The Moon literally turned itself inside out during early formation

Schematic illustration with a gravity gradient map (blue hexagonal pattern) of the lunar nearside and a cross-section showing two ilmenite-bearing cumulate downwellings from lunar mantle overturn. Credit: Adrien Broquet/University of Arizona & Audrey Lasbordes.

The story of how the Moon formed is one of fire and violence. Around four and a half billion years ago, there was no Earth or Moon. Both bodies formed out of the vaporized remnants of a giant collision between proto-Earth and another smaller planet the size of Mars. The powerful impact instantly melted the crust and outer part of the mantle of proto-Earth, mixing them with bits from the rogue planet. From the resulting cloud of debris, the Moon quickly formed. This explains why the isotopic composition of the Earth and the Moon is so strikingly similar.

Well, that’s the gist of it — but the devil is often in the details. There are still many things we don’t know about what happened between this violent early formation and where the Moon is today. A new study by researchers at the University of Arizona Lunar and Planetary Laboratory (LPL) is now peeling off another layer of this story.

What happened to the Moon

Initially, the moon was enveloped in a global ocean of molten rock. As this ocean cooled and solidified, it formed the Moon’s mantle and the familiar bright crust that lights up our night sky. Yet, hidden beneath this serene exterior, the Moon was anything but tranquil. Dense minerals, heavy with titanium and iron, began to crystallize from the last remnants of the magma ocean.

These minerals, including ilmenite, introduced gravitational instability. Being denser than the layers above them, they were destined to sink deeper into the moon’s interior. In doing so, the minerals mixed with the mantle, melted, and later reappeared on the surface as titanium-rich lava flows, the researchers claim. It’s a bit like the moon turned itself inside out.

“Our moon literally turned itself inside out,” said co-author and LPL associate professor Jeff Andrews-Hanna. “But there has been little physical evidence to shed light on the exact sequence of events during this critical phase of lunar history, and there is a lot of disagreement in the details of what went down — literally.”

The lunar near side with its dark regions, or “mare,” covered by titanium-rich volcanic flows (center) makes up the moon’s familiar sight from Earth (left). The mare region is surrounded by a polygonal pattern of linear gravity anomalies (blue in image on the right) interpreted to be the vestiges of dense material that sank into the interior. Their presence provides the first physical evidence for the nature of the global mantle overturn more than 4 billion years ago. Credit: Adrien Broquet/University of Arizona. — The lunar near side with its dark regions, or “mare,” covered by titanium-rich volcanic flows (center) makes up the Moon’s familiar view from Earth (left). The mare region is surrounded by a polygonal pattern of linear gravity anomalies (blue in image on the right) interpreted to be the vestiges of dense material that sank into the interior. Their presence provides the first physical evidence for the nature of the global mantle overturn more than 4 billion years ago. Credit: Adrien Broquet/University of Arizona.

The researchers carefully analyzed rock samples brought back by Apollo astronauts more than 50 years ago. They combined this with advanced satellite observations and theoretical models.

A heavy burden

This study picked up where Nan Zhang of Peking University in Beijing left off with a 2022 study. Previously, Zhang, who is also a co-author of the new study, published a computer model of the Moon’s composition. It predicted that titanium-rich material beneath the Moon’s near-side sank deep into the interior in a network of sheetlike slabs. The movements of these slabs can be envisioned like a waterfall, with each slab cascading down towards the lunar core.

But was this truly the case? Another prediction of the model was that not all the material sank; remnants formed in a geometric pattern of intersecting lines of dense titanium-rich rock beneath the crust. This was the smoking gun that the researchers were after.

In the recent study, researchers analyzed simulations of a dense layer of ilmenite, a titanium-iron oxide, sinking beneath the moon’s surface. They compared these simulations with linear gravity anomalies — unexpected variations in gravitational pull — identified by NASA’s GRAIL mission. Between 2011 and 2012, GRAIL’s twin spacecraft orbited the moon, mapping subtle changes in its gravitational field. These anomalies encircle a large, dark area on the near side of the moon, covered by volcanic flows called mare.

The study reveals that the gravity anomalies detected by GRAIL match the expected signature of ilmenite deposits. It seems that the Moon’s gravity field indicates the spread of ilmenite remains after most of this layer sank.

“Our analyses show that the models and data are telling one remarkably consistent story,” said Weigang Liang, the lead author of the study who performed the research while pursuing a PhD at LPL. “Ilmenite materials migrated to the near side and sunk into the interior in sheetlike cascades, leaving behind a vestige that causes anomalies in the moon’s gravity field, as seen by GRAIL.”

The Moon’s linear gravity anomalies are disrupted by its largest and oldest impact basins on the near side. So, the ilmenite layer would have sunk before these basins formed. This can only mean that the event happened over 4.22 billion years ago, influencing the volcanic activity observed on the lunar surface afterward.

Lopsided Mysteries and Future Explorations

Animation rotating moon — Credit: NASA/GSFC/Arizona State University.

The Moon’s asymmetry is another source of mystery. The far side crust is thicker and has a different composition than the near side. Its surface is also much paler, with fewer dark basalt splotches, and is covered in more craters.

The most peculiar thing is a region called the Procellarum KREEP Terrane, on the Moon’s nearside. This region is unusually rich in specific elements, which gives it its name: K (the atomic symbol for potassium), REE (rare-earth elements), and P (the atomic symbol for phosphorus).

The internal upheaval highlighted by the new study may explain why the moon is fundamentally lopsided, with the near side — the side always facing Earth — being so distinct from the far side (the side in perpetual darkness).

“Our work connects the dots between the geophysical evidence for the interior structure of the moon and computer models of its evolution,” Liang added.

“For the first time we have physical evidence showing us what was happening in the moon’s interior during this critical stage in its evolution, and that’s really exciting,” Andrews-Hanna said. “It turns out that the moon’s earliest history is written below the surface, and it just took the right combination of models and data to unveil that story.”

The new findings appeared in the journal Nature Geoscience.