In a new paper published in the Journal of Geophysical Research—Planets, a group of scientists led by Meng-Hua Zhu from the Macau University of Science and Technology suggest that our Moon, after its birth in a huge collision between the very early Earth and a Mars-sized object, was hit by a second huge planetoid shortly after its formation. That would explain why the Moon’s near side and far side are so different in topography, crustal thickness, and composition.
Of course, we never see the lunar far side from Earth because the Moon is tidally locked to our planet. Only in the Space Age has it become clear just how different the far side is, and it was not until this year that a spacecraft, China’s Chang'e 4, finally landed there.
Thanks to NASA’s GRAIL mission to map lunar gravity, we now know that the crust on the far side is about 20 kilometers thicker than on the near side of the Moon. The near side is dominated by mare—dark, basaltic plains formed by ancient volcanic eruptions. These lava outpourings about 3.5 billion years ago were so intense that the resulting gas produced a lunar atmosphere thicker than the current atmosphere on Mars and substantial enough to allow liquid water to remain stable on the lunar surface for perhaps millions of years. The ancient Moon might therefore have been habitable, and could possibly even have hosted microbial life.
Not only does the numerical modeling work by Zhu and colleagues explain the thicker crust on the lunar far side, it helps us understand why the composition of the Moon is similar to that of Earth in some respects, and dissimilar in others. It also accounts for the presence of potassium, rare-earth elements, and phosphorus (collectively known as KREEP) on the lunar surface, which is of high economical interest. Furthermore, Zhu’s Moon work will be relevant to other planets that have similar asymmetries (Mars, for example, is divided into southern highlands and northern lowlands).
The exploration of the Moon recently got kicked into higher gear with NASA’s Artemis program, which aims to land humans at the lunar South Pole by 2024 and establish a sustained human presence on or around the Moon by 2028. The selection of the South Pole as the preferred landing site is no coincidence. It’s ideal for astronomy, and would be a good place to hunt for astrobiological treasure. Organic molecules from the early Earth telling us about the origin of life on our planet could have been deposited in the lunar soil by rocks blasted off our planet by asteroid impacts. If so, they might still be waiting to be found beneath the lunar ice and regolith at South Pole locations such as Shackleton Crater.