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.
Meanwhile, tidal forces from Earth would have been causing both moons to migrate outward. When they reached about one-third of the Moon’s present distance (a process that would take tens of millions of years), the Sun’s gravity would have become a player in their orbital dynamics.
“The Lagrange points become unstable and anything trapped there is adrift,” Asphaug says. Soon after, the two moons collided. But because they were in the same orbit, the collision was at a relatively low speed.
“It’s not a typical cratering event, where you fire a ‘bullet’ and excavate a crater much larger than the bullet,” Asphaug says. “Here, you make a crater only about one-fifth the volume of the impactor, and the impactor just kind of splats into the cavity.”
In the hours after the impact, gravity would have crushed the impactor to a relatively thin layer, pasted on top of the Moon’s existing crust. “You end up with a pancake,” Asphaug says. The impact would have pushed the still-liquid KREEP layer to the Moon’s opposite side.
The best way to test the theory would be by analyzing the mineral composition of rocks from the far side of the moon. But, as Time magazine notes, no such mission is to occur soon to get such specimens. The likes of the the Lunar Reconnaissance Orbiter, or LRO, which is currently circling the moon, can detect a sense of the materials below it. NASA’s upcoming GRAIL, for Gravity Recovery and Interior Laboratory, mission could also provide more information about the Moon’s interior, by taking highly precise measurements of its gravity. But, as with seeing photos of some far-away place like the Temple of Apollo in Delphi and actually standing amid the ruins, there’s still no substitute for actually going to, in this case, the far side of the Moon.
Other scientists have called the two moon theory “elegant” and “provocative”; Peter Schultz of Brown University indeed describes the theorizing as “great fun.” For sure, if the earth ever did have two moons, think of the upset it would have on, for instance, myths about the formation of the universe. Such cosmological myths often describe how “first there was the sun and then the moon.” In ancient Greek mythology, the sun and moon are a twin brother and sister, Apollo (also known as Phoebus, which means “shining”) and Artemis (as known as Phoebe, the female equivalent of Phoebus). Ancient Greek scientists like Aristotle believed that the heavenly bodies orbited the earth, in a hierarchical order with the moon circling the earth, then Mercury, then Venus, then the sun, then Mars and outward to the “sphere of the Prime Mover.” With two moons, you’d need to make room for an interloper. Even more, with a second “sister” moon, wouldn’t Apollo and Artemis have had to have another sibling and been triplets, with the sun god outnumbered by his two sisters? I see the moons and the moons see me…
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Photo by armaggesin