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When Apollo 11 touched down on the lunar surface on 20 July 1969, it marked a turning point in humanity's relationship with the Moon.
We all know the monuments – the American flag. The crisply delineated bootprints. The bags of human waste left behind in the gunmetal-gray dust.
What we tend to forget are the boxes of rocks.
Between 1969 and 1972, the Apollo program hauled 382 kilograms (842 pounds) of lunar rocks and dust back to Earth.
Those materials answered decades-old questions, settled debates, and opened new avenues for research. To this day, the Apollo samples continue to shape our understanding of the Moon.
"Rather than being certain about anything, scientists were discussing different possibilities for the origin and geologic evolution of the Moon prior to the Apollo 11 mission," geologist David Kring of the Lunar and Planetary Institute told ScienceAlert.
Some of those possibilities included whether the Moon was a primitive body that had remained relatively unchanged since the early Solar System; whether its craters were blasted out by volcanoes or gouged by impacts; and whether its geological activity was ancient or ongoing.
The Apollo samples answered those questions straight away. They revealed a world with a core that had once harbored a global magma ocean, confirmed that impact cratering dominates the lunar surface, and helped establish the Moon's age at around 4.5 billion years.
"It is often said that lunar samples are the gift that keeps on giving." Geologist David Kring, LPI
They also paved the way for the Giant Impact Hypothesis, the leading explanation for the Moon's origin. It proposes that, rather than forming separately, the Moon was born from Earth itself after a colossal collision blasted molten debris into orbit around the newborn planet.
This completely recontextualizes Earth's relationship with the Moon. With the Giant Impact Hypothesis, the Moon was no longer just an object that happened to be hanging out with Earth – it had become a relic of Earth's earliest history.
"We go to the Moon to better understand its evolution and the geologic processes that continue to shape it today, but – in doing so – we are also learning more about our own home, Earth," Kring said.
Earth's early history is notoriously difficult to probe. Plate tectonics, volcanism, wind and water have relentlessly reshaped the planet's surface, erasing and concealing much of its earliest history.
The Moon is a different story.
Every impact, every lava flow, remains etched on the surface. Scars from later flows and impacts accumulate, creating a complex, layered archive of lunar history dating back billions of years – and it was all just sitting there on the surface, waiting for astronauts to bring it home.
"Truly transformative science lurks in those samples," Kring said.
Among the more groundbreaking insights derived from Apollo samples is the idea that the Moon may preserve evidence of the asteroid bombardment that assailed Earth around 4 billion years ago.
"I developed that hypothesis while analyzing lunar samples to determine the magnitude and duration of impact bombardment that resurfaced the Moon billions of years ago," Kring explained.
"Those same impact events, occurring even more frequently on Earth, vaporized Earth's seas, making surface conditions untenable for life. Yet the impact events also created vast, subsurface hydrothermal systems that were perfect crucibles for the origin and early evolution of life."
First proposed in 2000, the hypothesis has become an integral part of how scientists interpret impact features on Earth's surface – yet the clue came from a box of dull gray rocks from hundreds of thousands of kilometers from Earth.
"We continue to extract information from Apollo samples because analytical technology is constantly evolving and because our ideas, based on those results, continue to evolve," Kring said. "It is often said that lunar samples are the gift that keeps on giving."
Which is one reason scientists yearn to go back.
Science was not the primary motivation of the Apollo program. The race to the Moon was driven by Cold War rivalry between the United States and the Soviet Union. The program ended in 1972 after just six successful crewed landings, and humans have not set foot on the Moon since.
Yet, in those six missions, it completely reframed our understanding of the Moon, Earth, Earth's formation history, and the origins of life itself.
If we could learn all of that from a handful of missions more than 50 years ago, imagine what we could achieve with new equipment, new techniques, and the shoulders of Apollo to stand on.
"We need to clarify some types of geologic processes with samples recovered elsewhere on the Moon. Artemis will provide that opportunity. Once we understand those samples, we can return to the Apollo samples and begin to extract even more information from them," Kring explained.
"Exploration science is often an iterative process. That will certainly be true as results from Artemis are used to further enhance the value of the Apollo collection."
Related: We Will Never Get Tired of This Video of Astronauts Falling Over on The Moon
It's easy to think of Apollo's legacy as a planted flag, a message recorded in crackling audio, and a set of bootprints.
But the greatest legacy of Apollo may be far less glamorous: a few hundred kilograms of unassuming gray rocks that scientists are still learning from more than half a century later.
"[Apollo] inspired many to become scientists and engineers. But when I give public lectures, people have come up to me and say that Apollo inspired them, too, to pursue their dreams. I heard that from a neuroscientist. I heard that from a plumber. I heard that from people of many professions," Kring said.
"I hope Artemis inspires a new generation."
This article was fact-checked by Clare Watson and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
