Computer simulations examining thousands of scenarios have determined it would have taken about 20 mini-moons to create the one that orbits the Earth.
The researchers, from Israel, say the process may have taken millions of years.
It was previously believed that the moon was formed when a giant asteroid collided with the earth, knocking off a piece of it that ultimately failed to escape the Earth's gravity.
The scientists say the latest theory could explain why the moon appears to be made of material from Earth and not from outer space.
It was assumed that some of the asteroid would have remained attached to the piece that ended up in orbit.
The findings by Weizmann Institute of Science's Raluca Rufu were published in Nature Geoscience.
Co-author Hagai Perets of the Technion, Israel Institute of Technology in Haifa, said: "Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth.
Mr Rufu added: "It's likely that small moons formed through the process could cross orbits, collide and merge."
Gareth Collins, from London's Imperial College, said more evidence was needed in order to determine what happened.
In a companion article to accompany the theory, he said some of the moonlets must have been lost in space or failed to merge properly, so many more impacts may have been required.
|The Moon may have been made from about 20 moonlets|
Study suggests Earth once had many moonlets — until they merged to form the moon
The moon is the most obvious and familiar object in Earth's night sky — constant, consistent, predictable in its monthly cycles and its daily rising and setting. Astronomers understand the moon's movements so thoroughly that even a break from the routine, like an eclipse, can be anticipated 1,000 years in advance.
But we don't know the moon as well as we think. In fact, for years, astronomy has been in an uproar over the origin of Earth's only natural satellite, grappling to make sense of a model that seems increasingly unsatisfactory.
Now, a team of Israeli researchers has shaken up the debate by offering an entirely new explanation, published this week in the journal Nature Geoscience. They say the moon isn't a single chunk of rock but an amalgamation of nearly two dozen “moonlets,” one that was formed during a steady bombardment of Earth by several smaller bodies.
It's a major departure from the “giant impact model,” which was once the standard explanation for the moon's existence. That hypothesis proposes that the satellite came about during a single, violent collision between Earth and a hypothetical protoplanet called Theia. Theia sideswiped our planet roughly 4.4 billion years ago, scattering debris that eventually coalesced into the moon, which drifted away and started to circle the Earth.
The model explained the moon's modern migration — it's still receding, at a rate of about 4 centimeters per year. But it also had a problem: In the early 2000s, scientists examining lunar rocks brought back by the Apollo astronauts found the chemical composition of the moon was eerily similar to that of Earth. Its elements had the same ratio of isotopes as many on Earth do — and virtually no traces of Theia. How could a giant object create the moon and leave nothing behind?
“The whole giant impact model had been put into crisis several years ago,” Sarah Stewart, a planetary physicist at the University of California at Davis, told The Washington Post last year, “to the point where people thought it might be completely wrong because we couldn't make it work in its details.”
Scientists added other elements to the hypothesis in an attempt to resolve that chemistry issue. Maybe Theia was chemically identical to Earth? Maybe the collision vaporized both bodies, mixing their ingredients until they condensed back into the planet and moon we now know?
But every tweak seemed to make the giant impact model even more improbable.
“If you do that too many times, everyone in the room starts squirming,” Stewart said.
A more likely scenario, researchers argue in the new Nature Geoscience paper, is a series of smaller impacts. Lead author Raluca Rufu, a planetary scientist at the Weizmann Institute of Science in Israel, knew that the solar system's infancy was a chaotic time. Small bodies (a 10th the size of Earth or less) ran rampant in the system, bumping into things like rambunctious toddlers. Though a collision with a major protoplanet like Theia would have been rare, bombardment by these smaller bodies happened frequently.
Each collision would have sent a spray of debris into orbit around Earth, forming disks made mostly of material from Earth (rather than the impactor). Each disk then cooled and coalesced into a moonlet, which would migrate outward and glom onto other newborn small rocks, forming a growing moon. About 20 of these moonlets could have combined to create the satellite that orbits Earth today.
This version of the moon's origin story solves the big chemistry question that dogs proponents of the giant impact model. Creating a moon from many impacts dilutes the influence of each impactor on its chemistry — their individual signatures would have been lost amid all the Earth material each moonlet carried. That could explain why lunar samples are more like Earth than anything else in the solar system.
The idea for a many-impact model has been around since the 1980s, but no one was sure if such a process could create sufficiently large moonlets. With more than 1,000 computer simulations of potential ancient impacts, Rufu and her colleagues demonstrated that sizable moonlets are a fairly common outcome of these crashes.
Proving the second part of the hypothesis — that moonlets would bunch together, rather than drifting away or getting reabsorbed by the Earth — is Rufu's next challenge. Right now, she and her colleagues have no model for that merging process.
But already they have gotten the attention of other factions in the moon debate.
