But what if the Moon were the product of multiple impacts, rather than just one? Recent alternative models consider the Moon formed via tens of smaller impacts with Earth, rather than a single, giant impact. In this scenario, a relatively small impact creates a “moonlet” whose orbit spirals outward. A later impact produces another moonlet, and its outward migration could cause it to merge with the first. A full-sized Moon built up by many smaller impactors with a range of compositions is more likely to end up with an Earth-like composition than a Moon produced by a single impact. However, the problem with this theory is that moonlets formed by different impacts don’t necessarily merge. Instead, it’s more likely that such moonlets would get ejected from orbit or eventually collide with Earth.
A final question is whether lunar impact simulations have considered all important aspects of a Moon-forming collision. Prior studies have generally found similar outcomes even when different conditions and computational approaches are adopted. However, new research proposes that if Earth’s mantle was molten at the time of the giant impact — due to heating from a recent prior impact — then much more terrestrial material could be ejected into space, leading to a more Earth-like disk, even for a giant impact scenario.
Where do we go from here?
Thus, we find lunar origin models at a crossroads of sorts. On one hand, many once-uncertain aspects of the Giant Impact Hypothesis have been validated. Current planet-formation models predict that giant impacts were commonplace in the inner solar system as Earth grew. Thousands of increasingly sophisticated simulations have established that many (if not most) giant impact scenarios would produce disks and moons. The Moon’s lack of iron, which is difficult to explain in competing models like intact capture, results naturally from a large impact. This is because the material that coalesced into the Moon comes from the outer mantles of the colliding bodies rather than from their iron-rich cores.
However, explaining other characteristics still poses a difficult challenge. Specifically, it’s hard to account for the ever-growing list of elemental similarities between Earth and the Moon, as revealed by lunar samples. One would expect the collision of two planets to have left some trace of their compositional differences, and yet — at least based on current data — such differences are not evident.
Researchers have proposed many new, creative explanations for how an impact (or impacts) could have produced a Moon so chemically similar to Earth. However, the new ideas impose additional constraints. Thus, the impact theory still grapples with the question it faced nearly half a century ago: Would such an event have been likely, or does it require the Moon to be the product of a very unusual event?
Making headway depends on developments across several fronts. Researchers will need to employ next-generation models to link the varied origin scenarios to predict the Moon’s properties, which will then be tested by comparing them to observations.
Fortunately, the U.S. and other space-faring countries are planning upcoming Moon missions that aim to provide crucial new constraints. For example, new lunar samples may more fully reveal the Moon’s composition at depth. Improved measurements of lunar seismic activity and heat flow may better constrain the Moon’s internal composition and initial thermal state.
Ultimately, we will continue to pursue the answer for how our Moon came to be, not only so we can understand the history of our home world, but more generally, so we can learn what our nearest cosmic neighbor can teach us about the formation and evolution of inner planets — both in our solar system and beyond.