. What makes Earth denser than the Moon is its iron core (which is really big, larger than the Moon!).

The Moon has about the same density as mantle rock.

To know the answer we have to go back to what was discovered about the rocks brought back from The Moon (Luna). It was discovered that those rocks were of the same type as found in Earth’s crust but scientists were already sure that the Moon and Earth formed separately.

It took time but eventually it was concluded that, somehow, a big chunk of early Earth was thrown into space and into orbit around Earth (at much lower distance and at much faster orbital speed). It was also clear that meant the Earth had somehow survived an impact by a large object which struck the Earth just enough off-center to avoid blowing Earth apart completely.

A lot of computer power was applied to this theory and after running a number of theoretical models for that event, a model was found that matches the results completely. Now we have computer animations of the event which have appeared in documentaries about the Moon and Solar System, etc.

Apparently, because the moon is made mostly of mantle material that had already stratified in the primordial Earth and the Mars-sized impactor (Theia) that struck it to produce the moon. Most of Theia’s dense core merged with out own, whereas a chunk of its mantle and crust—along with a chunk of ours—was thrust into space. This accreted into two moons that later merged into one, leaving us with a single moon with a density similar to our upper mantle.

No molten metal core and smaller size.