On December 13, 1972, Apollo 17 astronaut Harrison Schmitt walked up to a boulder in the moon’s Sea of Serenity. “This boulder’s got its own little track, right up the hill,” he called to his commander, Eugene Cernan, pointing out the mark the boulder left when it rolled down a mountainside. Cernan bounded over to collect some samples.
“I’d rather not think about it,” Schmitt said.
The astronauts chiseled bits of the moon from the boulder. Then, using a rake, Schmitt scraped the powdery surface, lifting a rock later named troctolite 76536 off the regolith and into history.
That rock, and its boulder brethren, would go on to tell a story of how the entire moon came to be. In this creation tale, inscribed in countless textbooks and science-museum exhibits over the past four decades, the moon was forged in a calamitous collision between an embryonic Earth and a rocky world the size of Mars. This other world was named Theia, for the Greek goddess who gave birth to Selene, the moon. Theia clobbered Earth so hard and so fast that the worlds both melted. Eventually, leftover debris from Theia cooled and solidified into the silvery companion we have today.
But modern measurements of troctolite 76536, and other rocks from the moon and Mars, have cast doubt on this story. In the past five years, a bombardment of studies has exposed a problem: The canonical giant-impact hypothesis rests on assumptions that do not match the evidence. If Theia hit Earth and later formed the moon, the moon should be made of Theia-type material. But the moon does not look like Theia—or like Mars, for that matter. Down to its atoms, it looks almost exactly like Earth.
Confronted with this discrepancy, lunar researchers have sought new ideas for understanding how the moon came to be. The most obvious solution may also be the simplest, though it creates other challenges with understanding the early solar system: Perhaps Theia did form the moon, but Theia was made of material that was almost identical to Earth. The second possibility is that the impact process thoroughly mixed everything, homogenizing disparate clumps and liquids the way pancake batter comes together. This could have taken place in an extraordinarily high-energy impact, or a series of impacts that produced a series of moons that later combined. The third explanation challenges what we know about planets. It’s possible that the Earth and moon we have today underwent strange metamorphoses and wild orbital dances that dramatically changed their rotations and their futures.
To understand what may have happened on Earth’s most momentous day, it helps to understand the solar system’s youth. Four and a half billion years ago, the sun was surrounded by a hot, doughnut-shaped cloud of debris. Star-forged elements swirled around our newborn sun, cooling and, after eons, combining—in a process we don’t fully understand—into clumps, then planetesimals, then increasingly larger planets. These rocky bodies violently, frequently collided and vaporized one another anew. It was in this unspeakably brutal, billiard-ball hellscape that the Earth and the moon were forged.
To get to the moon we have now, with its size, spin, and the rate at which it is receding from Earth, our best computer models say that whatever collided with Earth must have been the size of Mars. Anything bigger or much smaller would produce a system with a much greater angular momentum than we see. A bigger projectile would also throw too much iron into Earth’s orbit, creating a more iron-rich moon than the one we have today.
Angular momentum is a quantity that describes the movement and mass of a rotating object or a system of rotating objects: the spinning Earth, the spinning moon revolving around the spinning Earth, and so forth. Angular momentum is always conserved, meaning it can only be gained or lost if something else gets involved.
Early geochemical studies of troctolite 76536 and other rocks bolstered this story. They showed that lunar rocks would have originated in a lunar magma ocean, the likes of which could only be generated by a giant impact. The troctolite would have bobbed in a molten sea like an iceberg floating off Antarctica. On the basis of these physical constraints, scientists have argued that the moon was made from the remnants of Theia. But there is a problem.
Back to the early solar system. As rocky worlds collided and vaporized, their contents mixed, eventually settling into distinct regions. Closer to the sun, where it was hotter, lighter elements would be likelier to heat up and escape, leaving an excess of heavy isotopes (variants of elements with additional neutrons). Farther from the sun, rocks were able to keep more of their water, and lighter isotopes persisted. Because of this, a scientist can examine an object’s isotopic mix to identify where in the solar system it came from, like accented speech giving away a person’s homeland.
These differences are so pronounced that they’re used to classify planets and meteorite types. Mars is so chemically distinct from Earth, for instance, that its meteorites can be identified simply by measuring ratios of three different oxygen isotopes.
In 2001, using advanced mass spectrometry techniques, Swiss researchers remeasured troctolite 76536 and 30 other lunar samples. They found that its oxygen isotopes were indistinguishable from those on Earth. Geochemists have since studied titanium, tungsten, chromium, rubidium, potassium, and other obscure metals from Earth and the moon, and everything looks pretty much the same.
This is bad news for Theia. If Mars is so obviously different from Earth, Theia—and thus, the moon—ought to be different, too. If they’re the same, that means the moon must have formed from melted bits of Earth. The Apollo rocks are then in direct conflict with what the physics insist must be true.
