“There is no dark side of the moon, really. Matter of fact, it’s all dark.”
—Pink Floyd, “Eclipse”
On June 25 a piece of the moon fell to Earth.
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It was a substantial piece: nearly two kilograms of rock and dust that were drilled out of the lunar crust by the Chinese Chang’e 6 lander, which had touched down on the moon’s surface a few weeks earlier. Stashed in a rocket-launched return capsule, those samples met up with the Chang’e 6 mothership in lunar orbit and then rode an Earth-return module on the 380,000-kilometer journey home, finally parachuting into Inner Mongolia for retrieval and study by eager scientists.
The feat was much the same as the lunar-sample return of the Chang’e 5 mission from 2020, save for one very important difference: this time—and for the first time ever, in fact—the material was from the moon’s far side, which is always turned away from Earth. That required extra steps, such as using dedicated satellites in lunar orbit to relay communications, but the scientific payoff could be worth the trouble. Researchers hope the minerals within these historic samples will help solve a long-standing mystery in planetary science: why the moon’s far side is so very different than its near side.
We only ever see one lunar face because the moon takes about the same amount of time to rotate once as it does to complete a single orbit of Earth. This is no coincidence; it’s linked to our planet’s strong tidal influence on our natural satellite. The result is that, more or less, we can divide the moon into two hemispheres: the side that always faces us—the near side—and the one that always points away—the far side.
Anyone who has gazed up at the moon is familiar with the near side’s most prominent features: many large and roughly circular dark patches against a backdrop of brighter terrain. Ancient astronomers dubbed these dark features “maria” (Latin for “seas”) because of their waterlike appearance from Earth. Yet they’re actually plains of basaltic volcanic minerals—solidified lava—that erupted out from beneath the surface long ago. The near side’s brighter regions are older, heavily cratered and more reflective highlands that rise from the plains.
Astronomers had long assumed the moon’s mostly unseen face was similar. But space-age reconnaissance shattered those expectations in 1959, when the Soviet Luna 3 spacecraft beamed back the very first image of the far side. Although it was grainy and fuzzy, the photograph was still clear enough to reveal a wildly different landscape. The far side was almost all rugged highland, with the near side’s sprawling maria shrunken to just a few scattered darker spots. Decades of follow-up observations only widened the shocking mismatch between the hemispheres. Gravity data from the two lunar-orbiting GRAIL (Gravity Recovery and Interior Laboratory) spacecraft, for instance, indicated that the far side’s crust was about 20 kilometers thicker on average than that of the near side.
Why were the two hemispheres so different? It’s tempting to think Earth’s tides played a role, yet nothing is ever that simple.
The cause must reach back to the moon’s birth. The current origin story, widely accepted by scientists, is called the giant impact hypothesis (also known as the big splat, which I rather like). In this scenario, a Mars-sized world—which astronomers call Theia, after the Titan who was the daughter of Gaia in Greek mythology—slammed into Earth at a grazing angle shortly after our planet’s formation 4.6 billion years ago. The immense release of energy tore Theia apart, sinking its core down into Earth’s depths and blasting its outer layers—plus lots of terrestrial material—into orbit.
All that superheated rock cooled rapidly in space and became the moon. Scientists still argue over the details, but our satellite may have coalesced from the debris in as little as a few months to a few years! At that time, the moon was much closer to Earth than today, perhaps only about one tenth the current separation—across the eons, tidal forces gradually pushed it farther out.
At that close distance, the tidal interaction between the two was fierce, and the newly created moon’s rotation may have become tidally locked in as little as a year. This is far faster than the crust could have formed and solidified, which means that the hemisphere dichotomy couldn’t be caused by Earth’s tidal pull. Something else must have happened to thicken the far side as the crust initially cooled.
Scientists tossed around a few ideas; none proved to be a perfect fits for the dichotomy we see today, however. Perhaps a second, smaller moon formed from the big splat’s debris as well, and it eventually collided with the bigger moon at slow speed and draped the far side with more material. Maybe physical processes churning inside the newborn moon allowed one side to get thicker.
Then, in 2014, a team of astronomers published a new, arguably better explanation. Their culprit was not our planet’s tides but Earth itself.
In all the models investigating the young moon’s evolution, none had completely taken into account that Earth loomed overwhelmingly large in the lunar sky. Shortly after the moon formed, Earth was so close that it would’ve taken up a staggering 40 degrees of angular diameter in the sky as seen from the moon, covering a celestial area 20 times larger than it does today.
Remember, at that time, Earth was hot. Theia’s impact would have vaporized much of our planet’s surface and melted the rest. For many centuries, if not millennia, the heat from rock boiling at about 2,000 degrees Celsius would’ve cooked the nascent moon’s near side, raising it to about 1,000 degrees C, while the far side would’ve cooled to frigid temperatures.
This has profound implications. Early on, when the moon was still entirely molten, it would’ve had a churning atmosphere of incandescent rock and metal. Hardy elements such as calcium and aluminum have a very high boiling point and would’ve stayed as gas on the moon’s roasting near side, whereas they would’ve easily condensed on the cooler far side. Those windswept elements would have interacted with others on the far side, forming relatively lightweight minerals such as feldspar, which would have floated on the surface to gradually create that side’s thicker crust.
Interestingly, orbital surveys do show more feldspar on the far side than on the near side. Also, this process could have concentrated other minerals on the near side, including radioactive ones that would have acted to heat the crust there and allowed magma to eventually burst forth. This would have flooded much of the near side’s surface to make the dark maria while leaving the far side mostly maria-free.
So does this solve the mysterious lunar dichotomy? It’s the best idea going right now, but of course more evidence is needed to support it. The far side material brought down from Chang’e 6 may provide some clues once scientists perform detailed chemical analyses.
It’s funny: sometimes people call the lunar far side its “dark side,” but this is a misconception. It gets just as much illumination as the near side in the two-week stretches of sunlight that make up half of each full lunar day. In that sense, the assertion in Pink Floyd’s song was right. This phrase can perhaps be redeemed by a more poetic interpretation: the “dark” side is the one we’re more in the dark about. We don’t know it as well because it’s unviewable from Earth and still mostly unexplored.
But that’s not as true as it once was. Soon we should have answers to questions we’ve asked since the start of the space age. We’ve mapped the moon’s far side, and now we even have samples of it to study. Flashes of insight are on the horizon, and a new era of discovery is dawning. Our understanding of the far side won’t stay dark forever.