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 computational approaches are adopted. However, a new paper proposes that if the Earth’s mantle was molten at the time of the giant impact — due to heating from a recent prior impact — it would have been more heated up more than previously predicted, 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) of such giant impacts would produce disks and moons. The Moon’s bulk 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 the Earth and 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 — for example that Theia must have had similar concentrations and flavors of both oxygen and tungsten, or that the angular momentum of the Earth-Moon system has drastically changed from its initial value. 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. It's not clear that existing models can account for all known traits of the Moon, including its volatile content and the tilt of its orbit relative to the plane of the solar system. 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, NASA and other countries are planning upcoming robotic and human Moon missions that hope to provide crucial new constraints. For example, new lunar samples may more fully reveal the Moon’s composition at depth, or 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 unravel what our nearest cosmic neighbor can tell us about the formation and evolution of inner planets — both in our solar system and beyond.