this is how the moon turned inside out

About 4.5 billion years ago, a small planet crashed into the young Earth. The debris flew into space. Slowly the debris coalesced, cooled, and formed a hard round sphere, our moon. Scientists agree that this is how it happened, but many details remain a mystery. Until now.

Not surprising, because most knowledge about the moon comes from samples that the Apollo astronauts took more than fifty years ago from the rocks they encountered, in combination with theoretical models. And that revealed something strange. The rock samples contained surprisingly high concentrations of titanium, but satellite observations later showed that this type of rock only occurs on the front of the moon, the side facing Earth. Astronomers now finally know more about how and why this is the case.

An ocean full of swirling magma
Because the moon formed quickly and was super hot, it’s likely that it was covered in a magma ocean. As the rock slowly cooled and solidified, it formed the mantle and bright crust that we see when we look at the full moon at night. But deeper below the surface, the young moon was completely out of balance. According to models, the last magma crystallized into, among other things, the mineral ilmenite, which contains titanium and iron.

“Because these heavy minerals are more dense than the mantle beneath them, an unstable gravity is created. You would expect this layer to sink deeper into the moon’s interior,” explains lead researcher Weigang Liang of the University of Arizona. That’s what happened initially, but it mixed with the mantle, melted and returned to the surface as the titanium-rich lava flows we see on the surface today.

Inside out
“Our moon literally turned itself inside out,” says researcher Jeff Andrews-Hanna, whose study in Nature popped up. “But there is little physical evidence that sheds light on the exact sequence of events during this critical phase of the moon’s history and there is much debate about the details of what fell down.”

Cross section of the moon with ilmenite sinking inwards. Image: Adrien Broquet/University of Arizona & Audrey Lasbordes

Because did this material sink as it formed, little by little, or did it sink all at once after the moon had completely solidified? And did it sink all over the moon and come back up at the leading edge, or did it move to the leading edge of the moon and then sink? Also, did it collapse into one large blur or into several small bubbles? “Without proof you can pick your favorite model. However, each model has profound implications for the geological evolution of our moon,” it said.

Thick layer of titanium
The researchers were guided by an earlier study from China. Those models predicted that the thick layer of titanium-rich material beneath the crust moved first to the moon’s leading edge, possibly caused by a massive impact at its far side. It then sank into the moon’s interior through a network of plateaus, somewhat like waterfalls. But as the material sank, it left a trail of titanium just beneath the crust.

“When we saw the model’s predictions, it was like a light bulb went off, because we saw exactly the same pattern when we look at the subtle variations in the moon’s gravitational field. It reveals a network of dense material hiding beneath the crust,” says Andrews-Hanna.

NASA measurements
The astronomers compared the simulation of a sinking ilmenite-rich layer to some small variations in gravity that two NASA spacecraft picked up as they orbited the moon in 2011 and 2012. These anomalies surrounded a vast dark region on the moon’s front covered in volcanic flows.

The authors found that NASA’s gravity measurements were consistent with the simulations of the ilmenite layers and that the gravity field can be used to map the distribution of the ilmenite remnants left behind after the majority sank.

Consistent story
“Our analyzes make it clear that the models and data tell a remarkably consistent story,” Liang said. “Ilmenite migrated to the moon’s front and sank into the interior via cascading steps. This left a trail that led to anomalies in the moon’s gravitational field, as observed by NASA.”

First the moon as we see it from Earth, then in blue the areas with titanium-rich volcanic flows and third in blue the gravity anomalies. Image: Adrien Broquet/University of Arizona

But not only what happened, but also when it happened, is now clearer: the gravitational anomalies were interrupted by the largest and oldest impact basins on the moon’s front and therefore must have formed earlier. Therefore, the ilmenite-rich layer sank more than 4.22 billion years ago. That’s consistent with how it contributed to later volcanism seen on the moon’s surface.

“For the first time, we have physical evidence showing what happened inside the moon during a crucial stage in its evolution and that is very exciting,” concludes Andrews-Hanna. “It turns out that the moon’s earliest history was written beneath its surface. It just needed the right combination of models and data to reveal that story.”