The Science of Apollo

Apollo 11 broke new ground for exploration, but along the way NASA squeezed as much science as it could out of this and subsequent missions landing on the Moon.

From President John F. Kennedy’s 1961 mandate to the moment when Apollo 17 departed the Moon in 1972, Apollo was about geopolitics. Nevertheless, NASA squeezed out as much science as possible out of the program, both in space and on the Moon, and the quantity and quality of the science only increased as the missions went on.

As Apollo 11 delivered Neil Armstrong and Buzz Aldrin to the lunar surface, the mission was the culmination of Apollo's engineering phase. Still, during their moonwalk, Armstrong and Aldrin collected geological samples — scientifically very useful samples that would inform scientists on the Moon’s history and composition and help to assure NASA that the lunar surface lacked organisms that might endanger terrestrial life. Armstrong and Aldrin also set up a simplified version of an instrument suite — the Apollo Lunar Science Experiment Package (ALSEP) — that would be deployed on subsequent missions.

The crew of Apollo 11 made good headway, but once JFK’s mandate was satisfied, science would play a greater role beginning with Apollo 12, when mission commander Pete Conrad and lunar module pilot Alan Bean spent two days on the lunar surface, performing a moonwalk on each day and collecting more material than their predecessors had.

Later, starting with Apollo 15, each landing crew conducted three days of moonwalks, riding a lunar buggy to cover greater distances on exploration that was especially heavy in field geology. To pull this off, the crews received hundreds of hours of training, giving them the equivalent of geology masters degrees — except for Harrison “Jack” Schmitt of Apollo 17, who already held a Ph.D. in geology. In the midst of all of this, NASA began crashing Saturn S-IVB stages and lunar module ascent stages into the Moon, generating moonquakes whose effects on seismometers — included in the ALSEP stations — would reveal information about the Moon’s interior.

The Importance of Impacts

The founder of NASA’s astrogeology program was Eugene Shoemaker, who had risen to prominence when he showed that the Barringer Crater in Arizona was the result of an impact event, not volcanism. The finding strengthened the idea that lunar craters, basins, and other lunar features had also been carved by impact events.

By examining samples from mare (lowland) and terra (highland) regions, scientists confirmed that impacts had been the dominant force shaping the Moon’s surface, as well as the surface of all of the inner planets billions of years ago. By dating samples, scientists were also able to calibrate age estimates determined by counting craters, which enabled them to date sites from which they couldn’t collect samples — including crater-covered sites of other planets.

Apollo science encompassed a range of topics: analysis of lunar dust, the potential of lunar resources, photography and lunar mapping, the Moon’s secular acceleration that takes it gradually farther from Earth, continental drift on Earth, space medicine, and radiation biology. But the importance of impacts was a major finding that addressed many longstanding questions about the Moon. One such question, for instance, was why do maria appear darker than the terrae?

Analysis of Apollo samples confirmed that the maria were covered in basalt, indicating that lava had spread following more recent, impacts that had cracked the crust deeply enough to release magna from the mantle.

The Moon’s Origin

The Apollo studies also addressed the question of the Moon’s origin. When Armstrong, Aldrin, and Michael Collins reached the Moon in 1969, three major hypotheses had been around for nearly a century. One was the co-accretion hypothesis, which posited that the Moon and Earth had formed together in space and were therefore chemically identical. However, the lunar samples that Apollo astronauts brought back to Earth demonstrated that the Moon was relatively deficient in iron. Early analysis also showed that the Apollo samples lacked volatile compounds, such as water.

Alternatively, some scientists had suggested that Earth had captured the Moon, which had originally formed as a separate body in our planet’s vicinity. In fact, analysis of the Apollo samples shows that the ratios of oxygen isotopes on the two bodies match, arguably solid evidence that both worlds formed at roughly the same distance from the Sun. However, the Moon’s deficiency of iron and lack of volatiles argue that it could not formed on its own in Earth’s vicinity.

Ultimately, these shortcomings led to the giant impact hypothesis, first proposed in 1975. The idea is that a Mars-size planet, since dubbed Theia, impacted early Earth after both worlds had already coalesced out of the Sun’s protoplanetary disk. They had enough time to separate into crust, mantle, and core. When Theia hit Earth, the cores of both worlds gave our planet its iron. Meanwhile, the impact debris, derived mostly from the impactor’s mantle, would have heated to high temperatures, boiling off volatiles like water, which explained the apparent lack of volatiles in the Apollo samples.

But analysis of the Apollo samples didn’t end in the 1970s — some samples were reserved for later study, with technology not yet in existence. In the early 2000s, analysis of lunar samples from Apollo 15 and 17 demonstrated the surprising presence of water in tiny amounts. Scientists have since modified the giant impact hypothesis to take the presence of water, and other findings into account. But such modifications are making the giant impact hypothesis increasingly complex. A more extreme take on the Moon's formation came in 2017 with the synestia hypothesis, a scenario in which the giant impact vaporized the entire early “Earth,” which later coalesced into the Moon and our current Earth. This scenario does not explain the water in the Apollo 17 and 15 samples, but other new hypotheses are emerging. The multiple small impact hypothesis, for example, would allow for water within the nascent Moon.

In the years to come, new lunar missions will provide more information, perhaps enough to answer the age-old question of how Earth’s natural satellite came to be.