Despite the fact that the Moon is Earth’s closest neighbor, scientists have been struggling with the mystery of its origin for centuries.
In the 1980s, astronomers suggested that this natural satellite formed after a catastrophic collision with the protoplanet Theia early in Earth’s history. However, a recent study by a team from the University of Pennsylvania suggests the so-called collision theory may be incorrect.
Did the Earth capture the Moon?
Instead, researchers claim that the Earth could have “simply” captured the Moon as it passed by, during a process known as binary exchange capture.
This radical theory suggests that the Moon may have formed as part of a “terrestrial double system” — a pair of rocky objects orbiting each other. When they passed within Earth’s gravitational zone, one body was pulled into orbit while the other was ejected into space.
The lead author of the study, Darren Williams, a professor of astronomy and astrophysics, noted: “No one knows for sure how the Moon was formed. For the past four decades, we had one theory about this. Now we have two.”
Why is the popular collision theory failing?
In 1984, astronomers gathered at a conference in Kona, Hawaii, to try to reach a consensus on how the Moon formed. Using 363 kg of lunar material brought back by NASA’s Apollo missions, scientists found the Moon’s chemical composition is similar to Earth’s but not identical. Based on that, researchers concluded the Moon likely formed from debris thrown up in a collision between a protoplanet and the young Earth.
This theory was popular because it fits many chemical clues. But it doesn’t explain everything.
For example, Professor Williams and his co-author Michael Zugger pointed out that if the Moon formed from a ring of debris slowly coalescing into a sphere, we’d expect it to orbit above Earth’s equator. Instead, the Moon’s orbital plane is tilted about seven degrees from the equatorial plane, the Daily Mail reported.
A fresh attempt to explain the birth of the Moon.
To find an alternative explanation, astronomers considered a phenomenon known as binary exchange capture. According to this idea, Earth could have “stolen” a rocky body from a passing binary pair and turned it into its satellite.
In support of this idea, Professor Williams cited Triton, Neptune’s largest moon. Modern theories suggest that Triton was captured by Neptune from the Kuiper Belt, where about 1 in 10 objects is a binary.
Like our Moon, Triton has a highly tilted orbit, deviating by roughly 67 degrees from Neptune’s equator. According to mathematical models, something similar could have happened with our Moon.
In an article published in The Planetary Science Journal, researchers calculated that Earth could have captured an object with a mass ranging from 1 to 10 percent of Earth’s mass. The Moon’s mass is 1.2 percent of Earth’s mass, which fits well within that range.
The only condition is that the double object would have had to pass within about 128,750 km of Earth at a speed no greater than 10,800 km/h. Although that seems incredibly fast, on the scale of the Solar System such movement is like a leisurely stroll.
The problem is that even at those moderate speeds, the Moon’s initial orbit would have been highly elongated, more like a comet’s path around the Sun. The researchers suggest that tidal forces could have gradually reshaped that elongated orbit into the more circular one we see today.
This theory has a number of advantages. For example, it could explain some chemical isotope differences between the Moon and Earth. At the same time, the researchers acknowledge their scenario will be difficult to prove and depends on several unlikely events happening together. Still, Professor Williams argues that capture through binary exchange is a viable alternative to the standard collision model and deserves further study.
