With the help of Apollo’s cache of lunar samples and data, researchers were introduced to tantalizing new clues and constraints for these three models. For instance, measuring the age of the oldest Apollo samples showed that the Moon must have formed some 4.5 billion years ago, only 60 million years or so after the first grains in our solar system condensed. This means the Moon came to be during the same early epoch that saw the birth of the planets.

From remote measurements of the Moon’s mass and radius, researchers also know its density is anomalously low, indicating it lacks iron. While about 30 percent of Earth's mass is trapped in its iron-rich core, the core of the Moon only accounts for a few percent of its total mass. Despite this substantial difference in iron, Apollo samples later revealed that mantle rocks from the Moon and Earth have remarkably similar concentrations of oxygen. And because these lunar and terrestrial rocks are significantly different than meteorites coming from Mars or the asteroid belt, it shows the Moon and Earth's mantle share a past connection. Additionally, compared with Earth, lunar rocks were also discovered to be more depleted in so-called volatile elements — those that vaporize easily upon heating — a hint that the Moon formed at high-temperatures.

Finally, researchers know that tidal interactions forced the Moon to spiral outward over time, which in turn caused Earth to spin more slowly. This implies the Moon first formed much closer to Earth than it is now. Precise measurements of the Moon’s position using surface reflectors placed during the Apollo program subsequently confirmed this, verifying the Moon's orbit expands by about an inch each year.

Giant Impact Hypothesis

As is not uncommon in science, the new Apollo data, which was originally intended to test existing theories, instead inspired a new one. In the mid 1970s, researchers proposed the Giant Impact Hypothesis. The new impact scenario envisioned that at the end of its formation, Earth collided with another planet-sized body. This produced a great deal of debris in Earth's orbit, which in turn coalesced into the Moon. The impacting planet would later be named “Theia,” after the Greek goddess who was the mother of the Moon.

The new theory seemed to reconcile multiple lines of evidence. If the material that formed the Moon originated from the outer layers of Earth and Theia, rather than from their cores, an iron-poor Moon would naturally result. A giant impact that struck Earth obliquely could also account for Earth’s rapid initial spin. Finally, the enormous impact energy associated with such an event would vaporize a substantial portion of the ejecta, accounting for the Moon's lack of volatile materials.