The Earth’s moon has existed since the early days of the solar system, but exactly when it came into being has been a matter of debate – dating based on lunar rocks differed significantly. Now a new reconstruction of early lunar history provides a solution to these contradictions. Accordingly, the Earth’s moon was formed 4.51 billion years ago. However, its crust largely melted again due to the initially much stronger tidal forces of the nearby Earth, as planetary researchers report. This melting also reset the “geological clock” in the form of minerals and isotopic composition of the lunar crust to zero – and thus simulated a younger age of the moon. The new model could also help clarify some other discrepancies in lunar geology.
The history of the moon begins with a massive collision early in the solar system. The protoplanet Theia, which was about the size of Mars, collided with the young Earth. As a result, the Earth was almost destroyed and huge amounts of vaporized material were ejected into space. The moon was formed from this debris orbiting the Earth. It was initially completely covered by an ocean of hot, liquid rock, which then solidified into the lunar crust over the following millions of years. But when these events happened and how old the moon is as a result has been controversial until now. While most analyzes of lunar rocks have suggested that the Earth’s satellite formed 4.35 billion years ago, other studies date its formation to around 4.51 billion years ago. Some individual zircons in particular seemed to indicate this older age.
Early heat due to tidal forces
But what is behind these discrepancies? A team led by Francis Nimmo from the University of California in Santa Cruz may now have clarified this question. For their study, they examined how the moon developed in its early days. “We are particularly interested in the phase when the distance between the Earth and the Moon was only about a third of the current distance,” explains Nimmo. Back then, the moon’s orbit was not only narrower, but also more elliptical than it is today. This caused the Earth’s gravitational influence to vary, resulting in stronger tidal forces than today. These compressed and stretched the moon’s interior alternately, thereby heating up the rock considerably. Something similar can still be observed today with Jupiter’s innermost moon Io. It owes its enormous volcanic activity and internal heat to Jupiter’s strong tidal forces. Nimmo and his team have now used a model to determine how strong this effect was on Earth’s early moon.
The calculations showed that the heat generated by tidal forces was enough to melt and overturn the entire mantle of the young moon. This led to the already solidified moon crust largely melting again – possibly even several times. Although this did not create a new magma ocean spanning the moon, large parts of the crustal rock could have liquefied, as the researchers report. This subsequent melting also influenced the “geological clocks” in the lunar crustal rocks. Because when the rock is molten, it exchanges isotopes with its surroundings. Its solidification ends this and, in a sense, “freezes” the current isotope level. The decay of these atoms then allows us to date the time at which the rock solidified. “The moon rock therefore does not reveal its original age, but only when it was last strongly heated,” explains co-author Thorsten Kleine from the Max Planck Institute for Solar System Research in Göttingen.
Geological clocks misaligned
This means that the subsequent melting of large parts of the lunar crust also distorted the geological clocks of the lunar minerals. “The strong volcanism may have restarted the moon’s geological clock and thus adjusted it,” explains Kleine. Evidence of the more distant lunar past is therefore only provided by a few heat-resistant zircons. They survived the melting phase unscathed because they were located in cooler areas of the lunar crust that had not been melted by volcanism – and reveal the true age of the Earth’s satellite. Accordingly, the moon would have to be between 4.43 and 4.51 billion years old. However, the melting phase shaped its crust until around 4.35 billion years ago – and thus led to the falsification of many previous dates. “In their diversity, the rock samples from the moon tell us its entire, eventful history. They report on its formation and its later violent volcanism. “We just haven’t read these instructions properly,” says Kleine.
The new findings also resolve several further contradictions about lunar geology. So far, its comparatively few large impact basins seemed to speak against the moon’s old age. Later volcanism now offers an explanation. “Lava from the interior of the moon could have filled the early impact basins and thus made them unrecognizable,” explains senior author Alessandro Morbidelli from the Collège de France in Paris. Another mystery was the composition of the lunar mantle. Their list of ingredients differs from Earth’s in key respects. However, if the moon’s interior was melted a second time, some materials from the mantle could have disappeared into the underlying iron core, thus explaining the relative lack of metal-loving elements in the lunar mantle. “The new results bring together all the pieces of the puzzle that previously didn’t fit together to form a coherent overall picture of the moon’s formation,” says Kleine.
Source: Francis Nimmo (University of California, Santa Cruz) et al., Nature, doi: 10.1038/s41586-024-08231-0