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.
image via (cc) flickr user markgregory
That wrench in the lunar-theory machine helped lend credence to a number of other, more isotopically plausible scenarios: Perhaps the moon was once part of Earth entirely, and was flung into orbit as our planet spun rapidly during its earliest state. Or perhaps our planet simply “snagged” a roving chunk of rock making its way through our solar system, locking it into a stable orbit around the Earth. To date, no single explanation has perfectly accounted for why our moon is the way it is, and how it got there. But now new research seems to indicate that the once questionable collision theory is, in fact, not only significantly more plausible than previously believed, but entirely likely.
In a paper published recently by Nature, Alessandra Mastrobuono-Battisti and Hagai Perets of the Technion-Israel Institute of Technology, along with Sean Raymond of the Laboratory of Astrophysics of Bordeaux, France describe a new series of simulations designed to explore how collisions between celestial bodies may have “fed” the creation of larger planets. They write:
We find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are statistically more similar to the planets they impact. A large fraction of planet–impactor pairs have almost identical compositions. Thus, the similarity in composition between the Earth and Moon could be a natural consequence of a late giant impact.
In other words, during our solar system’s early demolition derby days, odds are significantly higher than previously thought that we could end up with a collision which would result in the moon and the Earth being so chemically similar. How much higher? According to the researchers, anywhere from twenty to forty percent of the planet-impactor pairs met the criteria. Compare that with a 2007 Harvard simulation which put the number of objects hitting our planet with a similar isotopic makeup as our own at a measly one percent.
Which isn’t to say the collision-theory for the origin of our moon has been definitively proven. As Southwest Research Institute planetary scientist Robin Canup told sciencemag.org: “This is a very important piece of the puzzle.” Still, for a theory that had fallen out of favor with the scientific community, the team’s research offers a very significant boost. And as NASA readies itself for more missions to explore potentially life-sustaining moons like Jupiter’s Europa, steps toward understanding not only what we might find, but why we’re finding it, become all the more important.