The Moon, Earth’s constant companion, holds many secrets about the early days of our solar system, some of which have been elusive for scientists for years. One of the most intriguing aspects of lunar studies is determining the age of the Moon and understanding the geological history that shaped it. New findings published in Nature by an international team of researchers offer new insights into the Moon’s formation and its subsequent volcanic history, shedding light on previous contradictions in the Moon’s age. These revelations come from an in-depth study conducted by scientists from the University of California Santa Cruz, the Max Planck Institute for Solar System Research (MPS), and the Collège de France.
The study suggests that the Moon was formed around 4.43 to 4.51 billion years ago, a period in the early history of the solar system when planetary bodies were still undergoing violent interactions. The study’s findings resolve a longstanding mystery in lunar research: the inconsistency in the age of the Moon, as revealed by various lunar rock samples. Some rocks pointed to an age of 4.35 billion years, while others suggested the Moon’s formation occurred up to 4.51 billion years ago. These discrepancies have puzzled scientists for years, especially given the assumption that the Moon should be older than most of the samples suggested.
The key to understanding this issue lies in the extreme volcanic activity that occurred shortly after the Moon’s formation. The researchers propose that shortly after its birth, the Moon underwent an intense phase of volcanism driven by tidal forces. During this time, the Moon’s crust was subjected to extreme heat, and its interior was churned and melted, possibly several times. This volcanic activity played a pivotal role in resetting the Moon’s geological clock, so that much of its crust became geologically “young” despite the Moon itself being older.
The foundation for this conclusion lies in the Moon’s early history, which began with a catastrophic collision. A massive object, roughly the size of Mars, collided with the still-forming Earth. This collision generated enough heat to melt Earth entirely and sent large amounts of material into space. This debris eventually coalesced into the Moon. Initially, the Moon was covered by an ocean of hot, molten rock. Over time, it cooled, and the crust began to solidify. As the Moon moved further from Earth and entered its current orbit, which is approximately 384,400 kilometers from Earth, it continued to evolve. However, it was during the early phases of its existence, when it was much closer to Earth, that the most dramatic geological processes took place.
One of the most significant periods in this early history occurred when the Moon was much closer to Earth—about one-third of its current distance. This proximity meant that the Moon’s orbit was more elliptical, which caused the tidal forces between Earth and the Moon to be much stronger than they are today. These tidal forces would have been intense enough to heat the Moon’s interior, leading to massive volcanic activity. This process, similar to what is seen on Jupiter’s moon Io today, would have generated vast amounts of energy, resulting in a violent cycle of volcanism. The researchers argue that this volcanic activity melted much of the Moon’s crust and mantle, allowing heat from the interior to permeate and liquefy surface materials.
However, not all of the Moon’s crust melted at once. Instead, it was a gradual process, with some regions experiencing more intense volcanic activity than others. In some areas, lava flowed to the surface, while in others, magma was injected beneath the surface, heating the surrounding rocks. This volcanic history played a crucial role in the age discrepancies seen in lunar rocks. Lunar rocks, like those on Earth, contain radioactive isotopes that decay over time. The decay of these isotopes can be used to estimate the age of the rocks. However, if the rocks were subjected to extreme heat, their isotopic composition could have been altered, “resetting” the geological clock.
The extreme heat during the Moon’s early volcanic period would have caused most of the isotopic clocks in lunar rocks to reset, making them appear younger than their actual age. This explains why most lunar rock samples indicate an age of around 4.35 billion years—when the last major volcanic activity took place—rather than the Moon’s actual formation age of between 4.43 and 4.51 billion years. The few zircons (tiny crystals of zirconium silicate) found in lunar rocks were more resistant to the heat, and their isotopic clocks were not reset by the volcanic activity. These zircons provide evidence of the Moon’s more distant past and support the conclusion that the Moon itself is older than the volcanic crust that we see today.
The implications of these findings go beyond just resolving the lunar age mystery. The new research also offers explanations for other inconsistencies that have long puzzled scientists. For example, the Moon’s relatively low number of impact craters has raised questions about its age. If the Moon were as old as some researchers had suggested, it should have accumulated more craters from asteroid and comet impacts over billions of years. However, the volcanic resurfacing of the Moon may have played a role in erasing older craters. Lava from the Moon’s interior could have filled in ancient impact basins, making it difficult to recognize older impacts. This volcanic activity effectively reset the Moon’s surface, giving it a relatively younger appearance than expected for its true age.
The composition of the Moon’s mantle also presented an anomaly for scientists. The Moon’s mantle differs from Earth’s in several key aspects, and for years, researchers struggled to understand why. The new research proposes that the Moon’s mantle may have lost some of its substances to the iron core beneath it during the intense volcanic activity. If the Moon’s interior was partially molten after its formation, some of the material from the mantle could have migrated to the core, altering the composition of the Moon’s surface.
The new study offers a coherent explanation for these contradictions and fits all the pieces of the puzzle into a unified theory of the Moon’s formation and evolution. By combining calculations and observations of lunar rocks, the researchers have shown that the Moon’s formation occurred around 4.43 to 4.51 billion years ago, with significant volcanic activity shaping its crust around 4.35 billion years ago. This violent volcanism, driven by tidal forces, played a critical role in shaping the Moon’s surface and resetting the isotopic clocks of lunar rocks.
The implications of this study are far-reaching, not just for understanding the Moon’s history, but also for the broader study of planetary formation. The processes that shaped the Moon could offer insights into the early history of other planetary bodies in our solar system, especially those that have undergone intense volcanic activity or were formed through catastrophic collisions. The findings also highlight the importance of studying the geological history of celestial bodies to understand the early conditions of our solar system and the forces that shaped the planets and moons we see today.
Source: Max Planck Society