“The canonical model is in serious crisis,” said Sarah Stewart, a planetary scientist at the University of California, Davis. “It has not been killed yet, but its current status is that it doesn’t work.”
Stewart has been trying to reconcile the physical constraints of the problem—the need for an impactor of a certain size, going a certain speed—with the new geochemical evidence. In 2012, she and Matija Ćuk, now at the SETI Institute, proposed a new physical model for the moon’s formation. They argued that the early Earth was a whirling dervish, rotating through one day every two to three hours, when Theia collided with it. The collision would produce a disk around the Earth, much like the rings of Saturn—but it would only persist for about 24 hours. Ultimately, this disk would cool and solidify to form the moon.
Supercomputers are not powerful enough to model this process completely, but they showed that a projectile slamming into such a fast-spinning world could shear away enough of Earth, obliterate enough of Theia, and scramble enough of both to build a moon and Earth with similar isotopic ratios. Think of smacking a wet lump of clay on a fast-spinning potter’s wheel.
For the fast-spinning-Earth explanation to be right, however, something else would have to come along to slow down Earth’s rotation rate to what it is now. In their 2012 work, Stewart and Ćuk argued that under certain orbital-resonance interactions, Earth could have transferred angular momentum to the sun. Later, Jack Wisdom of the Massachusetts Institute of Technology suggested several alternate scenarios for draining angular momentum away from the Earth-moon system.
But none of the explanations were entirely satisfactory. The 2012 models still couldn’t explain the moon’s orbit or the moon’s chemistry, Stewart said. Then last year, Simon Lock, a graduate student at Harvard University and Stewart’s student at the time, came up with an updated model that proposes a previously unrecognized planetary structure.
In this story, every bit of Earth and Theia vaporized and formed a bloated, swollen cloud shaped like a thick bagel. The cloud spun so quickly that it reached a point called the co-rotation limit. At that outer edge of the cloud, vaporized rock circled so fast that the cloud took on a new structure, with a fat disk circling an inner region. Crucially, the disk was not separated from the central region the way Saturn’s rings are—nor the way previous models of giant-impact moon formation were, either.
Conditions in this structure are indescribably hellish; there is no surface, but instead clouds of molten rock, with every region of the cloud forming molten-rock raindrops. The moon grew inside this vapor, Lock said, before the vapor eventually cooled and left in its wake the Earth-moon system.
Given the structure’s unusual characteristics, Lock and Stewart thought it deserved a new name. They tried several versions before coining synestia, which uses the Greek prefix syn-, meaning together, and the goddess Hestia, who represents the home, hearth, and architecture. The word means “connected structure,” Stewart said.
“These bodies aren’t what you think they are. They don’t look like what you thought they did,” she said.
In May, Lock and Stewart published a paper on the physics of synestias; their paper arguing for a synestia lunar origin is still in review. They presented the work at planetary science conferences in the winter and spring and say their fellow researchers were intrigued but hardly sold on the idea. That may be because synestias are still just an idea; unlike ringed planets, which are common in our solar system, and protoplanetary disks, which are common across the universe, no one has ever seen one.
“But this is certainly an interesting pathway that could explain the features of our moon and get us over this hump that we’re in, where we have this model that doesn’t seem to work,” Lock said.
Among natural satellites in the solar system, Earth’s moon may be most striking for its solitude. Mercury and Venus lack natural satellites, in part because of their nearness to the sun, whose gravitational interactions would make their moons’ orbits unstable. Mars has tiny Phobos and Deimos, which some argue are captured asteroids and others argue formed from Martian impacts. And the gas giants are chockablock with moons, some rocky, some watery, some both.
In contrast to these moons, Earth’s satellite also stands out for its size and the physical burden it carries. The moon is about 1 percent the mass of Earth, while the combined mass of the outer planets’ satellites is less than one-tenth of 1 percent of their parents. Even more important, the moon contains 80 percent of the angular momentum of the Earth-moon system. That is to say, the moon is responsible for 80 percent of the motion of the system as a whole. For the outer planets, this value is less than 1 percent.
The moon may not have carried all this weight the whole time, however. The face of the moon bears witness to its lifelong bombardment; why should we assume that just one rock was responsible for carving it out of Earth? It’s possible that multiple impacts made the moon, said Raluca Rufu, a planetary scientist at the Weizmann Institute of Science in Rehovot, Israel.
In a paper published last winter, she argued that Earth’s moon is not the original moon. It is instead a compendium of creation by a thousand cuts—or at the very least, a dozen, according to her simulations. Projectiles coming in from multiple angles and at multiple speeds would hit Earth and form disks, which coalesce into “moonlets,” essentially crumbs that are smaller than Earth’s current moon. Interactions between moonlets of different ages cause them to merge, eventually forming the moon we know today.
