The Interior Structure of the Moon: What Does Geophysics Have to Say?

Samples of the Moon obtained from the Apollo and Luna missions, as well as from lunar meteorites found haphazardly on Earth, are a testament to the violent origin and early evolution of the Earth–Moon system. The prevailing paradigm for the origin of the Moon derived from these samples posits that a Mars-sized object collided with the proto-Earth about 4.5 billion years ago, placing in Earth orbit a large quantity of material that eventually accreted to form the Moon (Canup 2004). As a result of the huge amount of energy that would have been liberated in such an event, a significant portion of the Moon is thought to have been initially molten (Pritchard and Stevenson 2000). Fractional crystallization of this “magma ocean” would most likely have led to the formation of an anorthositic flotation crust and to the partitioning of a large portion of the Moon’s heat-producing elements into the last magmas that crystallized between the crust and mantle.