By Erin Weeks
About 65 million years ago, an asteroid the size of Manhattan collided with the Earth, resulting in the extinction of 75% of the planet’s species, including the dinosaurs.
Now imagine an impact eight orders of magnitude more powerful — that’s the shot most scientists believe formed the moon.
One of the leading researchers of the giant impact theory of the moon’s origin is Robin Canup, associate vice president of the Planetary Science Directorate at the Southwest Research Institute. Canup was elected to the National Academy of Sciences in 2012, and she’s also a graduate of Duke University — where she returned yesterday to give the fifth Hertha Sponer Lecture, named for the physicist and first woman awarded a full professorship in science at Duke.
According to the giant impact hypothesis, another planet-sized object crashed into Earth shortly after its formation 4.5 billion years ago. The catastrophic impact sent an eruption of dust and vaporized rock into space, which coalesced into a disk of material rotating around Earth’s smoldering remains (see a very cool video of one model here). Over time, that wreckage accreted into larger and larger “planetesimals,” eventually forming our moon.
Scientists favor this scenario, Canup said, because it answers a number of questions about our planet’s unusual lunar companion.
For instance, our moon has a depleted iron core, with 10% instead of the usual 30% iron composition. Canup’s models have shown the earth may have sucked up the molten core of the colliding object, leaving the dust cloud from which the moon originated with very little iron in it.
Another mystery is the identical isotopic signature of the moon and the earth’s mantle, which could be explained if the two original bodies mixed, forming a hybrid isotopic composition from the collision.
Canup’s models of the moon’s formation help us understand the evolution of just one (albeit important) cosmic configuration in our galaxy. As for the rest out there, she says scientists are just beginning to plump the depths of how they came to be. Already, the models show “they’re even crazier than the theoreticians imagined.”