How did Earth get its moon? A new theory has emerged

It is widely believed that the moon formed during a collision between Earth and a small planet known as Theia about 4.5 billion years ago. But now, experts are proposing a new theory about the origin of the moon. 

The research suggests that a young Earth may have snatched the moon during a close encounter with a binary system. 

Similarities between Earth and moon rocks

Over the course of six missions to the moon between 1969 and 1972, Apollo astronauts collected over 800 pounds of lunar rock and soil. 

Chemical and isotopic analysis of these samples revealed their similarities to Earth’s rock and soil: they were calcium-rich, basaltic, and dated to approximately 60 million years after the solar system formed.

Challenging the planetary collision theory 

Based on the Apollo data, planetary scientists who gathered at the 1984 Kona Conference in Hawaii reached a consensus that the moon formed from debris following a massive collision. This explanation of the moon’s origin has shaped scientific understanding for decades.

However, two researchers from Pennsylvania State University are challenging this long-held narrative. 

A study led by Professor Darren Williams and Professor Michael Zugger suggests that the moon was captured during a close encounter between Earth and a pair of rocky bodies.

Unresolved questions about the moon’s origin

“The Kona Conference set the narrative for 40 years,” Williams said. But certain unresolved questions remain. 

One of these concerns the moon’s orbit. If the moon had formed from the debris of a planetary collision and coalesced into a ring around Earth, it should orbit directly above the planet’s equator. Instead, the moon’s orbit is not aligned with Earth’s equator but is more in line with the sun.

Did Earth’s gravity capture the moon?

In the binary-exchange capture theory proposed by Williams and Zugger, Earth’s gravity separated the two objects in the binary system, capturing the moon while the other object drifted away. The captured moon then settled into the orbit we observe today.

This process isn’t unheard of in the solar system. Williams pointed to Triton, Neptune’s largest moon, as evidence of a similar event. 

It is believed that Triton was pulled into orbit from the Kuiper Belt, where an estimated 10% of objects are binary pairs. Triton’s retrograde orbit (opposite to Neptune’s rotation) and its tilt of 67 degrees from the planet’s equator suggest a capture event.

Dynamics of the moon’s orbit

Williams and Zugger calculated that Earth could have captured a satellite even larger than the moon – potentially an object the size of Mercury or Mars. However, they suggested that the resulting orbit may not have been stable enough to last.

The researchers explained that the moon’s orbit initially started as an elongated ellipse rather than a circle. Over time, tides on Earth influenced the shape of the orbit, causing it to change.

“Today, the Earth tide is ahead of the moon,” 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.”

Moon’s elliptical orbit became circular

However, when the moon was closer to Earth, as it would have been just after capture, the tides had the opposite effect. 

Williams and Zugger calculated that this initial elliptical orbit would have contracted over thousands of years, gradually becoming more circular. 

Eventually, the lunar spin locked into the moon’s orbit around Earth, a state that persists today.

At that point, the tidal evolution likely reversed, and the moon began slowly drifting away from Earth. 

Gravitational pull from Earth and the sun

Each year, Williams explained, the moon moves about three centimeters farther from Earth. Currently, the moon is 239,000 miles away, and at that distance, it feels a strong gravitational pull from both the sun and Earth.

“The moon is now so far away that both the sun and Earth are competing for its attention,” Williams said. “Both are pulling on it.”

Moon’s formation remains a mystery

Williams and Zugger’s calculations suggest that a binary-exchange captured satellite could exhibit the same behaviors as Earth’s moon. But Williams acknowledged that this theory is not necessarily the final word.

“No one knows how the moon was formed. 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,” he concluded.

The research is published in The Planetary Science Journal.

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