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How Lava Tides Shaped the Earth-Moon System

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The Earth and Moon have been gravitationally intertwined for billions of years, a relationship that has profoundly shaped both bodies. Each day, as Earth spins on its axis, the Moon’s gravitational pull generates the ocean tides, raising and lowering sea levels across the planet. This tidal interaction transfers energy from the Earth to the Moon, gradually slowing Earth’s rotation and causing its days to lengthen. At the same time, the Moon slowly spirals farther away from Earth. Although the changes are minute on human timescales, they accumulate significantly over geological epochs. For example, around 620 million years ago, a day on Earth lasted only 22 hours, and the Moon was at least 10,000 kilometers closer to our planet than it is today.

The record of this evolving gravitational dance can be traced in the geological history of Earth, but only up to about two billion years ago. Beyond that point, Earth’s ancient surface has been reshaped by processes such as tectonic activity, erosion, and volcanic eruptions, leaving little evidence of the Moon’s early relationship with Earth. To understand the distant past, scientists turn to computational models and theories of planetary dynamics to reconstruct the story.

We know that when Earth first formed about 4.5 billion years ago, it lacked a large moon. The current theory for the Moon’s formation is the giant impact hypothesis. According to this model, around 4.4 billion years ago, a Mars-sized protoplanet named Theia collided with the young Earth in a catastrophic impact. This collision ejected massive amounts of debris into orbit, which coalesced over time to form the Moon. The aftermath of this impact radically altered both the Earth and Moon, locking them into their dynamic relationship.

Interestingly, most computer simulations of the Theia impact suggest that the Moon initially orbited Earth at a much closer distance than we would expect, around 10 Earth radii (approximately 64,000 kilometers) compared to its current distance of about 60 Earth radii (384,000 kilometers). Early Earth lacked the vast oceans that today drive the tidal forces pushing the Moon outward. This raises the question: how did the Moon manage to move so far away from Earth in such a relatively short span of time?

A recent study proposes an intriguing solution: in its early history, Earth’s tides were not made of water but of molten lava. After the Theia collision, Earth’s surface was covered by a global ocean of molten rock. With the Moon positioned so close to Earth, its gravitational pull would have induced massive tidal forces in this lava ocean, creating immense waves of molten material. Because lava is much denser and more viscous than water, these tides would have been far more powerful than modern ocean tides.

These intense lava tides would have rapidly transferred angular momentum from Earth to the Moon. As a result, Earth’s rotation would have slowed down more quickly, and the Moon would have moved outward at a much faster rate than it does today. According to the study’s simulations, published on the arXiv preprint server, the Moon’s orbit could have expanded by as much as 25 Earth radii within just 10,000 to 100,000 years. This mechanism offers a plausible explanation for how the Moon reached its current distance more rapidly than water tides alone could account for.

The idea of lava tides has implications far beyond Earth’s history. Many exoplanets, particularly those orbiting close to their parent stars, may experience extreme heat that keeps their surfaces in a molten state for extended periods. For example, planets orbiting very close to their stars might retain global lava oceans for hundreds of millions or even billions of years. On these worlds, tidal forces caused by a nearby moon or the star itself could drive rapid changes in their rotation rates and orbital dynamics, similar to the early Earth-Moon system.

Simulations of such lava worlds suggest that tidal interactions could accelerate their evolution. For instance, lava tides could cause a planet to become tidally locked—where one side always faces the star—within just a few million years, compared to the billion-year timescale expected for planets with water tides. This accelerated locking process would significantly influence the planet’s climate and habitability. Tidally locked planets experience perpetual daylight on one hemisphere and eternal night on the other, creating stark temperature contrasts that would drastically shape any potential life.

If this model is correct, it could reshape our understanding of potentially habitable worlds. Most exoplanets orbit red dwarf stars, which are smaller, cooler, and more numerous than stars like the Sun. Red dwarfs account for about 75% of the stars in our galaxy, and their habitable zones—the regions where conditions might allow liquid water to exist—are much closer to the star than the Sun’s habitable zone. Planets in these tight orbits would have likely started as molten worlds, meaning that lava tides could have played a significant role in shaping their evolution.

This has profound implications for the search for extraterrestrial life. On tidally locked planets around red dwarfs, life would have to adapt to extreme environments. The dayside would be blisteringly hot, while the nightside would be freezing cold. Any habitable regions might exist in the “terminator zone,” the narrow band between light and dark, where temperatures could be more moderate. Life on such planets would likely differ greatly from anything on Earth, driven by the unique conditions of a tidally locked world.

The story of the Earth-Moon system serves as a reminder of the dynamic processes that shape planets and moons across the universe. From molten beginnings to the formation of life, the interactions between celestial bodies have profound and far-reaching consequences. By studying these processes, scientists gain not only a deeper understanding of our own planet’s history but also valuable insights into the nature of distant worlds and the conditions under which life might arise elsewhere in the cosmos.

Source: Universe Today