It might seem surprising that the exact origin of our moon is currently something of a mystery, but that’s the gist of a recent article in Nature:
Lunar-origin studies are in flux. No current impact model stands out as more compelling than the rest. Progress in several areas is needed to rule out some theories, support others or direct us to new ones.
We think something hit Earth 4.5 billion years ago, and the moon resulted, but after that the questions start.
It remains troubling that all of the current impact models invoke a process after the impact to effectively erase a primary outcome of the event — either by changing the disk’s composition through mixing for the canonical impact, or by changing Earth’s spin rate for the high-angular-momentum narratives.
Sequences of events do occur in nature, and yet we strive to avoid such complexity in our models. We seek the simplest possible solution, as a matter of scientific aesthetics and because simple solutions are often more probable. As the number of steps increases, the likelihood of a particular sequence decreases. Current impact models are more complex and seem less probable than the original giant-impact concept.
One solution, eagerly embraced by space buffs of course, would be a mission to Venus:
A clue may lie in Venus. The assumption that the Moon-forming impactor had a composition very different from that of Earth is largely based on what we know about Mars. We do not know the isotopic composition of Venus, the planet most similar to Earth in both mass and distance from the Sun. If Venus’s composition proves similar to that of Earth and the Moon, Mars would then seem to be an outlier, and an impactor composition akin to Earth’s would be more probable, removing many objections to the canonical impact.