The Moon may have been captured during a close encounter between a young Earth and a terrestrial binary (a system of the Moon and another rocky object), according to a new paper published in the Planetary Science Journal.
Darren Williams & Michael Zugger examined the concept of collision-less binary exchange for capturing massive satellites — comparable to and larger than the Moon — around terrestrial-mass objects either inside or outside the Solar System.
Over six missions to the Moon, from 1969 to 1972, Apollo astronauts collected more than 360 kg (800 pounds) of lunar rock and soil.
Chemical and isotopic analysis of that material showed that it was similar to the rock and soil on Earth: calcium-rich, basaltic and dating to about 60 million years after the Solar System formed.
Using that data, the planetary scientists who gathered at the Kona Conference in Hawaii in 1984 came to the consensus that the Moon formed from debris after a collision on the young Earth.
“The Kona Conference set the narrative for 40 years,” said Penn State’s Professor Darren Williams.
“But questions still lingered. For example, a moon that forms from a planetary collision, taking shape as debris clumps together in a ring, should orbit above the planet’s equator. Earth’s Moon orbits in a different plane.”
“The Moon is more in line with the Sun than it is with the Earth’s equator.”
“In the alternative binary-exchange capture theory. Earth’s gravity separated the binary, snagging one of the objects — the Moon — and making it a satellite that orbits in its current plane.”
“There is evidence of this happening elsewhere in the Solar System.”
“The reigning hypothesis in the field is that Triton, the largest of Neptune’s moons. was pulled into orbit from the Kuiper Belt, where one of every 10 objects is thought to be a binary.”
“Triton orbits Neptune in a retrograde orbit, moving in the opposite direction of the planet’s rotation.”
“Its orbit is also significantly tilted, angled 67 degrees from Neptune’s equator.”
Professor Williams and Penn State’s Professor Michael Zugger determined that Earth could have captured a satellite even larger than the Moon — an object the size of Mercury or even Mars — but the resulting orbit might not have been stable.
The problem is that the ‘capture’ orbit — the one the Moon follows — began as an elongated ellipse, rather than a circle.
Over time, influenced by extreme tides, the shape of the orbit changed.
“Today, the Earth tide is ahead of the Moon,” Professor Williams said.
“High tide accelerates the orbit. It gives it a pulse, a little bit of boost. Over time, the Moon drifts a bit farther away.”
The effect is reversed if the Moon is closer to Earth, as it would have been immediately after capture.
By calculating tidal changes and the orbit’s size and shape, the researchers determined that the Moon’s initial elliptical orbit contracted over a timescale of thousands of years.
The orbit also became more circular, rounding its path until the lunar spin locked into its orbit around the Earth, as it is today.
“At that point, the tidal evolution likely reversed, and the Moon began to gradually drift away,” Professor Williams said.
“Every year, the Moon moves 3 cm farther from Earth. At its current distance from Earth — 385,000 km (239,000 miles) — the Moon now feels a significant tug from the Sun’s gravity.”
“The Moon is now so far away that both the Sun and Earth are competing for its attention. Both are pulling on it.”
The team’s calculations show that, mathematically, a binary-exchange captured satellite could behave as Earth’s Moon does. But they’re not certain that’s how the Moon came to be.
“No one knows how the Moon was formed,” Professor Williams said.
“For the last four decades, we have had one possibility for how it got there.”
“Now, we have two. This opens a treasure trove of new questions and opportunities for further study.”
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Darren M. Williams & Michael E. Zugger. 2024. Forming Massive Terrestrial Satellites through Binary-exchange Capture. Planetary Science Journal 5 (9): 208; doi: 10.3847/PSJ/ad5a9a
