The Apollo 17 lunar module descent stage is causing tiny quakes on the Moon (Image: Public Domain / NASA / Harrison Schmitt)
When NASA astronauts Gene Cernan and Harrison Schmitt of the Apollo 17 mission departed the lunar surface on December 14, 1972, they left a few things behind — including a US flag, a Moon buggy, and the lunar module’s descent stage.
A new study by researchers from the California University of Technology has revealed that the latter regularly causes extra, tiny “moonquakes” which shake the lunar surface.
This finding comes, ironically, thanks to another thing Cernan and Schmitt left near their landing site — an array of four geophones used to conduct seismic experiments.
Reactivated between October 1976 and May 1977 for passive listening, these seismometers recorded thousands of subtle tremors on the Moon, the result of daily temperature variations.
Until now, however, the poor quality of the data had made a comprehensive analysis difficult — hiding the fact that some of the moonquakes were not quite what they initially seemed.
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Pictured: one of the geophones from the Apollo 17 Lunar Seismic Profiling Experiment (Image: NASA)
Having no atmosphere to insulate it like the Earth, the surface of the Moon experiences extreme temperature variations between day and night — transitioning from a toasty 250F (121C) at the height of the day to a nippy -208F (-133C) in the depth of the lunar night.
When the lunar surface heats up, it expands, and when it cools down again, it contracts. These movements result in cracking near the surface and tremors known as “thermal moonquakes”.
In their study, geophysicist Professor Allen Husker of the California University of Technology and his colleagues used techniques not available in the seventies — like machine learning — to clean up the Apollo 17 passive seismic data and undertake a more robust analysis.
The team found that thermal moonquakes occur with the regularity of clockwork, every morning and afternoon. The latter are the result of the Sun leaving its peak position in the sky, allowing the lunar surface to begin to cool off.
However, the team’s artificial intelligence model revealed that the seismic activity detected in the morning has a different profile — and are not regular thermal moonquakes at all.
The Apollo 17 crew — Ronald Evans (left), Harrison Schmitt (right), and Eugene Cernan (seated) (Image: NASA)
Using the data from the seismometer array to triangulate the source of the morning quakes, Husker and his team found that they were coming from the left-behind descent stage of the Apollo 17 lunar module.
Lying mere hundreds of feet from the geophones, this lander base comprises an octagonal prism some 14 feet across, mounted on four legs and with an attached ladder for astronauts to climb in and out of the module that sat atop it.
It contained the landing rocket, fuel tanks and storage space for equipment and the lunar rover — and was used as a launch pad for the ascent module.
The researchers believe that it is causing the extra moonquakes when it warms up and expands in the morning sunlight
Husker explained: “Every lunar morning when the sun hits the lander, it starts popping off!
“Every five-to-six minutes another one — over a period of five-to-seven Earth hours. They were incredibly regular and repeating.”
Pictured: Seismic signatures of regularly occurring moonquakes (Image: Civilini et al./ Journal of Geophysical Research: Planets)
While the finding may seem inconsequential, it can actually help NASA as they plan to return humans to the Moon in the coming years as part of the Artemis program.
Thermal moonquakes might be too subtle to be felt by an astronaut standing on the lunar surface, the study is proving insights into the kinds of thermal expansion and contraction that lunar landers, rovers, scientific apparatus and even future bases need to be able to tolerate.
Husker explains that the landers used in the other Apollo missions also likely experience similar temperature-driven stresses.
The reason we have only detected lander-related moonquakes with Apollo 17, however, is that the seismometers deployed by these missions operate at the wrong frequency to pick up on such subtle tremors.
Pictured: a photo of the Apollo 17 landing site, showing the lander and the ASLEP seismometers (Image: NASA)
Seismological studies of the Moon are — as on Earth — also a great way to get a glimpse of the structure beneath the surface. This is because seismic waves travel at different speeds.
As Husker adds, using moonquakes, “we will hopefully be able to map out the subsurface cratering and to look for deposits.”
(In fact, this was the original purpose of Apollo 17’s “Lunar Seismic Profiling Experiment” — which, NASA explains, used artificial moonquakes to “acquire data on the physical properties of the lunar near-surface materials”. These quakes were produced mainly via explosive charges, but the seismometer array also recorded the tremors produced when first the lunar ascent module blasted off — and later when the spent and vacated craft was allowed to crash back onto the Moon’s surface.)
Husker continued: “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.”
Water ice is of considerable interest, as it could potentially be broken down into its oxygen and hydrogen components in order to provide astronauts on future lunar missions with both life-sustaining air and a means to produce fuel.
The full findings of the study were published in the Journal of Geophysical Research: Planets.
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