Literally earth-shattering news: Once upon a very long time ago, the earth may have had two moons.
The two satellites were formed after a “Mars-size protoplanet” collided with the earth late in its formation period. Then, according to a recently published study in Nature, the two moons merged in a “slow-motion collision that took several hours to complete,” with the result a very visible difference between the two sides of the moon:
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.
Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, and Martin Jutzi, now of the University of Berne, used computer models to account for why the moon’s two sides are so different. In the course of their research, they theorized that “something ‘squished’ the late-solidifying KREEP layer to one side of the Moon [the nearside], well after the rest of the crust had solidified.” An impact, says Asphaug, is the most likely explanation.
Traditional theories have argued that “the infant Moon rapidly swept up any rivals or gravitationally ejected them into interstellar space.” The new theory rather contends that one body did survive, “parked in a gravitationally stable point in the Earth–Moon system.” After all, as Asphaug points out — and as anyone who’s been in an auto accident that’s more than a little fender-bender — a collision, by definition, only occurs on one side.
The scientists’ computer model suggests that the smaller “sister moon” was about one-thirtieth of the mass of the Moon, around 1,000 kilometres in diameter. So the smaller moon could have survived as a separate body long enough for its upper crust, and that of the (larger) Moon, to solidify, while the Moon’s inner KREEP layer remained liquid (just as — with apologies to any who find this comparison a bit mundane — while the top of a pan of brownies hardens after you take it out of the oven, the inside may still have more of the consistency of batter).The smaller moon was in what’s called a Lagrangian point, a “gravitationally stable point in the Earth-Moon” system about 60 degrees in front or behind the Moon’s orbit.
Photo by armaggesin
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