At the start of a talk at the Lunar and Planetary Science conference in Houston in March, Harrison Schmitt, one of the two astronauts who walked on the moon during Apollo 17, the last lunar mission, put up a picture of Neil Armstrong.
“Let’s pay tribute to this man,” said Dr. Schmitt, the only professionally trained scientist among the Apollo astronauts. A ballroom packed with scientists erupted in exuberant applause.
“Neil turned out to be the best field geologist on the moon,” he added. “Until Apollo 17, of course. In 20 minutes or so, he collected a fantastic suite of samples.”
Before Apollo 11, even simple questions about the moon confounded scientists. For instance, how old was it, anyway?
Once they started examining the 50 pounds of rocks and soil brought back by Armstrong and Buzz Aldrin, the answer quickly became clear: very, very old.
Dr. Schmitt said that had the Apollo program stopped then, with no additional landings, including his own, those first lunar samples would have been enough to forever reshape knowledge of the solar system.
Armstrong collected two types of rocks: basalts, which are hardened pieces of lava, and breccias, which are fragments of older rocks fused together. The landing site was within a flat lava plain, which was chosen because it appeared to be a safe place to touch down, not because it looked scientifically intriguing.
Nonetheless, the basalts rewrote solar system history. The relative amounts of certain long-lived radioactive elements within the rocks gave a range of ancient ages, unchanged since they cooled and solidified out of lava between 3.6 billion and 3.9 billion years ago. That is far older than almost all of the rocks on Earth, which have been churned, compressed, melted and resolidified over the eons. In fact, the moon rocks were nearly as old as the Earth and the solar system, which formed 4.5 billion years ago.
“Right there, we knew the moon was going to be, at least in part, the record for the early history of the Earth,” Dr. Schmitt said. “That was not clearly understood before Apollo 11. But it is clearly understood afterwards and now.”
Another major discovery lay within soil that Armstrong picked up and dropped into the collection box, because it was not packed full. The soil contained bits of a rock known as anorthosite. Just as ice floats on water, anorthosite, made of the mineral plagioclase, floats on magma.
Within half a year after Apollo 11, two teams of scientists, one at the University of Chicago, the other at Harvard, independently used the presence of anorthosite to come up with what was then a radical notion: The moon, the scientists proposed, had at one point melted into a global ocean of magma.
Buoyant anorthosite would then have risen to the surface while heavier materials, like iron, would have sunk to the core. Speculation of a lunar magma ocean, in turn, led to the hypothesis that the moon formed out of the debris from a collision between Earth and a Mars-size body.
“The concept, the phrase magma ocean, didn’t exist until Apollo 11,” Dr. Schmitt said in an interview. “That’s the way science moves.”
Rocks from later Apollo missions added evidence to the theory.
Armstrong’s soil also contained hydrogen, helium, nitrogen and carbon, much of which had been deposited by the solar wind, the stream of high-speed particles continually flying outward from the sun. A light version of helium, helium-3, is of particular future interest as fuel for fusion reactors, which could generate bountiful, nearly clean energy by combining atoms.
“It told us there were going to be tremendous amounts of potential resources for use in space, and possibly even on Earth,” Dr. Schmitt said.
Another far-reaching scientific legacy of the moon rocks gathered by the Apollo astronauts is how scientists used them to calibrate a technique of using craters to determine the ages of places in the solar system.
The concept is simple. Over time, impacts of asteroids, big and small, pocked the surface of the moon and elsewhere. But a layer of ice or lava can erase the craters and reset the clock. Thus, a heavily cratered surface is older than a smooth one. But while planetary scientists could see which places were older and which were younger, they did not know exactly how old any of them were.
With the dating of the rocks taken from Apollo 11’s landing site, scientists then knew the age of that patch of the lunar surface. Rocks from the other five Apollo landings set the ages of those corresponding regions, which then correlated with the different numbers of craters in each place.
The calibrated crater counts are now used to determine ages of bodies throughout the inner solar system.
The dating record still contains a huge two-billion-year gap, from one billion years ago to three billion years ago, because all of the Apollo missions touched down on older swaths of the moon. Scientists have tried to extrapolate the ages of younger regions, but different guesses provide a wide range of age estimates.
“Which is the correct chronology?” David Draper, NASA’s deputy chief scientist, asked. “That part of the curve is unconstrained. We desperately need new samples.”
Dr. Draper is part of a team that has proposed a small robotic mission called the Inner Solar System Chronology, or Isochron, which would grab five ounces of rock from a younger, smoother part of the moon and whisk it back to Earth, where scientists would determine the age of the sample.
Future robotic explorers may one day accomplish much more on the lunar surface than Armstrong could in 1969 with his space-suited hands holding a sampling stick.
But it took the humanity in that test pilot who moonlighted as a field geologist to pause from his collecting and take in the lunar landscape.
“It has a stark beauty all its own,” Armstrong said not long after taking his first steps on the moon. “It’s like much of the high desert of the United States. It’s different, but it’s very pretty out here.”