Mysterious gravity signatures are a window into the Moon’s early history
DOI: 10.1063/pt.ulry.chfr
In 2012 NASA’s GRAIL (Gravity Recovery and Interior Laboratory) mission mapped the gravity of the Moon. Naturally, the gravity was stronger above the mountains and weaker above the craters. But when astronomers removed the topographical contributions, GRAIL data revealed something curious where the topography was featureless: straight lines where the gravitational field is anomalously strong were surrounding a large, low region on the Moon. The cause, it seems, comes from something below the crust.
The near side of the Moon has a familiar dark region that is covered by titanium-rich volcanic flows. Rough circles in the gravitational map correspond to the craters of the region, but the strange, linear anomalies (blue) don’t have a visual counterpart. Instead, they are the signatures of dense material beneath the surface. The presence of titanium-rich materials sinking below the surface is the first physical evidence for a key event in lunar formation.
Adrien Broquet/University of Arizona
The Moon started as a magma ocean, roiling and boiling in the moments after it coalesced. Astronomers strongly suspect that the magma cooled quickly, crystallizing various minerals in mere hours. The last to solidify likely included such metals as titanium. They would have crystallized into the densest material in the interior, counterintuitively higher in the mantle than less dense materials. Because of that unstable arrangement, astronomers suspect that the Moon’s mantle overturned as the titanium-rich materials sank lower. In one model that Nan Zhang and colleagues at Peking University published in 2022 , dense materials in the Moon’s interior had migrated to the near side before forming sheet-like slabs and sinking deeper into the mantle. The theory of some sort of overturn has been widely accepted, but physical evidence has been hard to come by.
Weigang Liang and Adrien Broquet, working with Jeff Andrews-Hanna at the University of Arizona, recognized a similarity between the predicted remnants of those dense, titanium-rich slabs below the crust and linear gravity anomalies seen in the GRAIL data. Andrews-Hanna and his research team reached out to Zhang and began an investigation into whether the connection was real.
They simulated different geometries, depths, and densities of the titanium-rich material to compare with the GRAIL data, and they mathematically inverted the GRAIL data to determine the most likely material properties. Because they approached the work both from geophysical first principles and from the gravitational data, the results are independent. The conclusions from both investigations are consistent, indicating that sheet-like slabs of dense, titanium-rich material could create linear signatures in the gravitational maps.
Together, the results are long-awaited physical confirmation that there was an overturning in the Moon’s mantle. Moreover, they indicate that the overturning event would’ve happened at least 4.2 billion years ago, merely a few hundred million years after the Moon was formed. (W. Liang et al., Nat. Geosci. 17, 361, 2024 .)