Moon Origins Topic of Lively and Continuing Debate [Nature]

Although there is wide consensus on the theory that the moon formed through the collision of Earth and another object, the specifics of that theory are still not certain. And two new papers published in Science present two new significantly different takes on those specifics.

Credit: NASA
Credit: NASA

By putting different spins on the old impact model, two teams of researchers now report that they have solved the chemical conundrum and upheld the lunar origin theory. But they predict very different sizes for Theia: one smaller than Mars, and one four to five times bigger. “The two papers show that a wide variety of impact scenarios are possible,” says Sarah Stewart of Harvard University in Cambridge, Massachusetts, a co-author on one of the papers, both of which are published today in Science.

Both teams assume that Earth once rotated about twice as fast as it does now, because of the Moon-forming collision or previous impacts. Although a fast-spinning Earth is natural consequence of impacts, researchers had studiously avoided such a possibility because no one had figured out a way to slow Earth back down to its present-day rate of rotation. Stewart and her co-author Matija Ćuk, of the SETI Institute in Mountain View, California, show that a known but often-overlooked gravitational interaction between the Moon and the Sun could have drained the spin of a rapidly rotating Earth.

In the other study, Robin Canup of the Southwest Research Institute in Boulder, Colorado, one of the originators of the Mars-sized impact model, now considers a collision in which the impactor and the fledgling Earth had identical masses, about half of Earth’s current heft. When two equal-mass bodies collide, they would mix so thoroughly that the isotopic composition of the debris would be indistinguishable from that of either body. The impact would also have revved up Earth’s spin, and Canup invokes the same gravitational mechanism invoked by Stewart and Ćuk to slow it down.

You can read more about these new takes, as well as others currently being research in my Nature article Moon-forming impact theory rescued.