Our Moon may appear to glow peacefully in the night sky, but billions of years ago it was flattened by volcanic upheaval.
One question that has remained unanswered for decades is why there are more titanium-rich volcanic rocks, such as ilmenite, on the near side as opposed to the far side. Now, a team of researchers at the Arizona Lunar and Planetary Laboratory offers a possible explanation for this.
The lunar surface was once flooded by a bubbling ocean of magma, and once the ocean of magma solidified, there was a huge impact on the reverse side. The heat from this impact spread to the near side and made the crust unstable, causing layers of heavier and denser minerals at the surface to gradually sink deep into the mantle. They melted again and were erupted by volcanoes. The lava from these eruptions (more of which occurred on the near side) ended up in what are now titanium-rich flows of volcanic rock. In other words, the old face of the moon disappeared, only to resurface.
What lies beneath
The region of the Moon in question is known as the Procellarum KREEP Terrane (PKT). KREEP stands for high concentrations of potassium (K), rare earth elements (REE) and phosphorus (P). This is also where ilmenite-rich basalts are found. Both KREEP and basalts are believed to have first formed as the Moon cooled from its magma ocean phase. But the region remained hot because KREEP also contained high levels of radioactive uranium and thorium.
“The PKT region … represents the most volcanically active region of the Moon as a natural result of the high abundance of heat-producing elements,” the researchers said in a study recently published in Nature Geoscience.
Why is this region located on the near side while the other side lacks KREEP and ilmenite-rich basalts? There was one existing hypothesis that caught the attention of the researchers: It suggested that after the magma ocean solidified on the near side, the sheets of these KREEP minerals were too heavy to stay afloat. They began to sink into the mantle and down toward the mantle-core boundary. As they sank, these mineral sheets are thought to have left traces of material throughout the mantle.
If the hypothesis were correct, it would mean that there should be trace minerals from the solidified KREEP magmatic crust in sheet-like configurations beneath the lunar surface that could reach all the way to the edge of the core-mantle boundary.
How can this be tested? Gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) mission to the Moon may have had the answer. This will allow them to detect gravitational anomalies caused by the higher density of the KREEP rock compared to the surrounding material.
Surfacing
GRAIL data previously revealed that there is a pattern of subsurface gravity anomalies in the PKT region. This looked similar to the pattern that sheets of volcanic rock were predicted to make as they sank, so the research team decided to run a computer simulation of KREEP’s sinking to see how well the hypothesis matched the GRAIL findings.
Of course, the simulation ended up forming roughly the same pattern as the anomalies detected by GRAIL. The polygonal pattern seen in both the simulations and the GRAIL data most likely means that traces of heavier KREEP and ilmenite-rich basaltic layers remained below the surface as these layers sank due to their density, and GRAIL has discovered their remnant due to their greater gravitational pull. GRAIL also suggests that there are much smaller anomalies in the PKT region, which makes sense given that much of the crust is made up of volcanic rocks that are thought to have sunk and left behind remnants before melting and resurfacing through eruptions.
We already have an idea of when this phenomenon happened. Because there are impact basins dating back to about 4.22 billion years ago (not to be confused with the earlier impact on the far side), but the magma ocean is thought to have solidified before then, researchers believe the crust also started to sink before that time.
“The PKT boundary anomalies provide the most direct physical evidence for the nature of the post-magmatic ocean … mantle overturning and sinking of ilmenite into the deep interior,” the team said in the same study.
This is just one more piece of information about the evolution of the Moon and why it is so uneven. The nearby country once raged with lava that is now volcanic rock, much of which exists in streams called mare (which translates to “sea” in Latin). Most of this volcanic rock, especially in the PKT region, contains rare earth elements.
We can only confirm that there are indeed traces of ancient crust inside the Moon by collecting actual lunar material far below the surface. When the Artemis astronauts were finally able to collect samples of volcanic material from the Moon on the spotwho knows what will surface?
Nature Geoscience, 2024. DOI: 10.1038/s41561-024-01408-2
