New research attempting to explain why the Earth but not the Moon has so much iron splattered through its mantle has found that iron can be more easily vaporized during impacts than previously thought, and thus rained down on the planet during the early asteroid bombardment.
Principal investigator Kraus said, “Because planetary scientists always thought it was difficult to vaporize iron, they never thought of vaporization as an important process during the formation of the Earth and its core. But with our experiments, we showed that it’s very easy to impact-vaporize iron.” He continued, “This changes the way we think of planet formation, in that instead of core formation occurring by iron sinking down to the growing Earth’s core in large blobs (technically called diapirs), that iron was vaporized, spread out in a plume over the surface of the Earth and rained out as small droplets. The small iron droplets mixed easily with the mantle, which changes our interpretation of the geochemical data we use to date the timing of Earth’s core formation.”
The Moon’s gravity in turn wasn’t sufficient to pull its own iron vapor down. Thus, it does not have much iron in its mantle.