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New Analysis of Apollo-Era Moonquakes Shows the Moon Could Be Tectonically Active

A lobate scarp, a potential lunar fault line, bisects this image.
A lobate scarp, a potential lunar fault line, bisects this image.
Photo: NASA/GSFC/Arizona State University

Astronauts and Jeff Bezos-types hoping to set up shop on the Moon might have another challenge to worry about: moonquakes caused by tectonic activity.

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Apollo mission seismometers measured 28 shallow moonquakes between 1969 and 1977 that seemed to lack origin. Researchers re-calculated the epicenters of these quakes and found that eight of them occurred near small cliffs produced along fault lines. Combined with evidence of dust and boulders moving near these faults, the researchers conclude that the Moon is tectonically active.

“We’ve got these possibly active faults on the Moon, which means it isn’t this dead body,” Tom Watters, study first author from the Center for Earth and Planetary Studies at the Smithsonian Institution in Washington, DC, told Gizmodo. “It flies in the face of conventional wisdom, that the smaller a rocky body, the quicker it loses interior heat and becomes geologically inactive.”

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The Apollo seismometers measured lots of seismic activity, from meteorite-induced vibrations to rumbles as the cold crust expanded once the Sun began shining at the end of a lunar night. But 28 of those quakes were relatively shallow and powerful—one had a magnitude of 5.5, according to a NASA factsheet. The source of these quakes has remained a mystery for decades.

But in 2010, high-resolution images taken by the Lunar Reconnaissance Orbiter (LRO) mission revealed Moon-wide fault scarps, or cliffs tens to hundreds of feet high produced by motion along fault lines. These fault scarps appeared to be young—less than 50 million years old. Scientists wondered: How young were they, really? Are they still active?

The researchers started by applying a new algorithm—one that was designed for working with inaccurate data from Earth-based seismic networks with few sensors—in order to re-find the epicenters of the 28 Apollo quakes. They calculated at what distance from the fault scarp the shaking would be strongest, and compared this distance to the re-calculated epicenters. They built shake maps to show what forces would be experienced during moonquakes along these faults. They also compared the quakes’ timing to the distance between Earth and the Moon, to test how tidal forces between the two orbs could have influenced the seismic behavior.

The team found that eight of these quakes seemed to occur close to these fault scarps, according to the paper published Monday in Nature Geoscience. Taking all the models together and examining images showing evidence of recent moving debris on the Moon, the researchers hypothesized that the Moon must still be tectonically active.

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To be clear, the Moon does not have tectonic plates like Earth does. Instead, the Moon would contract as it loses heat, causing the land to rift and crack along faults. Scientists have already demonstrated evidence that Mercury experiences such tectonic behavior. But evidence is beginning to show that these rocky bodies stay warm, and continue to remain seismically active, for a long time, Watters said.

This is still a model-backed correlation (and others have analyzed and re-analyzed the Apollo moonquakes to try and find the source), and there’s more work to do to truly confirm the source of the seismic activity. Hopefully scientists will put more seismometers on the Moon to better localize the source of these quakes. Watters hopes the LRO team will be able to re-analyze fault scarps imaged by the orbiter since 2009 to find evidence of movement—that would be a smoking gun signature, he said.

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It shows just how much we have left to learn about our closest cosmic neighbor. And if we’re planning to built outposts on the Moon, we’ll probably want to know a bit more about these moonquakes.

Science Writer, Founder of Birdmodo

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DISCUSSION

We really need to send some more seismometers to the Moon as there is an intriguing pattern of Lunar ‘quakes. They appear to fall into two clusters - shallow ones less than 100km below the surface and much deeper ones more than 1000km down which appear to be periodic and are probably-influenced by the changing tidal forces on the Moon as it orbits the Earth.

But there is so little data to go on that we really need to know more about if these are genuinely two types of ‘quake or if things are more complex. The shallow cooling and shrinking of the Moon sounds like a perfectly reasonable explanation for these shallow ‘quakes along well-defined faults

Certainly thinking is moving towards considering the Moon to be somewhat tectonically active. A paper has just come out suggesting that partial melts might exist along the Moon’s Core/Mantle boundary. Something in the interior of the Moon is slowing earthquake waves as they travel through the deep interior, so some clever modelling of the temperature/pressure conditions deep inside the Moon to suggest anything up to 30% of the Mantle directly over the Core might be a crystal-rich mush at a temperature of between 1330-1470C. It’s highly-unlikely though that current conditions would allow these magmas to rise buoyantly towards the surface and create new volcanism - which is why the Moon looks dead.

  • Ananya Mallik, Tariq Ejaz, Svyatoslav Shcheka, Gordana Garapic. A petrologic study on the effect of mantle overturn: Implications for evolution of the lunar interior. Geochimica et Cosmochimica Acta, 2019; 250: 238 DOI: 10.1016/j.gca.2019.02.014

Having said that, there is some evidence for volcanism on the Moon within the last 100 million years, and perhaps within the last 10My.

  • Braden, S.E., Stopar, J.D., Robinson, M.S., Lawrence, S.J., van der Bogert, C.H., Hiesinger, H., 2014. Evidence for basaltic volcanism on the Moon within the past 100 million years. Nature Geoscience 7, 787. DOI: https://doi.org/10.1038/ngeo2252

And there is still the matter of Transient Lunar Phenomena - strange glows that have been seen on the surface of the Moon by even the very most reputable astronomers. Is something occasionally being erupted on the Moon?

https://en.wikipedia.org/wiki/Transient_lunar_phenomenon