Astronomers have unraveled a longstanding lunar mystery, discovering why the Moon presents an unusual chemical imbalance on its surface. A recent study suggests that a dramatic “mantle overturn” occurred when the Moon was in its infancy. The Moon’s then-upper layers plunged down, while lower materials surfaced, effectively inverting its mantle.
For years, experts have speculated on the possibility of a lunar mantle overturn. The latest evidence, led by planetary scientists Weigang Liang and Adrien Broquet from the University of Arizona, supports this theory through a gravity map analysis that aligns with the proposed mantle inversion model.
Simulations conducted by the researchers showcase that gravitational anomalies present on the Moon’s near side correspond well with ancient dense mineral deposits. The gravity patterns match computer models of the still-present remnants of heavy minerals like ilmenite.
The team’s paper describes the striking resemblances in the gravity anomalies’ patterns, magnitudes, and dimensions to those anticipated by geodynamic models with ilmenite-enriched accumulations at their heart. The study pinpoints the era this upheaval occurred at roughly 4.22 billion years ago, soon after the Moon’s violent birth from a cosmic collision involving Earth.
The Moon’s surface is notably peculiar, particularly in its KREEP Terrane – a geochemically unique area rich with potassium, rare earth elements, and phosphorus. This region coincides with the lunar maria, vast plains formed by volcanic activity and composed of heavy, ilmenite-rich basalt despite being overlaying less dense rock layers, which defies expectations based on density considerations.
Explanation for this arrangement may lie in the Moon’s early formation stages, during which a molten magma ocean cooled down, leading to the separation of the crust and mantle, with heavy minerals like ilmenite and the concentrated KREEP materials settling in respective layers.
The likelihood of these minerals sinking and then resurfacing due to warming raises the potential of an overturned mantle providing clarification for the KREEP deposits and volcanic titanium-rich basalts discovered on the surface.
However, a challenge to this theory comes from the lack of uniform distribution of these elements, as a significant concentration is located on the Moon’s near side. A possible reason could be the South Pole-Aitken Basin impact on the opposite side, a massive event that could have caused the lateral movement of ilmenite and KREEP materials, predominantly towards the near side.
The team corroborated their mantle overturn models with gravity data from NASA’s GRAIL mission, revealing a pattern of gravity anomalies that confirms the hypothesis and provides a timeline for when the overturn took place.
The researchers’ insights are documented in their publication in Nature Geoscience.
FAQs about the Moon’s Mantle Overturn
- What is a lunar mantle overturn?
- It is a process where the upper layers of the Moon’s mantle sink while lower layers rise, effectively reversing the mantle’s composition and structure.
- How was this mantle overturn discovered?
- Planetary scientists studied gravitational anomalies on the Moon through simulations, which matched patterns on gravity maps. These anomalies correlated with the presence of minerals like ilmenite that should have been buried if the Moon hadn’t undergone a mantle overturn.
- When did the Moon’s mantle overturn happen?
- According to the research, this event is estimated to have taken place around 4.22 billion years ago, shortly after the Moon was formed.
- Why is the Moon’s surface chemically asymmetrical?
- The asymmetry is due to the uneven distribution of certain elements, such as those found in the KREEP Terrane, which may have been caused by mantle overturn and other processes like the South Pole-Aitken impact.
- What was NASA’s GRAIL mission?
- GRAIL (Gravity Recovery and Interior Laboratory) was a mission involving a pair of spacecraft that mapped the Moon’s gravity to help scientists understand its internal structure and composition.
Conclusion
The Moon’s unique geological features continue to fascinate and puzzle scientists. With the revelation of the mantle overturn event billions of years ago, researchers have unlocked a crucial piece of the lunar formation puzzle. This discovery not only sheds light on the Moon’s past but also enhances our knowledge of planetary sciences and the dynamic processes that shape celestial bodies throughout the cosmos.