The answer to that question came only with the six landing missions on the Moon made by the Apollo astronauts between 1969 and 1972, as well as the samples returned by the Soviet Union’s robotic probes, and the exhaustive analysis by scientists of these materials returned from the lunar surface. Finding evidence about the geological origins and development of the Moon had been a central element of the analysis of these samples. The core questions included: “How old is the Moon, how was it formed, and what is its composition?”
Scientists worldwide shared data and samples from the Moon missions, comparing results and geological details. While it took more than a decade, data from these explorations led eventually to a consensus on the origins of the Moon. Indeed, if there is one dramatic moment—as opposed to myriad important but mundane events—in the history of lunar science it is the 1984 conference in Kona, Hawaii, in which scientists around the world presented papers on the sole topic of how the Moon originated.
So contentious had the question of lunar origins been prior to the American and Soviet Moon exploration efforts, that many scientists just threw up their hands in frustration at ever being able to develop a reasonable hypothesis. Confusion ruled among scientists about the Moon’s origin as competing schools battled among themselves for dominance of their particular viewpoint in the textbooks. Some even expressed concern that determining the Moon’s origins should be the single most significant scientific objective of Project Apollo, thinking of it as a hopeless objective.
Their concern was legitimate based on what had gone before. Prior to the 1960s the origin of the Moon had been a subject of considerable scientific debate and careers had risen and fallen on championing one of three principal theories:
- Co-accretion—a theory which asserted that the Moon and the Earth formed at the same time from the Solar Nebula.
- Fission—a theory that asserted that the Moon split off from the Earth.
- Capture—a theory that held that the Moon formed elsewhere and was subsequently drawn into orbit around the Earth.
The data supporting these various theories had been developed to an amazingly fine point over time but none of these theories actually explained enough open questions to convince a majority of planetary scientists.
The new and detailed information from the Moon’s explorations pointed toward an impact theory—which suggested that the Earth had collided with a very large object (perhaps as big as Mars and named after the fact “Theia”)—and that the Moon had formed from the ejected material of both Theia and the Earth.
This proved to be a theory that fit the fact that although the Earth has a large iron core the Moon does not, because the debris blown out of both the Earth and the impactor would have come from iron-depleted, rocky mantles. Also lending credence to this theory, although the Earth has a mean density of 5.5 grams/cubic centimeter the Moon’s density is only 3.3 g/cc, which would be the case were it to lack iron, as it does. The Moon has exactly the same oxygen isotope composition as the Earth, whereas Mars rocks and meteorites from other parts of the Solar System have different oxygen isotope compositions. While there were some details left unexplained by this conclusion, the impact theory came out as the scientific consensus and is now widely accepted.
This “big whack” theory, as it was called, explained well what was learned about the geology and selenogony of the Moon during the Apollo program. Historian of science Stephen Brush has commented:
According to this hypothesis, the Earth was struck by a fairly large (perhaps Mars-size) body, resulting in ejection of material that stayed in terrestrial orbit and subsequently condensed to form the Moon. At a conference in Hawaii in 1984, the giant impact hypothesis was thoroughly discussed and gained at least provisional acceptance from most of the experts present, although many details remained to be worked out.
Lunar scientist Paul D. Spudis recently concluded on this issue: “The giant-impact hypothesis appears to explain, or allow for, several fundamental relations—not just bulk composition, but also the orientation and evolution of the lunar orbit…Part of the reason for this model’s current popularity is doubtless because we know too little to rule it out: key factors such as the impactor’s composition, the collision geometry, and the Moon’s initial orbit are all undetermined.” In the end, further research is required, for “as it turned out, neither the Apollo astronauts, the Luna vehicles, not all the king’s horses and all the king’s men could assemble enough data to explain circumstances of the Moon’s birth” to everyone’s satisfaction.
Thereafter, the impact theory made its way into the textbook arena. First it was mentioned as one of several theories, but by 1990 it had gained precedent as the preferred explanation in the undergraduate curricula. In this case, much has changed in terms of scientific knowledge about the Moon because of the Apollo effort.