The Moon is shrinking and triggering new tectonic activity, including moonquakes that could impact future lunar surface environments. A study published Wednesday in The Planetary Science Journal details the first global map of small ridges in the lunar maria – dark, basaltic plains – as evidence of this ongoing activity.
Researchers from the National Air and Space Museum team identified these small mare ridges (SMRs) as relatively young and widespread across the lunar maria. By studying how SMRs form, the team also identified new sources of moonquakes that could influence the location of future lunar landing missions.
While both Earth and the Moon experience tectonic forces, they behave differently. Earth’s crust is divided into plates that collide, separate and grind against each other, forming mountains, deep ocean trenches, and volcanic activity around the Pacific Ring of Fire. The Moon, however, lacks plate tectonics. Instead, stress builds within its single crust, continuously compressing it. This stress results in distinctive surface features, such as lobate scarps – ridges formed when the crust is compressed and one section is thrust over another along a fault. These scarps are common in the lunar highlands and have formed within the last billion years, representing roughly 20 percent of the Moon’s history.
Evidence of the Moon’s gradual shrinking began to emerge in 2010, with research led by Tom Watters of the Center for Earth and Planetary Studies. As the Moon’s interior cooled, its surface wrinkled, creating compressive forces that formed the lobate scarps in the highlands. However, these scarps alone could not explain all of the Moon’s relatively recent contraction features. Another class of surface forms, the small mare ridges (SMRs), has also been identified.
SMRs form from the same compressive forces that create lobate scarps, but are located specifically within the lunar maria. Researchers systematically mapped these ridges across the maria to study their role in recent tectonic activity. “Since the Apollo era, we’ve known about the prevalence of lobate scarps across the lunar highlands, but this is the first time scientists have documented the widespread prevalence of similar features across the lunar maria,” said Cole Nypaver, a postdoctoral research geologist at the Center for Earth and Planetary Studies and lead author of the article.
“This research helps us gain a complete global perspective on recent lunar tectonism, which will lead to a better understanding of its interior, thermal history, seismic activity, and potential future moonquakes,” Nypaver added.
The team compiled the first comprehensive catalog of SMRs, identifying 1,114 previously undocumented SMR segments on the near side of the Moon’s maria. This brings the total number of known SMRs on the Moon to 2,634. Their analysis indicates that the average age of SMRs is approximately 124 million years – comparable to the average age of lobate scarps (105 million years), as determined by previous research led by Watters and colleagues. This similar age suggests that, like lobate scarps, SMRs are among the youngest geological features on the Moon.
The study also shows that SMRs formed along the same types of faults as lobate scarps. In some areas, highland scarps transition into SMRs within the maria, reinforcing the idea that both structures share a common origin.
Combined with existing data on lobate scarps, the new SMR catalog provides a clearer picture of the Moon’s recent contraction and tectonic evolution. “Our discovery of young, small ridges in the maria, and the identification of their cause, complements the global picture of a dynamic, shrinking Moon,” said Watters.
Previous research by Watters has linked the tectonic forces forming lobate scarps to recorded moonquakes. Since SMRs form through the same type of faulting, moonquakes may occur wherever these ridges are present across the lunar maria. Expanding the map of potential moonquake sources provides scientists with new opportunities to study the Moon’s interior and tectonic behavior. However, it also highlights the seismic risks facing future astronauts exploring or inhabiting the lunar surface.
“We are in a highly exciting time for lunar science and exploration,” said Nypaver. “Upcoming lunar exploration programs, such as Artemis, will provide a wealth of new information about our Moon. A better understanding of lunar tectonics and seismic activity will directly support the safety and scientific success of these and future missions.”