On August 1, 1971, while exploring the eastern edge of the lava plain known as Mare Imbrium on the silent, serene lunar surface, Apollo 15 astronauts David Scott and James Irwin found something remarkable: a profoundly old piece of lunar crust, a relic more than four billion years old that carried clues to the moon’s formation. Seeing the glint of ancient crystals embedded in what would later be called the Genesis rock, Scott immediately knew its potential importance for solving the mystery of how the moon was made. “I think we found what we came for,” he radioed to mission control as he and Irwin retrieved the rock and placed it in a bag. It would become a key part of what is the Apollo program’s greatest scientific legacy.
Studies of the Genesis Rock and the nearly 400 kilograms of other samples hauled back to Earth by the Apollo astronauts overturned our understanding of lunar history. In what amounted to a scientific reboot, these precious samples nullified the then prevailing theories—that the moon had been gravitationally captured by Earth or had formed alongside it—while revealing important new details, such as the fact that the newborn satellite had been covered by a magma ocean.
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The immense energy required to form the moon’s magma ocean pointed to a radical new idea for lunar origin: the notion that Earth’s closest companion had formed from a giant impact, a collision between the proto-Earth and another planetary body. The concept built on calculations showing that growing planets would collide with one another, as well as the curious fact that the moon’s composition is uncannily similar to that of Earth’s rocky mantle. Some researchers even proposed that such an impact had set the young Earth’s spin, establishing what would become our planet’s 24-hour cycle of day and night. The canonical giant impact hypothesis that emerged from these early studies proposes that a glancing collision with a Mars-size body created a hot disk of rocky debris around Earth. The moon then coalesced from the disk—a scenario that can explain the moon’s large mass and dearth of water and other volatiles.
Yet the giant impact hypothesis is not without flaws. Chief among them is the astounding chemical relationship between Earth and the moon. These two bodies are made from the same source material, as if they are planetary twins, whereas the canonical hypothesis predicts the moon should mostly be made of its Mars-size progenitor. That progenitor should differ in composition from the proto-Earth because planets growing from the disk of gas and dust around the young sun would each incorporate distinctive mixes of building blocks based on their orbital location. Scientists can discern these differences by making very precise measurements of the relative abundances of isotopes in rocks, yielding unique “isotopic fingerprints” for every planetary body in the solar system—except for Earth and the moon, which, bizarrely, appear to be almost the same.
This isotopic crisis has haunted the giant impact hypothesis for decades, but no better explanation has emerged for the lunar origin. Now, however, in another scientific reboot we have discovered that most giant impacts do not make a planet surrounded by a debris disk. In fact, most giant impacts do not make a planet at all. Instead they make an entirely new class of astronomical object, a transient hybrid between planet and disk called a synestia that could explain many of the moon’s most mysterious features.
Hiding in plain sight
The discovery of synestias traces back to a few years ago, when we (Lock and Stewart) were puzzling over whether or not a giant moon-forming impact had set the length of Earth’s day. That diurnal cycle is linked to the giant impact by a fundamental law of physics, the conservation of angular momentum. Going back in time, the moon was closer to Earth, and to conserve angular momentum, Earth spun faster—much faster: it would have had a five-hour day. Other scientists had found that a grazing giant impact by a Mars-size body could set the total angular momentum of Earth and the moon. But if something else had set Earth’s length of day, then the moon-forming event could have had more (or less) angular momentum, opening the door for a much wider array of possible impact scenarios. And a giant impact with more angular momentum and more energy could, in very rare cases, lead to an equitable mix of material from the two colliding bodies, potentially explaining Earth and the moon’s status as isotopic twins.
Examining this problem in simulations of about 100 different scenarios for a high-energy, high-angular-momentum, giant moon-forming impact, we were confronted by seemingly non-sensical results. Our plots of each postimpact scenario did not show the tidy division between “planet” and “disk” that we had expected. The postimpact planets were hot and huge, their rocky mantles partially vaporized and puffed up to more than 100 times Earth’s present-day volume, so swollen that they became connected to the encircling disk. The resulting objects did not look like normal planets or disks anymore but instead something in between. In a flash of insight, we realized that these giant impacts were making something new. But we could not immediately understand what it was. We did not know what to call it at the time, but we had seen our first synestia.
