Once a leading theory of the origin of the Earth-Moon system, the Capture Theory possessed the virtues of simplicity and intuitive plausibility. The numerous instances of moons with retrograde orbits supported it. The lunar orbit’s three moments of inertia were consistent with a past very eccentric orbit, which fit a capture1. However, the Moon would have had to come from a different part of the solar system to account for its very depleted iron compared to the Earth’s iron, which meant that it would approach the Earth at a high velocity that would prevent capture. Researchers searched in vain for a braking mechanism that would slow it down so it could be captured. Still, the accumulated evidence and arguments made the Capture Theory a viable one.
But around 1980, the Giant Impact Theory came to the fore. It held that Theia, a Mars-sized planet, struck the Earth, with the debris from the collision accreting to constitute the Moon. Giant Impact would account for the nearly identical oxygen isotope ratios of the Earth and Moon. Because it lent itself to modeling, researchers were able to adjust the parameters repeatedly in response to objections and new evidence. For instance, instead of a head-on impact, they posited an oblique blow that would leave the Earth largely intact while adding angular momentum. Over time, the Giant Impact Theory thus gained credibility and overcame doubts. It emerged as the standard theory and eventually became an almost completely dominant one. It took on various permutations–for instance, multiple lesser impacts; but debate over them took place within the framework of a Giant Impact, with Capture Theory ignored. Journals practically stopped publishing on the Capture Theory.
In particular, the oxygen isotope ratios from the Apollo 17 deep core samples matched those of Earth’s basalt so closely that they must, it was thought, have come from Earth, presumably via a Giant Impact. Searches of meteorites found not a single instance of the nearly identical ratios. These considerations led the great majority of scientists to accept the Giant Impact Theory and reject the Capture Theory.
A Revised Theory
Still, one could ask: If the Capture Theory was wrong, why were some of the arguments and evidence that supported it so reasonable and hard to refute? Also, the sole highly persuasive evidence of the Giant Impact Theory was the oxygen isotope ratios. Did Giant Impact belong to the category addressed by the observation of Arthur Conan Doyle: “when a fact appears contrary to a long train of deductions, it invariably proves to be capable of bearing some other interpretation”? Here, instead of a long train of deductions, were an array of evidence and a number of arguments.
The Capture Theory needed a major reformulation to make it competitive again. So it became part of other theorizing in which the Moon and Mercury were originally parts of a combined planet–Merculuna–that was pulled by Jupiter’s gravity from the outer solar
system. As Merculuna passed Jupiter, it heated up by tidal friction; and Jupiter extracted the Moon from its north polar region (leaving the blue depression in the image). That would account for the excess iron in Mercury and the small amount of iron in the Moon. As a result, the Moon would no longer have to form far from the Earth but could form in exactly the same original location.
Sequential Formation
According to the concept of Sequential Formation, the Earth began as part of the planet Terramars in a special niche in the outer solar system. Then, shortly before 4.5 BYA, Terramars was pulled inward by Jupiter’s gravity, turned molten from tidal heating, and separated into the Earth and Mars, which Jupiter’s gravity tore from the Pacific Basin. Immediately, the same materials and processes began to form the planet Merculuna in exactly the same niche. After 3.9 BYA, Merculuna was pulled inward by Jupiter’s gravity along the same trajectory as Terramars, separating into Mercury and the Moon as it passed Jupiter. The smaller mass of Merculuna compared to Terramars, even though the time available for its formation was far longer than that for Terramars, suggests that supplies of the materials to compose it might have come to an end early on. Since the Moon is estimated to have formed around 4.51 BYA,2 both Terramars and Merculuna seem to have formed swiftly, with Terramars promptly pulled inward from the niche while a fully formed Merculuna lingered there for hundreds of millions of years until it, too, was pulled inward. Therefore, only the Earth, Mars, Mercury, and the Moon shared such closely matching oxygen isotope ratios. Rapid formation periods for Terramars and Merculuna would keep the niche’s conditions more stable and thus would help ensure nearly identical oxygen isotope ratios between the Earth and the Moon.
As the Earth moved past Jupiter into the inner solar system, it was braked by Jupiter’s gravity before escaping. The same braking occurred with the Moon as it followed the Earth’s trajectory 500 million years later. So it ended in roughly the same orbit and with roughly the same velocity as the Earth, leading to capture. In other words, the origin of the Earth-Moon system was a three-step process: first Sequential Formation, second Separation, and third Capture.
Sequential Formation explains why the oxygen isotope ratios of the Earth and Moon are nearly identical, just as the Giant Impact Theory does. But a Revised Capture Theory decisively outperforms the Giant Impact Theory in terms of matching the special features of the Moon: its heavily cratered surface, the near side/far side dichotomy (including surface features and crustal depths), the complementary low iron of the Moon and high iron of Mercury, the identical very low albedos of Mercury and the Moon, the lunar magnetic field, the remarkable properties of Oceanus Procellarum (the rectangular separation point between Mercury and the Moon 
in the image), lunar melt inclusions, the displacement of the center of mass of the Moon, thermal layering, and an orbit that reveals high past eccentricity. These features are just what we should expect of a Moon extracted from the north polar region of Merculuna. They neatly complement the features of Mercury.
Accreted debris from a Giant Impact fails to explain these features.
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Kenneth J. Dillon is a historical and scientific researcher. See his biosketch at About Us. See also an explanation of the planet Terramars and its fate. The origins of the Earth, Mars, Mercury, and the Moon play a role in Dillon’s novel of discovery science Rosemarie.
