While some of the details
are still being worked out, it’s generally agreed that the Moon
formed when a Mars-sized body collided with the early Earth. Some
of the debris put into orbit by the collision would then go on to
condense into the Moon.
One of the consequences of this is that the early Moon spent a
lot of its history being bombarded by this debris, a process that
should have left its surface molten. This magma ocean would only
solidify slowly as the bombardment wound down, and the process of
solidification should have left a mark on the Moon’s composition.
So far, indications of this have been difficult to come by. But
now, there are indications that the Chang’E-4 mission to the
Moon’s far side has finally spotted some of the Moon’s mantle,
which contains signs of its magma ocean.
The end of an ocean
At first glance, the end of a magma ocean might seem simple:
molten rock solidifies, leaving behind a solid body. But different
minerals have their own melting points and densities, which can
cause the ocean to become stratified. Ultimately, it’s thought that
the densest minerals will solidify at the base of the ocean, while
the crust would be formed from lighter material that could solidify
while floating on top of the remaining magma. Thus, we’d expect to
see certain minerals on the surface and a different group of
minerals deep in the mantle.
Source: FS – All – Science – News
China’s Chang’E-4 may have landed near pieces of the Moon’s interior