Simulating a Different Birth of the Moon

Birth of the moon

Planetary scientists in Israel are using linked computers to unravel the secrets of how the moon formed.

Earth’s moon is special. At 2,000 miles across,  it’s the largest moon in the solar system compared to the planet it orbits. But questions about the birth of the moon have always been a topic of contention among planetary scientists.

Knowing the answer would help scientists understand the formation of Earth and the solar system, as well as other planets and solar systems, said Raluca Rufu, a planetary scientist at the Weizmann Institute of Science. She said it can even help know where to search for extraterrestrial life.

“Without the moon, life may not have evolved on Earth,” she said. “It stabilized our orbit, giving us a stable climate.”

Birth of the moon
The multiple-impact hypothesis proposes that the moon was formed by debris kicked up by many high-velocity collisions with smaller objects. Image courtesy of NASA/JPL/Space Science Institute.

The leading explanation has always been the “giant impact theory,” also known as the “Big Whack.” That theory states something roughly the size of Mars collided with the early Earth, and the material ejected into space formed a disc that eventually clumped into the object that lights up the night skies today. However, this doesn’t explain how the Earth and the moon are almost identical in composition. What happened to the leftovers of the other body?

An alternate explanation, the “multiple-impact hypothesis,” says that the moon was formed by debris kicked up by many high-velocity collisions with smaller objects left over from the formation of the solar system. Scientists know this happened often when the Earth was forming, including the six-mile-wide asteroid whose impact led to the extinction of the dinosaurs.

Spread out over millions of years, these would have each ejected rubble into space that eventually merged into a single orbiting body. (Researchers have floated other explanations, such as the Earth “stole” the moon from Venus, but some kind of impact was most likely involved.)

Simulating the Sky Falling

The multiple-impact hypothesis was first proposed in 1989, but no one had tested it until last year, when Rufu and fellow planetary scientist Oded Aharonson, joined forces with Hagai Perets of the Technion–Israel Institute of Technology.

The researchers used the institute’s high-performance computer cluster, with 624 CPUs and a total of more than 5,000 processor cores and 2.9 TB of memory. (Home computers and laptops typically have anywhere from one to eight processor cores.)

They simulated a total of 864 impacts by bodies ranging from one-hundredth to one-tenth the mass of Earth. Each simulation was so calculation-intensive it took a couple of days to run on 12 processors.

Their findings suggest the multiple-impact hypothesis checks out. Each simulated impact formed a floating disc of debris that eventually coalesced into what’s called a “sub-lunar moonlet.” Gravity then pulled in material kicked up by later impacts, like a growing ball of Play-Doh, until the result was something in the size range of the moon.

“We are imagining a half-dozen or dozen impactors over a period of 60 to 100 million years,” said Aharonsen. Smaller objects would leave fewer traces of themselves behind, he added. Some objects likely traveled very fast,  kicking up more of the proto-Earth material that ended up in the moon.

Birth of the moon
The moon may have been formed by a floating disc of debris, growing like a ball of Play-Doh. Image courtesy of NASA/JPL.

Who Needs a Supercomputer?

The simulation shows how scientists are leveraging the power of linked computer clusters to do work that would otherwise be impossible without access to a supercomputer, said Intel’s Pradeep Dubey, a Fellow at Intel Labs.

“In the old days, there used to be a big difference [between supercomputers and clusters]”, he said. “Now, that distinction is less meaningful.”

While the average supercomputer still out-performs setups like the one in Israel, Dubey said, computer clusters are much more affordable and accessible — and they’re becoming more and more powerful.

Academics and other researchers are using data-driven simulations to improve healthcare, test economic theories, search for signs of alien life or design safer cars without putting actual lives at risk.

Other researchers at the Technion-Israel Institute are putting the cluster to work to search for previously missed exoplanets and to understand the moods of Jupiter’s atmosphere, said Rufu from the Weizmann Institute of Science.

The next phase of the moon project includes using simulations to look more closely at exactly how the ejected debris gloms together into larger and larger moonlets, she said, a process that could have been more common during the formation of the solar system than previously thought.

Luckily the solar system is mostly cleared out of large wandering bodies, Rufu said, so our modern moon is probably the final version. “You can sleep OK at night.”

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