Early Earth's Oceans of Magma Accelerated the Moon's Departure

The Earth and Moon have been locked in a gravitational dance for billions of years. Each day, as the Earth turns, the Moon tugs upon the oceans of the world, causing the rise and fall of tides. As a result, the Earth's day gets a little bit longer, and the Moon gets a little more distant. The effect is small, but over geologic time, it adds up. About 620 million years ago, a day on Earth was only 22 hours long, and the Moon was at least 10,000 km closer than it is now.

Evidence for this evolving dance in the geological record only goes back about two billion years. Beyond that, the Earth was so very different that there simply isn't enough evidence to gather. So, instead, we must rely on computational models and our understanding of dynamics. We know that when the Earth formed, it had no large moon. Then, about 4.4 billion years ago, a Mars-sized protoplanet named Theia collided with our world to create the Earth-Moon system. What’s interesting is that most of the computer simulations for this collision generate a Moon that is much closer to the Earth than we’d expect. Early Earth didn’t have vast oceans, so there were no water tides to drive the Moon to a larger orbit. So how did the Moon get to its present distance?

The potential structure of a lava planet. Credit: Farhat, et al.

A new study argues that back then the Earth did have tides, but they were made of lava, not water. Just after the Great Collision, the Earth would have been covered in an ocean of hot lava. With the Moon so near, the lava would have experienced strong tides. Since lava is much denser than water, the effects of the tide would have been much greater. The Earth's rotation would have slowed down much faster, and the Moon would quickly become more distant. Based on their simulations, the authors argue that the Moon's distance would have increased by 25 Earth radii in just 10,000 to 100,000 years. This would explain how the Moon moved toward its present distance range rather quickly.

The idea of tides on an ocean world also has implications for planets around other stars. Planets that form very close to their sun would be extremely hot, and many of them could have lava oceans for a billion years or more. Simulations of such worlds show that lava tides would accelerate the spin dynamics of such a world and could cause them to become tidally locked on a million-year timescale instead of a billion-year timescale. If this model is correct, it would have a significant impact on potentially habitable worlds. Most exoplanets orbit red dwarf stars, since red dwarfs make up about 75% of the stars in our galaxy. The habitable zone of red dwarfs is very close to the star, meaning that many of them would have begun as lava worlds. This would mean most potentially habitable worlds would have one side always facing the sun, while the other side is forever in the cold. Life on these worlds would be very different from what we see on Earth.

Reference: Farhat, Mohammad, et al. “Tides on Lava Worlds: Application to Close-in Exoplanets and the Early Earth-Moon System.” arXiv preprint arXiv:2412.07285 (2024).

Introduction

The process of lunar migration is a significant phenomenon in the Earth-Moon system, influenced heavily by gravitational interactions and the tidal forces generated by Earth's rotation. The intriguing interplay between early Earth's geological composition and its gravitational relationship with the Moon offers insights into the evolution of both celestial bodies. The foundation of this dynamics can be traced to the *giant impact hypothesis*, which theorizes the formation of the Moon from the debris ejected during a colossal collision between the early Earth and a Mars-sized body dubbed Theia. However, new evidence points to a different critical component in this narrative: the presence of magma oceans on early Earth.

The Giant Impact Hypothesis

The giant impact hypothesis remains one of the most widely accepted explanations for the moon's origin. According to this theory, between 20 to 100 million years after the formation of the solar system, Theia collided with the early Earth. This violent encounter resulted in a significant portion of Earth's mantle being ejected into orbit around the planet, which eventually coalesced to form the Moon. This monumental event not only marked the inception of our natural satellite but also started a complex journey of lunar migration.

The Role of Magma Oceans

Initial findings suggested that Earth's primordial molten state played a vital role in shaping the current Earth-Moon dynamic, particularly the geographical and geological characteristics of our planet. During the Hadean eon, the Earth was largely molten, with vast oceans of magma. The current study posits that these magma oceans would have significantly influenced the tidal interactions between the Earth and the Moon.

The potential structure of a lava planet. Credit: Farhat et al.

Simulation models indicate that instead of a water-driven tidal system, the early Earth had tidal forces incumbent upon a molten surface. These conditions would create significantly stronger responses compared to the tidal forces experienced by the current ocean waters. This peculiar state means that the transfer of angular momentum from the Earth to the Moon would have occurred at a much more accelerated rate, enabling the Moon to drift away from Earth more rapidly.

Angular Momentum and Tidal Forces

Angular momentum is a fundamental principle that dictates how objects rotate and orbit around one another. In the case of the Earth and Moon, tidal interactions act as a medium for the transfer of angular momentum. As the Earth rotates, it generates tidal bulges that are pulled by the Moon’s gravity. Under normal circumstances, these forces would result in a gradual increase in the Earth-Moon distance over an extended period; however, the presence of a molten surface accelerates this interaction.

Impact on Earth's Rotation

In conclusion, the study's revelation about Earth's lava tides suggests that the Moon's gradual departure from Earth isn't merely an elongated process over millions of years. Instead, it suggests a rapid migration influenced heavily by the denser, more responsive magma oceans. This phenomenon would have caused the Earth’s rotation to slow at a significantly accelerated pace and allowed the Moon to achieve its current orbital distance far more quickly than previously believed.

Implications for Exoplanets

These findings also bring intriguing insights into the dynamics of exoplanets situated close to their stars. In these scenarios, such as what could exist in lava ocean environments, the same principles of angular momentum and tidal interactions would apply, potentially altering planetary rotation periods and migration patterns significantly. This knowledge shapes a broader understanding of habitability criteria when assessing potential life-harboring planets orbiting red dwarf stars.

Conclusion

Understanding the intricate behaviors of celestial bodies and their evolutions opens new avenues in the search for life beyond Earth. This new knowledge may also allow researchers to gauge the potential habitability of rocky planets, especially those undergoing similar early developmental processes like that of Earth. The rapid lunar migration due to early lava tides illustrates a compelling narrative of how dynamic processes could shape planets and their celestial counterparts.

References

1. Farhat, Mohammad, et al. “Tides on Lava Worlds: Application to Close-in Exoplanets and the Early Earth-Moon System.” arXiv preprint arXiv:2412.07285 (2024).
2. Never a Miscommunication - Brian Koberlein.

For More Information

For more insights into planetary dynamics and research surrounding the Earth-Moon system, please have a look at other articles on Uniiverse Today or reach out to astrophysics blogs.