Early Earth may have had two moons
Richard Lovett for Nature:
Earth once had two moons, which merged in a slow-motion collision that took several hours to complete, researchers propose in Nature today.
Both satellites would have formed from debris that was ejected when a Mars-size protoplanet smacked into Earth late in its formation period. Whereas traditional theory states that the infant Moon rapidly swept up any rivals or gravitationally ejected them into interstellar space, the new theory suggests that one body survived, parked in a gravitationally stable point in the Earth–Moon system.
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The Moon’s visible side is dominated by low-lying lava plains, whereas its farside is composed of highlands. But the contrast is more than skin deep. The crust on the farside is 50 kilometres thicker than that on the nearside. The nearside is also richer in potassium (K), rare-earth elements (REE) and phosphorus (P) — components collectively known as KREEP. Crust-forming models show that these would have been concentrated in the last remnants of subsurface magma to crystallize as the Moon cooled.
What this suggests, [Erik] Asphaug says, is that something ‘squished’ the late-solidifying KREEP layer to one side of the Moon, well after the rest of the crust had solidified. An impact, he believes, is the most likely explanation.
Interesting new hypothesis by Erik Asphaug from UC Santa Cruz and Martin Jutzi of the University of Berne. It’s a neat idea, and it could be true, but we still don’t know enough about the formation of the Moon to know for sure. Hopefully we’ll learn more from future lunar missions.
Oh wait!