Credit: Jen Christiansen
To understand what we were seeing, we went back to first principles, reexamining fundamentals such as the working definition for a “planet.” A planet is defined in part by its spheroidal shape, which comes from the body’s self-gravity being strong enough to deform the rock as if it were a fluid. And planets rotate around together, with only small variations arising from any internal dynamics. We used a fluid dynamics code to calculate what happens to an Earth-like planet as its rocky mantle is slowly heated, watching as our models showed the planet swelling up as its rocks began to vaporize. At the extreme temperatures after a giant impact, the body resembles a gas giant—hot enough that it lacks a true surface, just a thick rock vapor atmosphere that becomes denser with depth. If such a world rotates with a five-hour day, it maintains a roughly spheroidal shape even as it balloons in size as the temperatures rise.
But if the planet is rotating even faster, as it heats up, something surprising happens. As the planet’s equator expands, it reaches a point at which the equator rotates as fast as if it were in orbit. We call this point the corotation limit. With just a little more heat, material will then flow from the planet’s equator into orbit. Suddenly, a fin of vapor protrudes from its equator, and the planet becomes something else. Unlike a planet, it is no longer a simple spheroid. Furthermore, unlike a planet, it no longer rotates cohesively, instead featuring an inner corotating region and an outer region that rotates more slowly. After some thought, we chose to name this new celestial creature a synestia, after Hestia, the Greek goddess of the hearth and home—because we believe Earth used to be one of these fiery objects. (The “syn” emphasizes the synergy that exists between all the interconnected material in the planet and the disk.) A synestia is what a planet turns into when heat and spin force it to exceed the limit of a spheroidal shape.
Soon we were manufacturing synestias by the hundreds in our computer models, heating spinning planets beyond the corotation limit. Synestias can have a wide range of shapes and sizes, depending on how mass, energy and angular momentum are distributed throughout the body. The properties of a synestia depend on how it was made. Gently heating a planet makes a synestia that looks like a squat flying saucer, but giant impacts make huge puffy synestias shaped more like doughnuts or cream-filled pastries. Armed with a better understanding of how these objects arise and manifest, we began digging through all our previous simulations of giant impacts and finding synestias there, too. It turned out that we had been making synestias by accident for years. In fact, most scientists working on giant impacts unknowingly had synestias sitting in their modeling data, just waiting to be recognized as strange objects new to science.
The fact that no one had noticed them earlier had been a matter of misplaced expectations. In the range of possible moon-forming giant impacts, the energy and angular momentum of a canonical Mars-size impact are too low to produce a synestia. By focusing on the Mars-size impactor, the entire field—generations of scientists—had been misled into thinking that a planet and disk were the standard outcome of giant impacts.
For us, the next obvious step was modeling how often synestias should emerge from the complex process of planet formation. We developed techniques to map out which impacts could transform planets into synestias. By comparing these results with models of growing planets, we have found that synestias are not extremely rare oddballs but are actually a very common but transient feature of young planetary systems. Indeed, our simulations suggest that most of the universe’s rocky planets may have transformed into synestias one or more times during their formation. We now believe that most giant impacts forming an Earth-mass body will also make a synestia. In a flash, we had discovered a missing piece in the cosmic history of planets.
Back to the moon
And yet the motivating question remains: Could a synestia explain our moon’s unique relation to Earth? A synestia is a very different environment for lunar accretion than a traditional circumplanetary disk. We have found that forming the moon from a synestia offers solutions to many of the issues that have plagued the giant impact model for lunar origin.
A synestia’s surface temperature is set by the boiling point of rock, which is about 2,300 kelvins (nearly 3,700 degrees Fahrenheit) at its low-pressure outer edge. There, cooled by radiating heat to space, rock vapor from the moon-forming synestia would have condensed to droplets of magma that rained down into its interior. The rate of the magma rain would have been 10 times that of the most intense rainfall ever measured on Earth. In this scenario, the moon would begin as a small orb of molten rock and metal—some of the material that was not vaporized in the initial impact. Dwarfed by the synestia’s immensity, the nascent moon would, in fact, have orbited within the synestia’s glowing, vaporous depths, surrounded by vast quantities of high-pressure gaseous rock, growing with each absorbed droplet of falling magma rain. The synestia would shrink as it cooled, so that after tens of years, it would have sufficiently contracted for its outer edge to recede within the orbit of the moon. In that moment, the moon would emerge, born from the dying synestia.
This story may explain why Earth and the moon are isotopic twins because the synestia formed from the vaporized and well-mixed material derived from the two colliding bodies. Furthermore, the synestia’s torrential magma rains and turbulent vapors would have driven even more mixing throughout a large fraction of the body. If the synestia was sufficiently well mixed, the moon would have acquired the same isotopic ratios as Earth.
