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

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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.<br>Our Moon’s nearside and farside. The farside is more cratered and lacks dark plains which are abundant on the nearside. Images: NASA / LRO / ASU | Graphic: Jatan MehtaObservations 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.<br>Crustal thickness on the Moon’s nearside and farside as inferred from NASA’s GRAIL mission data Illustration: NASA / JPL / GSFC / MIT / IPGPScientists 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.<br>The molten Moon<br>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.<br>Illustration of a young Moon with a global magma ocean and newly forming crust. Image: CCTV+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.<br>The ~2500-kilometer wide South Pole-Aitken basin region (dashed white circle) on the Moon, with some notable craters marked within. Image: Lunar and Planetary InstituteResearchers 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...