Our Moon is two-faced. Science missions unravel its layers.

It wasn’t until 1959 that any being on Earth saw our Moon’s farside for the first time. Because the Moon is tidally locked to Earth, we only ever see its familiar nearside face. In the early days of space exploration, the Soviet Luna 3 and spacecraft succeeding it transmitted the first lunar farside images. While these captured images at low resolutions by today’s standards, the photos nevertheless revealed a far more cratered hemisphere than the Moon’s nearside. It looked more akin to Mercury and Jupiter’s moon Callisto than the gray world in our skies. The Moon’s farside also lacks its nearside’s familiar dark splotches, volcanic plains formed about 3 to 4 billion years ago. To this day, scientists don’t know exactly why our Moon has two distinct hemispheres.

Observations by more spacecraft later on only deepened the dichotomy. Several lunar orbiters showed that the Moon’s farside rocks have a noticeably different chemical composition from nearside ones, meaning they formed in different ways. The twin NASA GRAIL orbiters launched in 2011 revealed that the farside crust is about 20 kilometers thicker on average than the nearside.

Scientists are especially interested in this lunar dichotomy because it’s tied to the Moon’s formation and evolution, in itself tied to Earth. Scientists think Earth and the Moon have a shared origin. The Moon likely coalesced from the orbital shards of a gigantic Earth impact; after a Mars-sized object named Theia collided with the newly born Earth about 4.5 billion years ago. While our planet’s active geological processes have erased hints of Earth’s makeup in those early years, material from the infant Earth that formed the Moon remains largely preserved on our airless cosmic companion. This is even more true for the lunar farside where large regions haven’t been volcanically altered. Studying rocks here can provide scientists with insights not just on the Moon’s split-history but into baby Earth as well. Understanding what caused our Moon’s two faces will also allow us to better gauge why Mars is asymmetric too, as are some other bodies in the Solar System.

The molten Moon

Researchers think Earth may be responsible for lending the Moon’s nearside a thinner crust than its farside. When the Moon coalesced as a molten body from leftover debris of Theia’s collision with Earth, it was about 15 times closer to us than it is today. It became tidally locked soon after, likely within ~100 days, a time that barely registers on geologic timescales. The Earth was still extremely hot from the force of the planetary collision. The Moon’s nearside constantly faced early Earth’s fuming temperatures of about 2500°C, and thus remained molten for longer than the farside. Catalyzed by other factors such as tidal heating, the nearside magma cooled and crystallized into rocky crust slower than the farside. Some crustal elements like aluminum and calcium continued vaporizing on the molten nearside and condensing on the relatively cooler farside, thickening its crust.

At first, scientists thought that having a thinner nearside crust allowed magma inside the Moon’s mantle to ascend and volcanically erupt on the surface more easily than on the farside. This would explain the nearside’s abundant, dark volcanic plains. However, the farside South Pole-Aitken Basin (SPA)—the largest, deepest impact crater on the Moon—refutes that as the only explanation. The asteroid or cometary impact that created the ~2500-kilometer wide basin carved deep into the lunar crust, and possibly excavated some of the mantle too. Later sizable impacts on top of the basin carved further into the surface, making it very thin compared to the rest of the farside. And yet only 3 to 4% of SPA floor is covered in basaltic lava plains. Crustal thickness thus alone cannot explain the contrast in volcanic activity between the Moon’s hemispheres.

Researchers then thought that since the nearside crystallized slower than the farside, its crust and mantle may have accumulated more heat-producing and radioactive elements such as potassium and thorium. High-temperature experiments on Earth suggest that the presence of such elements would lower the melting temperatures of rocks in the nearside mantle, producing about 4 to 13 times more magma than below the farside. Simulations have supported this by showing that about four billion years ago, Oceanus Procellarum, the largest volcanic plain on the nearside, should’ve been hundreds of degrees hotter than the farside crust. Because of the temperature difference, nearside impacts would’ve thus produced deeper and larger craters overall than the farside, making the nearside crust thinner and increasing volcanic activity on the surface.

The deepest lunar basin and nearby Chandrayaan 3

A sample return mission to the South Pole-Aitken Basin has been assessed as the highest priority for lunar exploration in multiple Planetary Decadal Surveys, a report produced every 10 years by the US scientific community to guide future NASA missions. On-ground measurements and sampling of key areas within SPA will help scientists determine the nature and ages of farside crustal and mantle material. Coupled with equivalent results from nearside studies, it will help us know the exact conditions under which the Moon & Earth formed and evolved.

Earlier this decade, two NASA-funded studies identified good surface locations to collect and study potential pieces of the Moon’s mantle in SPA. NASA then planned to launch the Endurance-A mission later this decade, which would see a robotic rover traversing 2000 kilometers across the basin. It would measure rock and soil composition at key locations, and collect about 100 kilograms of samples to bring to Earth. However, even four years after a formal recommendation and consideration, the mission is not yet approved. It will not be ready to fly until next decade.

India’s Chandrayaan 3 mission landed on the Moon’s nearside at the near-polar location of 69.37°S, 32.32°E in August 2023. While not a farside mission, the site’s proximity to the SPA meant that the region’s history and some of its material are linked to the large farside basin. The Chandrayaan 3 rover Pragyan made lunar soil composition measurements using its X-ray spectrometer, which bolstered our Moon’s molten origin scenario from Theia’s impact. There’s a small chance the rover might have stumbled upon lunar mantle material. Some recent analysis found that the mission’s landing site materials originated from a relatively magnesium-rich lunar crust and regolith, representing a rarer aspect of lunar evolution studies and feeding into our understanding of SPA’s history.

