The Moon was geologically active between 3.7 and 2.5 billion years ago, experiencing quakes, volcanic eruptions, and outgassing.
Thanks to the Moon being an airless body, evidence of this past has been carefully preserved in the form of extinct volcanoes, lava tubes, and other features.
While the Moon has been geologically inert for billions of years, it still experiences small seismic events due to tidal flexing (because of Earth's gravitational pull) and temperature variations. These latter events happen regularly and are known as "moonquakes."
Thanks to the Apollo missions, scientists have measured this activity using seismometers placed on the surface.
In a recent NASA-funded study, a team of researchers from the California Institute of Technology (Caltech) reexamined the seismic data with a machine-learning model.
This revealed that moonquakes occur with precise regularity, coinciding with the Sun rising to its peak position in the sky and then slowly setting. In this respect, moonquakes are like a "Lunar Alarm Clock," which could be useful for future missions and lunar settlers!
The NASA-funded research was led by Francesco Civilini, a postdoc graduate from the California Institute of Technology (Caltech), now at the NASA Marshall Space Flight Center. He was joined by Renee Weber, a Planetary Scientist at the Marshall Space Flight Center, and Allen Husker, a Geophysics Research Professor with Caltech's Division of Geological and Planetary Sciences.
The paper that describes their findings, "Thermal Moonquake Characterization and Cataloging Using Frequency-Based Algorithms and Stochastic Gradient Descent," appeared on September 5th in the Journal of Geophysical Research – Planets.
Unlike tidal flexing in the Moon's interior, moonquakes result from temperature changes in the lunar crust (thermal quakes). The airless environment on the Moon essentially means that heat from the Sun is not retained, nor does sunlight lead to the gradual warming of the surface.
As a result, the crust is heated to temperatures of up to 120 °C (250 °F) during the peak of the day and drops to a low of -133 °C (-208 °F) at night. This causes the crust to expand and contract rapidly, triggering small seismic events. In 1972, astronauts from the Apollo 17 mission placed seismometers on the Moon to measure this activity.
The sensors collected data over a period of eight months (October 1976 to May 1977), which remained largely untouched until recently. For their purposes, Civilini and his team reanalyzed this lunar seismic data with the help of a machine-learning model.
Their analysis showed that thermal quakes occur with precise regularity every afternoon as the Sun leaves its peak position in the sky and the surface begins to cool rapidly.
However, the model also detected seismic signatures in the morning that looked different from evening quakes.
The researchers were able to triangulate the source of the activity and found that the morning tremors were coming a few hundred meters away from seismometers – from the Apollo 17 lunar lander itself!
Every morning, as sunlight reached the vehicle, its surface would expand, causing vibrations in the ground that were detected by the seismic array.
"Every lunar morning, when the Sun hits the lander, it starts popping off," explained Husker in a Caltech press release. "Every five to six minutes, another one, over a period of five to seven Earth hours. They were incredibly regular and repeating."
This data could have significant implications for future missions to the Moon, including NASA's Artemis Program. Though thermal quakes are too small to be felt by anyone on the lunar surface, these findings provide vital data that could inform the design of future landers and equipment.
It could also inform the structure of future bases, like the Artemis Base Camp, the International Lunar Research Station (ILRS), and the ESA's proposed Moon Village – where composite materials would be used instead of alloys to avoid triggering local quakes.
In addition, seismic activity is a good way to probe the interiors of celestial bodies, which can be used to infer the interior structures of celestial bodies and locate materials (like water ice) underground. Said Husker:
"We will hopefully be able to map out the subsurface cratering and to look for deposits. There are also certain regions in craters at the Moon's South Pole that never see sunlight; they are permanently shadowed. If we could put up a few seismometers there, we could look for water ice that may be trapped in the subsurface; seismic waves travel slower through water."
And while there is no plate tectonics or volcanic activity on the Moon, researchers still have many questions about the Moon's internal structure.
"It's important to know as much as we can from the existing data so we can design experiments and missions to answer the right questions," Husker added.
"The Moon is the only planetary body other than the Earth to have had more than one seismometer on it at a time. It gives us the only opportunity to thoroughly study another body."