“I applaud the group,” Robin Canup, an astrophysicist at the Southwest Research Institute and a backer of the single-impact hypothesis, told the New Yorker. “They’ve convinced me that maybe it’s now worth considering. Suddenly, the multiple-impact scenario looks equally probable — or improbable, depending on your perspective.”
Was the moon formed from a bunch of moonlets all mashed together?
JANUARY 9, 2017 —A steady fixture in the night sky, the moon seems to have always been present. As such, it lies at the center of many spiritual beliefs, cultural symbols, and calendars. But it hasn't always been there. So how did the moon form?
Scientists long thought that a giant celestial body, about the size of Mars, slammed into the early Earth, spraying debris out into space. That debris eventually formed the moon, according to that classic model.
But as scientists examined the composition of moon rocks more and more closely over the years, it dawned on them that the giant-impact model, as that canonical story is called, may be too simplistic. Although researchers have been trying to tweak that model to fit the data, they have yet to settle on the specifics of the lunar origin story.
Now a new model that is being added to the mix may reframe researchers' thinking about those ancient lunar origins.
It might not have been just one impactor that hit the early Earth, a team of researchers now suggests. Instead, a series of smaller collisions may have formed moonlets which, in turn, merged into the final, larger moon that orbits Earth today, according to a paper published Monday in the journal Nature Geoscience.
The only problem with the classic, giant-impact model is the composition of the moon itself, says study lead author Raluca Rufu, a planetary PhD student at the Weizmann Institute of Science in Israel. Researchers think that for that model to work smoothly, the moon would have to be made up predominantly of material from the Mars-sized object that impacted Earth, not bits of the Earth. When scientists compared the chemical composition of the moon rocks to that of Earth, it was a near perfect isotopic match.
Initially it seemed reasonable that the impactor and Earth could have had the same composition, says Robin Canup, an astrophysicist at the Southwest Research Institute in Boulder, Colo., whose own research has focused on the formation of planets and satellites but was not part of this work.
But over the past decade it has become clear that that isn't such a safe assumption – thanks, in large part, to a paper published in 2007. Instead, it seems that most objects in the solar system have distinct isotopic signatures, so the impactor would have likely been distinct, too. And that, Dr. Canup says, became the "fly in the ointment" for the canonical model.
Researchers have proposed other variations of the great-impact model to explain the isotopic signatures of the moon and Earth. One model, for example, has the impact being so violent that it vaporized the Earth's mantle and the impactor, melding the material so much so as to explain the nearly matching isotopic signatures. But such a tweak would make other physical aspects, like the Earth's current spin rate, much more difficult to explain.
According to Ms. Rufu's calculations, her model might be able to resolve that problem.
"What we are suggesting in this paper is that the moon itself was accreted in steps," she explains in a phone interview with The Christian Science Monitor. First, an object between the size of the moon and Mars impacts the Earth. The debris from the collision, as in the giant-impact scenario, forms a disk around the planet. Over time, that material accretes into a small moonlet.
Millions of years later, or so, another impact occurs. Again, a disk forms, and then a moonlet. Eventually the moonlets merge to form a bigger moon. Then this process happens again, again, and again. Rufu calculates that it would take about 20 impacts and moonlets to form the final moon that orbits Earth today.
This model resolves the isotopic puzzle in two ways, Rufu explains. First, because the objects are smaller, they can travel at a higher velocity and therefore mine more materials from Earth, making the disks a higher percentage of Earth material. And secondly, when many different objects are mixed together, the average material becomes more similar, much like mixing paints becomes a muddier brown.
"I was skeptical when I first heard about the idea," Canup tells the Monitor in a phone interview. Mainly, she says, she had wondered whether the model could explain the Earth's spin rate.
"The current angular momentum in the Earth-moon system implies that when the moon first formed, the Earth was rotating with about a four or five hour day. And that's a pretty rapid rotation. And a single, off-center impact of about a Mars-sized body is the simplest way to impart that kind of spin," Canup explains. But if there were many impacts, they would have hit the Earth at random, in multiple locations, which would likely slow down the Earth's rotation – not speed it up.
When Rufu and her colleagues simulated possible strike sequences, they found that most still had the Earth rotating too slowly. But there were a few scenarios in which these impacts could actually get the Earth rotating at the right speed.
For Canup, that is enough to make the model a contender.
"I think this made a pretty compelling case," she says of the paper. "I'm impressed and I applaud them for what I would call a real out-of-the-box idea. All of the other ideas we've been pursuing, while they have their differences, have all focused on the idea that a single impact produced the moon. And all of those existing models have at least an element, and in some cases multiple elements, that we think are low-probability. Given that, exploring out-of-the-box ideas is very important and needed, and I see this as one."