In a groundbreaking discovery, researchers have revealed that the ridges on the Moon, known as small mare ridges (SMRs), are relatively young and are widely distributed across the lunar maria, the dark plains visible from Earth. By understanding the formation of these features, scientists have pinpointed new potential sources of moonquakes, which could impact the landing sites of future lunar missions.
Differentiating Lunar and Terrestrial Tectonics
While both Earth and the Moon experience tectonic activity, their mechanisms differ significantly. Earth's crust consists of moving plates that collide and separate, leading to the formation of mountains, deep ocean trenches, and volcanic activity. Conversely, the Moon lacks plate tectonics; instead, stress accumulates within its single, unbroken crust, resulting in unique geological formations. A notable example is the lobate scarps, which form when the crust compresses and one section is pushed over another along a fault line. These features are prevalent in the lunar highlands and date back to the last billion years, representing the most recent 20% of the Moon's geological history.
The Shrinking Moon and the Emergence of SMRs
In 2010, Tom Watters, a senior scientist emeritus at the Center for Earth and Planetary Studies, discovered evidence indicating that the Moon is gradually shrinking. As the Moon's interior cools, its surface contracts, generating compressional forces that lead to the formation of lobate scarps in the highlands.
However, lobate scarps do not account for all of the Moon's recent contraction features. The research team identified another geological class: small mare ridges (SMRs). These ridges arise from the same compressional forces as lobate scarps, but they are located exclusively in the maria. The team undertook a systematic mapping of these ridges to explore their role in the Moon's recent tectonic activity.
"Since the Apollo missions, the presence of lobate scarps has been well-documented in the lunar highlands, but this is the first time we have comprehensively recorded similar features in the lunar maria," stated Cole Nypaver, a postdoctoral research geologist at the Center for Earth and Planetary Studies and the lead author of the study. "This research provides a more complete understanding of recent lunar tectonism, enhancing our knowledge of the Moon's interior and its thermal and seismic history, as well as the potential for future moonquakes."
Cataloging the Young Ridges
The research team compiled the first extensive catalog of SMRs, identifying 1,114 previously unrecognized segments across the near side of the lunar maria, bringing the total number of known SMRs to 2,634.
Their findings suggest that the average age of an SMR is approximately 124 million years, closely aligning with the average age of lobate scarps, which is 105 million years. This similarity indicates that SMRs are among the Moon's youngest geological features.
The study also reveals that SMRs form along the same types of faults as lobate scarps. In some areas, scarps from the highlands transition into SMRs within the maria, reinforcing the notion that both structures share a common origin. Together with existing data on lobate scarps, the new SMR catalog provides a comprehensive view of the Moon's recent contraction and tectonic evolution.
"Our identification of these young, small ridges in the maria, along with understanding their origin, completes a global perspective of a dynamic and contracting Moon," Watters emphasized.
Implications for Moonquakes and Future Exploration
Previous research by Watters linked the tectonic forces that create lobate scarps to recorded moonquakes. Given that SMRs are formed through similar faulting processes, moonquakes may also occur across the lunar maria wherever these ridges are present.
Expanding the mapping of potential moonquake sources opens new avenues for scientists to study the Moon's interior and tectonic behavior, while also highlighting possible seismic risks for future astronauts who may explore or reside on the lunar surface.
"We are entering an exhilarating era for lunar science and exploration," Nypaver remarked. "Upcoming lunar missions, such as Artemis, promise to yield a wealth of new information about our Moon. A deeper understanding of lunar tectonics and seismic activity will directly enhance the safety and scientific success of these and future endeavors."