Three Key Factors That Likely Shaped the Moon’s Formation in Earth’s Early History

Recent theories suggest that multiple collisions with Earth might better elucidate the Moon’s origin than the traditionally accepted single massive impact 4.5 billion years ago, potentially addressing one of its greatest enigmas.

Tracing the Moon’s origin has proven challenging. The prevailing theory is that it formed early in the solar system’s evolution due to a catastrophic collision between Earth and Theia, a Mars-sized body, and its formation likely originated closer to the sun than Earth’s current position. This impact would have expelled debris that ultimately coalesced into the large natural satellite we recognize today. At that period, matter around the sun was highly intermixed, making collisions frequent.

However, this prevailing model encounters complications, as the chemical compositions of Earth and the Moon are remarkably similar, suggesting that the Moon should retain more material from Theia than our planet does. “This presents a significant dilemma for the standard model,” comments Philip Carter, a researcher at the University of Bristol, UK.

Carter and his team propose a paradigm shift, suggesting that a series of impacts with Earth over millions of years may provide a more coherent explanation for the compositional similarities between Earth and the Moon. They propose that three or more significant impacts in the early solar system, involving bodies from the size of the modern Moon to those approaching Mars in size, could account for the Moon’s creation as we observe it today.

In this revised model, each impact creates smaller moons, known as microsatellites, orbiting Earth. Over eons, these smaller bodies would progressively merge under gravitational attraction, forming a singular large entity. “They will be drawn to one another and collide,” explains Carter. “The probability of sustaining a stable system with multiple large moonlets is exceedingly low.”

Previous models also posited multiple impacts as the origin of the Moon; however, they typically required a more rigorous series of impacts than this current framework. “After three significant collisions, we introduced sufficient mass into orbit to form a full Moon,” stated Carter.

Robert Citron, a researcher at the Southwest Research Institute in Colorado, suggests that fewer impacts might be more favorable since too many collisions could displace smaller satellites from Earth’s orbit and hinder Moon formation. However, as more impacts occur, the compositional alignment between Earth and the Moon increases, accurately reflecting their current similarities. “When multiple impacts are involved, you are averaging out more influencing factors,” Citron notes.

The unique relationship between Earth and the Moon underscores the necessity of understanding the Moon’s formation. “It is a remarkably distinctive satellite,” Citron emphasizes. “Its size relative to Earth is vast, whereas the moons of Mars appear minuscule in comparison to Mars, and the moons of gas giants are diminutive compared to their planets.”

Establishing which hypothesis is correct necessitates more intricate modeling to assess the impact’s intensity on Earth and the volume of material expelled. Carter remarks, “Calculating all these details remains exceedingly complex.” He adds, “Personally, I prefer the multi-impact model over the traditional single-impact theory.”