Mystery solved: why the two sides of the Moon are so different

It's like being able to look inside a watermelon without having to cut it open. You can tell if it's ripe, if it has large or small seeds, if it contains a lot or a little water... all just by examining it from the outside. That's precisely what scientists have been doing for years, although not with watermelons, but with moons , asteroids, and even entire planets. To achieve this, they harness the subtle but powerful force of gravity, and thanks to it, they are able to access the secrets hidden within.

Two recent NASA studies, published in Nature and Nature Astronomy , are excellent examples of how the analysis of gravitational data collected by orbiting spacecraft is revolutionizing our understanding of planetary structures. And they all don't even have to land on their surfaces. Although the Moon, our natural satellite, and Vesta , a giant asteroid that resides in the main belt between Mars and Jupiter, are very different celestial bodies, both investigations have used a similar technique to reveal previously unseen details about their internal compositions.

For the lunar study, published in Nature , researchers developed a new gravitational model of our satellite that takes into account the tiny variations in its gravitational field throughout its elliptical orbit around Earth.

These fluctuations cause the Moon to deform slightly due to the 'tidal force' exerted by our planet, a phenomenon known as 'tidal deformation'. This subtle lunar 'flexing' provides crucial information about its deep internal structure. It would be like squeezing a rubber ball in your hand. Its shape would change due to the pressure. Similarly, the Earth exerts gravitational 'pressure' on the Moon , causing it to stretch and contract slightly as it orbits. How the Moon responds to this pressure—how it deforms—depends on the distribution of mass within its interior. A stiffer interior, in fact, will deform less than a more flexible one.

Thanks to their sophisticated computer model, the researchers produced the most detailed lunar gravity map to date. An extremely precise mapping of lunar gravity that, by the way, is not only useful for this kind of scientific study, but is also an invaluable tool for future space missions .

The achievement was made possible through an exhaustive analysis of data collected by NASA's GRAIL (Gravity Recovery and Interior Laboratory) mission . The mission's twin spacecraft, named Ebb and Flow, orbited the Moon from December 2011 to December 2012, measuring the minute variations in its gravitational field with astonishing precision.

One of the most intriguing findings of this study focuses on the differences between the near side of the Moon (the one it always shows us) and its far side. While the former is dominated by vast, dark plains, known as lunar seas, and are made of molten rock that cooled and solidified billions of years ago, the far side is much more mountainous and rugged, with few seas.

Some theories suggest that the cause of these disparities could be intense volcanism on the near side. The process would have caused the accumulation of heat-generating radioactive elements deep within the mantle. This is something the new study confirms, providing the strongest evidence to date for this hypothesis.

“We found that the near side of the Moon flexes more than the far side ,” explains Ryan Park of NASA’s Jet Propulsion Laboratory in Southern California, leader of both studies, “which means there’s something fundamentally different about the internal structure of the two sides. When we first analyzed the data, the result surprised us so much we didn’t believe it. So we repeated the calculations many times to verify the findings. In total, this represents a decade of work.”

Comparing their results with other existing models, Park's team found a small but significant difference in the amount of deformation between the two lunar hemispheres. The most likely explanation is that the near side is made of materials originating from a warmer region of the mantle. This, moreover, is strong evidence of the volcanic activity that shaped the near side's surface between 2 and 3 billion years ago.

In the second study, published in Nature Astronomy , the researchers applied a similar technique to analyze the rotational properties of Vesta , a celestial body much smaller than the Moon. Using radiometric data from NASA's Deep Space Network and images from the Dawn spacecraft, which orbited the asteroid between July 2011 and September 2012, the team discovered something surprising about its internal structure.

Until now, the prevailing theory suggested that Vesta, like terrestrial planets such as our own, should have well-defined internal layers: a rocky crust, a mantle, and a dense iron core. However, the new findings showed that Vesta's interior could be much more uniform , with a very small or even nonexistent iron core.

To understand how this conclusion was reached, it is necessary to understand the concept of 'moment of inertia'. Imagine a skater spinning with her arms extended. If she draws them in, bringing them close to her body, her spinning speed increases. This is because her moment of inertia decreases as she brings mass (her arms) closer to her axis of rotation. Similarly, by measuring the way Vesta 'wobbles' as she spins, scientists can determine her moment of inertia, a property very sensitive to the distribution of mass within her. A low moment of inertia would indicate a concentration of mass toward the center, while a high moment of inertia would suggest a more even distribution.

Park's team's measurements revealed that Vesta fits the second possibility, suggesting a more even mass distribution and a very small, or even absent, dense core. The finding challenges previous theories about Vesta's formation .

Typically, gravity causes the heavier elements to sink toward the center of a planetary body over time, as happened, for example, with Earth's liquid iron core. Vesta's more homogeneous structure could indicate that it never formed distinct layers or that it formed from the fragments of another planetary body following a massive impact.

It's important to note that this approach of using gravity data to infer the internal structure of celestial bodies is not unique to the Moon and Vesta. In 2016, Ryan Park himself applied the same technique to data from the Dawn mission to study Ceres, the dwarf planet also located in the asteroid belt. The results of that study suggested a partially differentiated interior for Ceres.

More recently, Park and his team extended this methodology to Jupiter's volcanic moon Io. Using data collected by NASA's Juno and Galileo spacecraft during their flybys of the Jupiter moon, and combining them with ground-based observations, the scientists measured subtle changes in Io's gravity as it orbits Jupiter, a massive planet that exerts a powerful tidal pull. Their findings revealed that Io is unlikely to possess a global magma ocean, a hypothesis that had previously been considered.

"Our technique," Park says, "is not limited to Io, Ceres, Vesta, or the Moon. There are many opportunities in the future to apply it to the study of the interiors of intriguing planetary bodies throughout the Solar System."