A team of scientists from the Lunar and Planetary Laboratory at the University of Arizona has published a new research paper on the evolution of the Moon's interior. The team highlighted the instability of the Moon's ancient interior and its chaotic early development. They stated that the Moon literally turned upside down after separating from the Earth, according to the research paper published in the journal Nature Geoscience. Co-author Jeff Andrews-Hanna, an associate professor at the Lunar and Planetary Laboratory (LPL) at the University of Arizona, explained in a press release: "Our Moon literally flipped upside down. However, there was little physical evidence to illuminate the precise sequence of events during this critical phase of lunar history, and there is much disagreement about the details of what literally happened."
**How Did the Moon Form?**
About 4.5 billion years ago, a small planet collided with the Earth. The impact caused molten rocks to spread far enough into space that they began to orbit around our planet. Over time, those rocks solidified to form our Moon. Although we do not have direct evidence of this impact event, the global scientific community largely agrees that this is the most likely scenario for the formation of our celestial neighbor. Lunar rock samples collected by Apollo astronauts provided hints and new insights into the Moon's ancient history. For example, the basalt lava rocks extracted from the Moon’s surface contained surprisingly high concentrations of titanium. Subsequent satellite observations of the Moon’s surface showed that these volcanic rocks are more prevalent on the near side of the Moon. Now, the new model from the University of Arizona team sheds light on why this is the case.
The Moon may have been covered by a magma ocean when it first formed. As the molten rocks cooled, they formed the Moon's mantle and outer crust. However, beneath the surface, the small Moon was incredibly unstable. Computer models suggest that the magma beneath the crust likely crystallized into dense minerals, including ilmenite that contains titanium and iron. Leading the study as part of his PhD at the Lunar and Planetary Laboratory, Weigang Liang explained: "Because these heavy metals are denser than the underlying mantle, they create gravitational instability, and this layer is expected to sink deeper into the Moon’s interior.”
In the thousands of years that followed, the dense material sank into the Moon's interior, where it mixed with the mantle and melted. However, it eventually returned to the surface via titanium-rich lava flows that can be seen today. The scientists found that the Moon's gravitational field can be used to map the distribution of the remnants of ilmenite formed after the sinking of dense material into the Moon's mantle. Liang stated: "Our analyses show that the models and data tell a remarkably consistent story. Ilmenite materials migrated to the near side and sank within in plate-like waterfalls, leaving remnants that cause anomalies in the Moon's gravitational field, as we have seen from NASA."
Based on their findings, the scientists believe that the ilmenite-rich layer sank more than 4.22 billion years ago. This discovery aligns with volcanic activity later in the Moon's development.