A synestia can also explain several other lunar mysteries that the canonical giant impact hypothesis does not. For example, although the moon has the same isotopic fingerprint as Earth, it does not have exactly the same chemical composition. The moon has lower abundances of extremely volatile elements, such as hydrogen and nitrogen, and moderately volatile elements, such as sodium and potassium, as compared with Earth. These peculiar features are not definitively explained by the canonical hypothesis. Yet they arise naturally from “baking” a growing moon at a few thousand degrees in the “oven” of a synestia.
More volatile elements would have preferentially stayed in the vapor of the synestia, so the moon would never have acquired Earth-like abundances of these elements. The volatile elements that stayed in the vapor would be carried inward with the shrinking synestia to become part of Earth. With help from our colleagues Misha Petaev and Stein Jacobsen, both at Harvard University, we demonstrated that the pattern and abundance of moderately volatile lunar elements can be explained by the moon coming into chemical equilibrium with the vaporized elements inside the synestia. Simply put, being born in a synestia naturally explains why the moon has a similar composition to Earth but has a lower abundance of volatile elements. Our simple recipe for making the moon’s chemistry is as follows: vaporize two colliding planetary bodies, mix well and bake at 4,000 degrees Celsius (more than 7,000 degrees F) in a convection oven for 10 to 100 years.
Finally, synestias may explain otherwise mysterious quirks in the moon’s orbit. Strangely, the moon does not orbit Earth in the same plane in which Earth orbits the sun, which is called the ecliptic plane. Instead the moon’s orbit is inclined to the ecliptic plane by about five degrees. The tilt of the orbit is why we do not have total lunar eclipses every month but only on the rare occasions when Earth, the moon and the sun align. Yet after a giant impact, if the moon formed from a circumplanetary disk or a synestia, the naive expectation would be that it should be orbiting in the ecliptic plane. So why is the lunar orbit inclined?
A new model for how the orbit of the moon changes with time by SETI Institute theorist Matija uk and his colleagues can explain both the inclination of the lunar orbit and the length of Earth’s day. The giant impact may have knocked the proto-Earth on its side and produced a synestia with its rotation axis tilted close to the ecliptic plane. The moon would have formed in the plane of Earth’s equator, with its orbit also tilted far from the ecliptic. Over time, resonant interactions with the sun would have pulled the rotation axis of Earth more upright to its present-day 23-degree tilt. Earth’s spin would have been slowed in the process, with our planet being pushed slightly farther away from the sun to conserve angular momentum. As the moon dissipated its orbital energy by raising tides on Earth, it would slowly move away from the planet, decreasing the lunar inclination to the ecliptic to its present orientation. Thus, a single giant impact that created a tilted synestia could explain many of the key dynamical characteristics of Earth and its satellite.
In sum, the synestia’s natural elegance and explanatory power have rescued the giant impact hypothesis—and permanently changed the playing field for studies of the origin of the moon.
Fulfilling apollo’s legacy
Without the data from the rocks collected by the Apollo astronauts, we could have been satisfied with an incomplete, or even erroneous, idea for how the moon was created. The challenge of explaining the data led to the discovery of synestias. Now our new challenge is to further develop our understanding of synestias and their role in planet formation. We are only at the beginning of this quest.
Our model of a moon-forming synestia can be tested by improving its chemical and isotopic predictions for lunar composition. We are still learning from the samples collected by the Apollo missions—half a century of progress in instrumentation is allowing the extraction of more accurate and detailed data. But the Apollo samples are a limited resource with enormous gaps in coverage and completeness. More than ever, we need rocks from the lunar mantle to build better chemical models for the moon’s bulk composition. Returning to the moon to obtain samples from the mantle, parts of which should be exposed in and around massive impact craters, will let us make fresh predictions for that vital measurement. Meanwhile rocks right here on Earth may provide additional important clues for lunar origins. It has recently been realized that the deepest regions of Earth’s mantle contain traces of material that survived the moon-forming giant impact. Whatever process formed the moon could not have erased these chemical records. By combining data from Earth and the moon, we hope to piece together our view of the synestia that made both bodies.
Help in understanding synestias may also come from beyond our solar system. So far we have seen them only as mathematical objects on our computer screens, but synestias may not remain a purely theoretical notion much longer. Many telescopes, in space and on the ground, are staring at the heavens in search of exoplanets silhouetted against the bright faces of their stars. Because their shapes are very different from a spherical planet, synestias would cast unusual shadows on our telescopes. Other new and emerging facilities are snapping baby pictures of planets around very young stars that may still be in the giant impact stage of formation. Perhaps some of those snapshots will reveal a puffy, glowing doughnut of rock vapor where a planet used to be. Soon we may glimpse our first natural synestia and witness a near replay of the creative destruction that led to the formation of our own Earth and moon.