It's A Hard Knock Life For Lunar Rocks

"It wouldn't look very good if we went to the moon and didn't have something to do when we got there- Max Faget


Over the past two and a half years, I've been dedicating a full week of posts on the Astronomical Returns Instagram to each of the Apollo missions as they hit their 50-year anniversaries. Since this past week was Apollo 15's 50th birthday, lunar geology instantly came to mind! 

All of the Apollo lunar landings were of course enormous technological achievements, but the more I've learned about them, the more it surprises me how much of the heavier geological science was only done on the later missions. Each of the crewed Apollo missions was given a letter designation to specify the nature of the mission: Apollo 11 was a G-type mission, meaning it's primary goal was simply to get boots on the moon, and not much else. Indeed, Neil and Buzz only spent two and a half hours on the surface, barely enough time to scoop some rocks, deploy the flag, chat with Nixon, and head back inside. Apollos 12-14 were H type missions, meaning their mission objectives were more ambitious and involved more precise landings and scientific instruments. Yet even with 2 EVAs, without the Lunar Rover they could only explore as far as they could walk and could only carry back 30-40 kgs of moon rocks. The task of really diving into the geology of the moon would fall on Apollo 15, the first of the J missions

Apollo 15 Lunar Module Pilot Jim Irwin with the LRV in the mountains of Hadley-Apennine

The Apollo astronauts were largely pilots, not scientists; training time leading up to a mission was a precious commodity that needed to be optimized, and most of them preferred to focus on flight objectives rather than learning about rocks. To bolster interest, NASA brought on geologists Lee Silver and Farouk El-Baz, and lucky for them Commander David Scott quickly took to the geology lessons and wanted to make sure he and crew members Jim Irwin and Al Worden gathered as much scientific data as possible. Fortunately, all three had been the backup crew of Apollo 12, so they were already familiar with the CSM/LM and could focus more of their training on science. Soon, the astronauts were making regular visits to Arizona and New Mexico, where Silver would accompany Scott and Irwin on long expeditions training them how to identify various rocks and minerals, while Worden would fly overhead in an airplane learning how to call out geographic features from above. Worden also spent significant time with El-Baz, studying lunar maps and practicing how he'd describe features on the moon while in lunar orbit to relay the most accurate information possible to geologists back on Earth. And after NASA had narrowed down the final landing site to two choices, Hadley-Apennine or Marius Crater, Scott pushed heavily for Hadley-Apennine due to its greater variety of geological features, and ultimately got his way

Irwin (left) and Scott (right) examining a rock formation at Cococino Point, Arizona

Once on the moon, Scott and Irwin got the chance to flex their newfound geology knowledge. In general there are two broad types of moon rocks: basalt, an igneous rock formed from the lava flow of ancient lunar volcanos, and breccia, a sedimentary rock formed from the remnants of smashed meteors and other minerals that got glassed and fused together (both basalt and breccia exist on Earth too). But beyond that, there are tons of subtleties in the regolith that reveal the history of the moon. From looking at just a single grain of sand, rounded edges would suggest a past where water flowed freely and moved the sediment around. With no liquid water to speak of, and no atmosphere to blow the dust around, the particles the moon remain jagged and hazardous to spacesuits and machinery. Furthermore, the surface of the moon varies widely in its age. The lowlands visited by the previous Apollo missions were relatively younger at 3 billions years old, and there are still yet-unexplored regions that may be as young as 1 billion years old. But Apollo 15 was looking for the other end of the spectrum in the highlands of Hadley-Apennine: a specific mineral called anorthosite, thought to be over 4 billion years old and come from the primordial lunar crust. When Scott and Irwin found their precious anorthosite sample, later dubbed the Genesis Rock, you can literally hear the excitement in their voices:

Scott: "Guess what we just found? Guess what we just found! I think we found what we came here for!... I think we might have ourselves something close to anorthosite, because it's all crystalline. What a beaut!"

Irwin: "That is really a beauty"

Basalt (left) and breccia (right), both from the moon. Contrast the basalt's porous structure, often seen in igneous rocks, with the breccia's speckled hodgepodge of minerals often seen in sedimentary rocks

So what was the big deal about anorthosite anyway? Well stepping back for a moment - even as recently as the mid-20th century, before the Apollo moon landings, there was still a ton of uncertainty about the origins of the moon. Some scientists speculated that perhaps the moon was some wandering body that got captured by Earth's gravity. But others believed in the Giant Impact Hypothesis, which suggests that 4.5 billion years ago, the young Earth collided with a Mars-sized planet (dubbed Theia) which blasted a ton of material into Earth's orbit that gradually coalesced into our nearest cosmic neighbor

For the highlands on the moon to be covered in anorthosite, there had to have been a time in the moon's history where its entire surface was completely molten. That way, this light, white igneous rock would float to the surface while denser materials would sink into the magma and down to the core. If the moon had indeed been formed out of a violent, planetary-scale collision, that would explain how it remained molten for much of its early history, and how all this anorthosite came to be. So Apollo 15's Genesis Rock provided crucial evidence for the Giant Impact Hypothesis and developed our modern understanding of the moon's origins

Other evidence for the Giant Impact Hypothesis: the moon's core is proportionally really small, relative to other solar system bodies. During the Earth-Theia collision, Earth would've absorbed most of Theia's core, leaving little leftover for the moon

Lastly, one other interesting fact about the lunar samples returned from the Apollo missions is that the vast majority of them remain untouched. While over the years NASA has carefully doled out samples to labs across the country and around the world, about 80% of humanity's moon rocks are still locked away in airtight vaults meant to preserve them for as long as possible, so that future scientists with new questions and more advanced technology will still have samples to work with. As much as I commend NASA for their foresight, part of me is sad they were right in predicting that 50 years after Apollo, moon rocks would still be an incredibly precious scientific commodity. Maybe in another 50 years, once the Artemis Program establishes a moon base and commercial lunar expeditions are a regular occurrence, lunar soil will finally be cheap as dirt

Lunar samples still in their storage vaults at Johnson Space Center

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