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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