Scientists have made a geological map of Chandrayaan 3’s near-polar landing region and found it to be 3.7 billion years old. The region has been significantly altered since its formation by subsequent impacts and their material ejections. Zooming in a bit, other researchers assessed the mission’s landing site to lie above an ancient crater which spans about 160 kilometers across and is up to 4.4 kilometers deep. This inference is primarily based on ejecta trails around the landing site as imaged by the mission’s rover coupled with high-resolution views of the larger region from the Chandrayaan 2 orbiter. Scientists think the ancient crater has been filled with material ejected from subsequent crater-forming impacts in the south polar region. These deposits include swaths of possible mantle material displaced here by the SPA-forming impact.

The first farside samples transform lunar science

The world’s first lunar farside landing was achieved by China, wherein the Chang’e 4 spacecraft lander in a crater within SPA in 2019. However, the crater’s lava plains cover much of the ancient crust, and the mission’s instruments weren’t designed to probe the lunar dichotomy. It wasn’t China’s only mission to the farside though. In 2024, the Chang’e 6 spacecraft collected samples from within SPA’s Apollo impact crater, another farside first. Two years later, fervent studies of the samples have transformed our understanding of the Moon’s evolution by anchoring key events in the lunar timeline. Below is a look at some key discoveries related to advancing our understanding of the Moon’s nearside-farside dichotomy in particular.

• One Chang’e 6 sample study found evidence of a distinct volcanic eruption about 4.2 billion years ago amid materials otherwise comprising younger, 2.8-billion-year aged basalts. Scientists concluded that the former was due to the presence of heat-producing and radioactive elements whereas the latter’s eruptions happened despite the lunar interior beneath the landing site being deprived of such volcanic catalysts. Combined with studies of 2-billion-year young volcanic samples fetched by the Chang’e 5 mission, these findings validated heat-producing and radioactive elements as a catalyzing factor in the nearside’s volcanic activity being greater than the farside.

• Until recently, all direct evidence of our Moon being covered in a global magma ocean shortly after its formation came from Apollo and Luna samples sourced from nearside equatorial and near-equatorial regions. Surface measurements made by Chandrayaan 3’s Pragyan rover in 2023 extended this hypothesis’ validity to high-latitude regions on the nearside. But we lacked any such tactile measurements from the Moon’s farside. Recently though Chinese researchers studying two grams of Chang’e 6 samples confirmed the presence of key chemical elements that are compatible with a fully molten young Moon. Chang’e 6 thus lent unequivocal credence to said hypothesis, anchoring a key event in the Moon’s origin and evolution.
• In another critical study, researchers analyzed 1600 fragments from five grams of Chang’e 6 samples and found 20 relevant pieces to determine the truest age yet of the SPA basin as being 4.25 billion years. This has allowed scientists to now place stringent constraints on when the farside and nearside crusts formed, and thus plan future measurements to evaluate the refined models against.

• The expectations of scientists that the SPA-forming impact would change the physical and chemical makeup of the Moon’s mantle down to hundreds of kilometers was also validated. The dominant Chang’e 6 basalts from lava erupted ~1.4 billion years after SPA’s formation do exhibit a unique makeup compared to nearside volcanic lunar samples. A study of 16 fragments scooped up by Chang’e 6 found them severely lacking elements such as titanium and thorium. More such findings across SPA and the farside would help build a clearer picture of the lunar mantle.

• Another study analyzing 578 particles revealed for the first time that the Moon’s farside mantle contains less water than within the nearside, another factor in the contrasting farside-nearside volcanic activity. The finding added to the debate on the topic by lending a solid and unique credence to the hypothesis that our Moon indeed lost most of its water during its fiery formation. CASC’s news release on the study noted how Francis McCubbin, NASA’s Astromaterials Curator and a peer reviewer of the paper, called the work “a landmark study on the water abundance of the lunar farside.”

Chang’e 6 has not only transformed our understanding of the Moon’s farside and its evolution but it has also helped scientists worldwide determine the next set of finer measurements to perform with future missions as we continue unraveling the history of our Moon, Earth, and the Solar System. 🌙

Future missions

The lunar farside will be a destination for several upcoming missions, albeit most will perform cosmic radio astronomy instead. NASA had planned to have farside geology related instruments on one commercial lander part of its CLPS program study the pristine Schrödinger crater on the Moon’s farside, particularly the region’s internal structure and heat flow. The payloads were contracted to be sent on ispace US’ first CLPS mission through Draper Laboratory. However, the contract to carry NASA payloads has been cancelled, and was announced as such on July 15. It’s possible NASA reallocates the flight-ready payloads to another CLPS lander but we’ll have to wait and watch for future announcements following feasibility studies.

Later this decade or early next though, India’s Chandrayaan 4 & Chandrayaan 5 spacecraft and several crewed missions from China and the US’ Artemis program aim to explore south polar sites, which are intertwined scientifically to the SPA and the lunar farside’s internal structure. Other than findings from on-ground measurements, there will studies of samples from Chandrayaan 4 and crewed missions, which are expected to contain pieces of ancient polar crustal and potential mantle materials. Much like Apollo, studying a fresh set of novel samples will likely transform not just how we look at our Moon and Earth but also the Solar System and beyond. Luna’s pristine interior compared to other geologically active worlds means it helps researchers understand the baseline layer of processes that drive the interior evolution of rocky worlds everywhere.

Another key site could be Moscoviense, a large lava plain on the farside. Missions here will likely be robotic but studying how its volcanic rocks differ from the nearside plains will not only inform scientists about nearside-farside volcanic contrasts but also lend more insights into how the two mantles evolved differently.

Being the most accessible and well preserved body in our Solar System to study fundamental planetary processes, our Moon beckons. 🌗