An old theory gets new life as research points to a planetary collision as our Moon’s origin
Earlier this year, during a segment for the QVC home shopping network, fashion designer Isaac Mizrahi struggled to define what the moon actually is (“the moon is a planet, darling”) and faced, as a result, the wrath of the internet for not knowing—as I’m sure all of you do—that the moon is, obviously, Earth’s only natural satellite. As turns out, though, actual scientists (which Mr. Mizrahi is certainly not) have had their own troubles defining what the moon is, or more accurately—why it is the way it is.
For years, the prevailing explanation for the origin of Earth’s moon has been that at some point in our solar system’s early history an object roughly the size of Mars collided with—or more likely, careened off of—Earth. That object, the theory goes, was entirely destroyed upon impact, but its shattered remains eventually coalesced into what is now our moon. What scientists have found, though, is that on an isotopic level the moon is surprisingly similar to the Earth’s mantle. That’s problematic for the collision theory. If the moon is comprised of the remains of whatever hit us, it should display chemical traits similar to that object, rather than the Earth. For the moon to so closely resemble our planet, scientists theorized that the collision object itself would have needed an isotropic composition similar to our own—something their research indicated was extraordinarily unlikely.