Planetary scientists were receptive when her paper was published last year; Robin Canup, a lunar scientist at the Southwest Research Institute and a dean of moon-formation theories, said it was worth considering. More testing remains, however. Rufu is not sure whether the moonlets would have been locked in their orbital positions, similar to how Earth’s moon constantly faces the same direction; if so, she is not sure how they could have merged. “That’s what we are trying to figure out next,” Rufu said.
Meanwhile, others have turned to another explanation for the similarity of Earth and the moon, one that might have a very simple answer. From synestias to moonlets, new physical models—and new physics—may be moot. It’s possible that the moon looks just like Earth because Theia did, too.
The moon is not the only Earth-like thing in the solar system. Rocks like troctolite 76536 share an oxygen isotope ratio with Earth rocks as well as a group of asteroids called enstatite chondrites. These asteroids’ oxygen isotope composition is very similar to Earth’s, said Myriam Telus, a cosmochemist who studies meteorites at the Carnegie Institution for Science in Washington, D.C. “One of the arguments is that they formed in hotter regions of the disk, which would be closer to the sun,” she said. They probably formed near where Earth did.
Some of these rocks came together to form Earth; others would have combined to form Theia. The enstatite chondrites are the detritus, remnant rocks that never combined and grew large enough to form mantles, cores, and fully fledged planets.
In January, Nicolas Dauphas, a geophysicist at the University of Chicago, argued that a majority of the rocks that became Earth were enstatite-type meteorites. He argued that anything formed in the same region would be made from them, too. Planet-building was taking place using the same premixed materials that we now find in both the moon and Earth; they look the same because they are the same. “The giant impactor that formed the moon probably had an isotopic composition similar to that of the Earth,” Dauphas wrote.
David Stevenson, a planetary scientist at the California Institute of Technology who has studied lunar origins since the Theia hypothesis was first presented in 1974, said he considers this paper the most important contribution to the debate in the past year, saying it addresses an issue geochemists have grappled with for decades.
“He has put together a story which is quantitative; it’s a clever story, about how to look at the various elements that go into the Earth,” Stevenson said. “From that, he can back out a story of the particular sequence of Earth’s formation, and in that sequence, the enstatite chondrites play an important role.”
Not everyone is convinced, however. There are still questions about the isotopic ratio of elements like tungsten, Stewart points out. Tungsten-182 is a daughter of hafnium-182, so the ratio of tungsten to hafnium acts as a clock, setting the age of a particular rock. If one rock has more tungsten-182 than another, you can safely say the tungsten-filled rock formed earlier. But the most precise measurements available show that Earth’s and moon’s tungsten-halfnium ratios are the same. “It would take special coincidences for the two bodies to end up with matching compositions,” Dauphas concedes.
Understanding the moon—our constant companion, our silvery sister, target of dreamers and explorers since time immemorial—is a worthy cause on its own. But its origin story, and the story of rocks like troctolite 76536, may be just one chapter in a much bigger epic.
“I see it as a window into a more general question: What happened when terrestrial planets formed?” Stevenson said. “Everybody is coming up short at present.”
Understanding synestias might help answer that; Lock and Stewart argue that synestias would have formed apace in the early solar system as protoplanets whacked into each other and melted. Many rocky bodies might have started out as puffy vapor halos, so figuring out how synestias evolve could help scientists figure out how the moon and other terrestrial worlds evolved.
More samples from the moon and Earth would help, too, especially from each mantle, because geochemists would have more data to sift through. They would be able to tell whether oxygen stored deep within Earth is the same throughout, or if three common oxygen isotopes preferentially hang out in different areas.
“When we say that Earth and the moon are very close to being identical in the three oxygen isotopes, we are making an assumption that we actually know what the Earth is, and we actually know what the moon is,” Stevenson points out.
New tweaks to solar system origin theories, which are often based on complex computer simulations, are also illuminating where planets were born and where they migrated. Scientists increasingly suggest we can’t count on Mars to tell this story, because it may have formed in a different area of the solar system than Earth, the enstatites, and Theia. Stevenson said Mars should no longer be used as a barometer for rocky planets.
Ultimately, lunar scientists agree that the best answers may be found on Venus, the planet most like Earth. It may have had a moon in its youth, and lost it; it may be very similar to Earth, or not. “If we can get a lump of rock from Venus, we can answer this question [of the moon’s origins] very simply. But sadly, that is not on anyone’s priority list right now,” Lock said.
Absent samples from Venus, and without laboratories that can test the unfathomable pressures and temperatures at the heart of giant impacts, lunar scientists will have to keep devising new models—and revising the moon’s origin story.