How Moon’s mantle may have formed after planet smashed into Earth

It is believed that 4.5 billion years ago a young Earth was smashed into by another proto-planet called Theia. The resulting debris formed our Moon. Scientists now think they have a better understanding of how this may have formed the Moon’s mantle.

Understanding these mechanisms might help make sense of how other rocky planets like Mars or Earth formed. Mantle sits between the core and crust of rocky planets and plays the crucial role in many geological phenomena including volcanism and the movement of tectonic plates.

The Theia hypothesis was first proposed nearly 80 years ago. It is supported by observations including the fact that rocks on Earth and the moon have the same isotope signatures, and the Moon’s orbit having approximately the same period as the Earth’s rotation.

New insights published in Nature Geoscience attempt to explain the early stages of how the Moon’s mantle formed.

The Moon’s crust and underlying mantle probably formed when the molten magma ocean that once covered it, crystallised into a solid. But this process has been a mystery until now.

One theory is that the last dregs of the magma formed dense minerals including ilmenite – a mineral containing titanium and iron. These layers would have formed between the crust and the mantle.

The ilmenite-rich layer is then thought to have sunk further into the mantle due to gravity. This “mantle overturn” could help explain why some volcanic rocks on the Moon are so rich in titanium.

Evidence for lunar mantle overturn has not been found until now.

Simulations of an ilmenite-rich layer sinking into the Moon’s mantle were compared to gravitational data from NASA’s GRAIL (Gravity Recovery And Interior Laboratory) mission which has been mapping variations in the lunar gravitational field for more than a decade.

Gravity gradient map of the lunar nearside and cross-section showing mantle
Gravity gradient map of the lunar nearside and cross-section showing two ilmenite-bearing cumulate downwellings from lunar mantle overturn. Credit: Adrien Broquet & Audrey Lasbordes.

Anomalies centre on regions of past lunar volcanism. This data was then combined with the ages of basins on the Moon’s surface to work out when mantle overturn could have taken place.

The researchers found that the simulations of mantle overturn of ilmenite layers match up with gravitational data. They found that the ilmenite-rich layers sank more than 4.22 billion years ago, in geological terms, just after the Moon formed.

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