When you gaze up at the Moon from Earth, you are always looking at its near side, with those familiar dark patches, known as maria, that have inspired countless stories over the centuries. So when Soviet and American missions glimpsed the far side of the Moon for the first time in the 1960s, it came as a surprise that this foreign landscape was so different in appearance, with barely any traces of maria.
The origin of this remarkable distinction, known as the nearside-farside lunar asymmetry, has puzzled scientists for decades. Now, a team led by Matt Jones, a PhD candidate at Brown University, proposes that a cataclysmic impact that created one of the largest craters in the solar system, the South Pole–Aitken (SPA) basin, may have forged the stark asymmetry, which the researchers note “has remained unexplained since its discovery in the Apollo era,” according to a paper published on Friday in Science Advances.
“Giant asteroid impacts deposit huge amounts of heat in planetary interiors,” said Jones in an email. “Awareness of this phenomenon and its consequences have become more widespread among planetary scientists over the last decade or two, but there's a lot more work to be done.”
Jones was inspired to dig into this tantalizing enigma by his PhD advisor Alexander Evans, an assistant professor and planetary scientist at Brown, and co-author of the study, who has researched the evolution of these impact basins on the Moon and their potential role in the lunar asymmetry.
“When I started my PhD, I was thrilled to take the lead on such an impactful study (pun intended) as explaining the lunar compositional asymmetry,” Jones said. “In my PhD work, I'm eager to contribute to major questions about the early evolution of rocky planets while I develop my analytical and computational skills. The physics-and computing-intensive nature of this project made it the perfect opportunity for me.”
The lunar near side is decorated with iconic features like Mare Tranquillitatis, where humans first stepped foot on the Moon, and Mare Imbrium, which forms the right eye of the traditional image of a “Man in the Moon.” These maria were created by lava that flowed some four billion years ago, producing patches of smooth and dark terrain that contrast with the cratered landscapes surrounding them.
Much of the lunar nearside is also rich in the so-called KREEP signature, which is an acronym for the chemicals potassium (atomic symbol K), rare Earth-elements (R-E-E), and phosphorus (atomic symbol P). In the new study, Jones and his colleagues suggest that a massive impact in the Moon’s infancy may have asymmetrically distributed these volatile materials, along with the heat-producing radioactive element thorium, to the lunar nearside, where they eventually helped to drive the volcanism that created the maria.
The team points to the SPA basin, a crater that stretches for an astonishing 1,600 miles across the south lunar region, as a key piece of evidence for their hypothesis. This immense formation was blasted into existence by a projectile about 4.2 billion years ago. The crash was powerful enough to excavate material buried more than 100 miles under the Moon’s surface, spraying ejecta around the lunar south pole.
In simulations of the impact and its aftermath, Jones and his colleagues demonstrated that the SPA impact could have also spurred convection within the Moon’s mantle. This process erupted into volcanism, fueled by KREEP materials, hundreds of millions of years later. The location of the impact, among other factors, ended up affecting the nearside far more than the farside, according to the team’s models.
“We expected to find a plausible link between the SPA impact and the lunar compositional asymmetry, but we were blown away that the link showed up in every scenario we tested,” Jones explained. “My surprise grew when I started to realize how consistently our model fits with a wide array of previous, independent studies. In scientific research, it isn't common to find such a clear connection between phenomena that persists across all scenarios. I feel lucky as a graduate student to be the lead author on an exciting study like this.”
In addition to shedding light on the weird idiosyncrasies of the Moon, the hypothesis could “provide insight to the giant impact origin hypothesis” on Mars, which has been proposed to explain the difference between the north and south hemispheres of the red planet, according to the new study.
The mystery of the Moon’s asymmetry may be further unraveled by upcoming missions to the lunar south pole, such as China’s robotic Chang'e 6 mission, which is tasked with returning the first rock samples from SPA, as well as NASA's Artemis program, which aims to send humans back to the lunar surface.
With these new endeavors, “lunar science will advance in leaps in bounds,” Jones said. “Chemical analyses of samples returned from SPA will help clarify our model of the lunar compositional asymmetry. Another boon would be to expand the network of seismometers on the Moon. Seismometers allow geoscientists to ‘image’ the interiors of planets, and better seismometer coverage leads to better image resolution.”
“Adding seismometers to the farside of the Moon to supplement the three currently on the nearside would give lunar scientists a better picture of the Moon's present-day interior, which we use to constrain certain aspects of the models in our study,” he concluded.
Update: This article has been updated to include comments from lead author Matt Jones.