Some initial working models were found, upon analysis to... well, not work. One early idea was that what is now the Moon sheared away from the Earth while it was molten because of centrifugal force, but the viscosity of molten rock is too high (or the rotational speed of the Earth is way too low) for that to be feasible. Another possibility was the gravitational capture of a pre-formed body, but that makes it hard to explain the Moon's nearly perfect circular orbit. (Captured objects -- a likely candidate is Neptune's moon Nereid -- tend to have highly elliptical orbits and/or orbits not parallel to their host planet's rotation, because there's no reason to suppose that their capture occurred at any particular angle.)
A big clue came from isotopic analysis of lunar rocks, which found that the ratios of isotopes for several different elements were nearly identical to terrestrial rocks, arguing for a common source. The prevailing theory is that the Moon formed when, about 4.5 billion years ago, the proto-Earth was struck by a Mars-sized planet -- named Theia, after the Greek Titan who was the mother of Selene, the goddess of the Moon -- which caused a blob of material to shear away, propelling it into orbit where it coalesced into what we see today on a clear night.
Artist's depiction of the collision between Theia and the proto-Earth [Image is in the Public Domain courtesy of NASA/JPL]
The reason the topic comes up is because of a paper that appeared this week in Nature that I found out about from a friend and loyal reader of Skeptophilia. A team led by geophysicist Qian Yuan of Arizona State University took a look at two large low-velocity provinces (LLVPs) in the Earth's lower mantle -- dense regions where seismic waves slow down, and which are hypothesized to have a significantly higher iron oxide content than the rest of the mantle -- and their models support the astonishing idea that these are the remnants of Theia.
It's wild that there are still relics discernible, between the violence of the collision and the fact that 4.5 billion years have passed since it happened. You'd think this would be plenty enough time to stir the mantle and homogenize the material Theia brought in with whatever was present in the proto-Earth. But Yuan et al. think that the collision's energy was mostly dissipated into the upper mantle, allowing the remnants of Theia's core to sink into the lower mantle without mixing completely -- where the pieces are still detectable today.
Like all good science, the Yuan et al. paper raises some interesting questions, such as what effect the collision had on the rest of Earth's evolution. "A logical consequence of the idea that the LLVPs are remnants of Theia is that they are very ancient," said Paul Asimow, of the California Institute of Technology and senior author of the paper, in an interview with Science Daily. "It makes sense, therefore, to investigate next what consequences they had for Earth's earliest evolution, such as the onset of subduction before conditions were suitable for modern-style plate tectonics, the formation of the first continents, and the origin of the very oldest surviving terrestrial minerals."
So that's today's cool scientific research, which I can say without fear of contradiction is pretty close to earthshattering. Think about that next time you see our companion's ghostly white light in the night sky -- that despite its tranquil appearance, it may well have been born from a collision of almost unimaginable violence, billions of years ago.