Most scientists agree that a major collision 4.5 billion years ago with the protoplanet Theia formed the Earth-Moon system. A new study from the University of Nevada argues that the chaos of that collision in the early days of the Earth-Moon system may have created polar cislunar “moons.”
While changing conditions in the Earth-Moon system no longer allow this, the discovery could have major implications for astronomers studying exoplanets, some of which may even mimic early conditions on Earth.
About 4.5 billion years ago, when Earth was only about 100 million years old, a Mars-sized protoplanet named Theia smashed into our planet, ejecting cargo that eventually returned to Earth’s surface, formed the Moon, or escaped the newly formed Earth-Moon system entirely.
Putting the pieces together for this catastrophe hasn’t been easy, but evidence of possible remnants of Theia in Earth’s mantle and analysis of moon rocks have combined to confirm it. Now, a new study from the University of Nevada adds some complexity to this extremely chaotic moment in Earth’s history, suggesting that for a short time, the Earth-Moon system hosted multiple polar moons, also known as circumbinaries. However, these “moons” were only possible because of the Moon’s close proximity to Earth immediately after the planet’s collision with Theia. The results of this study have been uploaded to the non-peer-reviewed preprint server arXiv.
“Circumbinary orbits that are polar or highly inclined to the Earth-Moon orbit are subject to two competing effects: nodal precession around the Earth-Moon eccentricity vector and solar-induced Kozai-Lidov oscillations of eccentricity and tilt,” the paper says. “While we find that no stable polar orbits exist around the Earth-Moon orbit with the current semi-axis, polar orbits were stable immediately after the Moon’s formation, at a time when there was a lot of debris around the system.”
Nodal precession means that particles in orbit around the Earth-Moon system slowly “precess” (slowly, continuously changing the orbital parameters of a body) around the angular momentum vector. Kozai-Lidov oscillations, on the other hand, are largely a dynamical phenomenon involving a binary system and disturbances introduced by a distant third body.
After calculating these competing effects, the particles orbiting in the polar region were found to be the most stable. That’s because when the Moon first formed (which some scientists think may have taken just a few hours), it was only 5 percent farther from Earth than it is today, an average of about 238,900 miles. Over billions of years, however, tidal forces began to push the Moon farther and farther into its present-day orbit, eventually erasing these stable polar regions entirely. It’s possible that this polar material was also pushed by the increasing eccentricity of the Earth-Moon system.
“Polar circumbinary material may contribute to the increase in the binary system’s eccentricity,” the paper says. “If a significant mass of material ended up in polar or librational orbits, the Earth-Moon binary system’s eccentricity could increase as a result of its interaction.”
Although these polar moons have virtually no effect today, confirming their existence in Earth’s early years has profound implications for the study of exoplanets beyond our solar system. The research team explained that a planet-moon system with a highly eccentric orbit would likely host one of these circumbinary polar “moons,” which would in turn drive a retrograde procession — that is, an orbit moving backwards.
Finding one such moon among the billions of stars in the galaxy is like catching a glimpse of the creation of our home planet.
2% of Earth’s mass may be the remnants of the giant collision that created the Moon. 4.5 billion years ago, Earth experienced a cataclysmic rendezvous with an ancient planet called Theia. Evidence of the collision is still buried deep in our crust.
Deep beneath the Earth’s crust, along the mantle-core boundary, lie dense, teardrop-shaped geological anomalies known as large low-shear-velocity provinces.
While there are many hypotheses to explain these anomalies, the giant impact hypothesis is the favorite. This concept states that these blobs are actually remnants of the Earth’s collision with the protoplanet Theia, which led to the formation of the Moon, 4.46 billion years ago.
A new study provides detailed modeling showing that the Teian material may make up two percent of Earth’s mass along the core-mantle boundary. Two newly formed sister planets, harmoniously sharing the same solar orbit, suddenly collide due to gravitational perturbations in the early solar system. But from this chaotic encounter emerges an endless bond that lasts 4.46 billion years, and the only planet that supports life (that we know of) is Earth.
Over the past 50 years, this catastrophic collision has gone from being one of many hypotheses to a leading candidate for explaining how the Earth-Moon system formed. Now, a new study by an international team of scientists says evidence of this primordial collision can still be found in the Earth’s lower mantle.
Hongping Deng, an astrophysicist at the Chinese Academy of Sciences, used a new fluid dynamics technique called Meshless Finite Mass (MFM) to model this ancient collision. The results were published in the journal Nature.
“Previous studies have overemphasized the structure of the debris disk (the Moon’s precursor) and overlooked the impact of the giant impact on the early Earth,” Deng said in a press release. “Our results challenge the traditional view that the giant impact homogenized the early Earth. Instead, the giant impact that formed the Moon appears to be the source of heterogeneity in the early mantle and marks the starting point of Earth’s 4.5 billion-year geologic evolution.”
The two largest examples of this heterogeneity (proto-Earth had a higher silicon content than Theia) are found in two large low-shear-velocity provinces (LLVPs) located deep beneath Africa and the Pacific Ocean. These vast, dense regions along the mantle-core boundary have been detected using seismic measurements, and scientists have even mapped them in detail. Some theories suggest that these regions represent old tectonic plates or an anomalous magma ocean at the base of the mantle. But the possibility is also beginning to emerge that these clumps are remnants of the proto-planet Theia.
In 2021, University of Arizona graduate student Qian Yuan, now a postdoctoral fellow in the Department of Geophysics at Caltech, theorized that iron-rich (and therefore initially denser) Taian material moved into the lower mantle and formed these LLVPs after long-term mantle convection.
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Dan’s new simulations confirm that the early Earth experienced post-impact mantle stratification, meaning the mantle was swirled with material from both Theia and Earth, while the core remained unaltered. The simulations also show that material from Theia equal to two percent of Earth’s mass eventually ended up in this lower region of the mantle.
“By accurately analyzing a wider range of rock samples, combined with more sophisticated giant impact models and Earth evolution models, we can infer the material composition and orbital dynamics of the primordial Earth… and Theia,” Yuan said in a press release. “This allows us to constrain the entire formation history of the inner solar system.”
Earth is a unique gem in the solar system and, as far as we can tell, in the known universe. Understanding the early days (or first million years) during which our planet formed can provide valuable insight into what made our Earth so special in the first place.
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