Fiber-Optic Networks Could Reveal the Moon’s Inner Structure – GWC Mag

When the Apollo astronauts landed on the Moon, they brought seismometers with them. Beginning in 1969, they established seismic stations at six sites that remained in operation until 1977. Information beamed back by these stations illuminated the Moon’s interior for the first time, and scientists are still learning from it today because despite their age, the Apollo data are still the best and only seismological information we have from the Moon.

Distributed acoustic sensing, or DAS, which turns ordinary fiber-optic cables into dense networks of seismometers, could change that. New research published in Seismological Research Letters points to DAS as a promising, cost-effective, and scientifically fruitful way to build seismometer arrays on the Moon. Scientists simulated lunar seismograms and showed that by “stacking” the many thousands of measurements made along a DAS array, it may even be possible to catch the echoes of seismic waves bouncing off the Moon’s core—despite the lunar crust’s unfortunate tendency to scatter seismic waves. A DAS array could thus help settle the question of whether the Moon’s core is liquid or solid, among other geophysical mysteries.

“People are using communication cables to do seismology [on Earth]. So why not on the Moon?”

“It’s a very different way of doing seismology than we’ve traditionally done in the past, which is why people are really excited about it,” said Nicholas Schmerr, a seismologist at the University of Maryland who was not involved in the study.

“I think the paper is very good,” agreed planetary seismologist Raphaël Garcia of the Institut Supérieur de l’Aéronautique et de l’Espace in Toulouse, France. Garcia was also not involved in the new research. “People are using communication cables to do seismology” on Earth, he reasoned. “So why not on the Moon?”

Mysteries of the Moon

Thanks to the Apollo seismometers, we know the Moon has at least three distinct layers: a silicate crust, an olivine mantle, and some kind of small, metal-rich core. Knowing about the interior composition of a planetary body can tell scientists a lot about its history—and because the Moon is Earth’s companion, learning about its interior reveals new chapters in Earth’s story, too. For instance, the Moon’s undersized core suggests that the Moon formed from light, metal-poor material spewed from Earth when it impacted another planet-sized body.

But there’s a lot we still don’t know about the Moon’s geological guts.

One problem with understanding the Moon’s interior is actually on the surface. The loosely packed, dusty regolith that blankets the lunar surface scatters seismological waves, and scattering creates a cacophony of “coda” noise following the first arrival of waves from a moonquake. Such noise drowns out the true signals of weaker seismic waves arriving later, including seismic waves that bounce off the lunar core before reaching the surface.

Additionally, we currently have good seismic data from only the nearside of the Moon, where the Apollo astronauts landed.

DAS on the Moon

To better understand moonquakes, scientists need more seismometers on the Moon—ideally, dense arrays of them. But even on Earth, building such arrays can be technically fussy and financially costly.

Wenbo Wu, a scientist at Woods Hole Oceanographic Institution and coauthor of the new paper, joined his colleagues in wondering whether DAS arrays might be a way forward for lunar seismology.

DAS observations made along about 40 kilometers of fiber would be enough to pick up seismic waves reflecting from the Moon’s core.

DAS seismometers are essentially just lengths of fiber-optic cable attached at one end to an “interrogator” device. The interrogator shoots pulses of laser light down the fiber and measures the signal that scatters back from the fiber’s natural imperfections to track how the cable deforms as seismic waves pass by. Each section of the cable acts like its own seismometer, turning a simple length of fiber into a seismological array.

When Wu’s team simulated lunar seismograms, they found that stacking the DAS observations made along about 40 kilometers of fiber would be enough to pick up seismic waves reflecting from the Moon’s core.

By stacking, Wu said, “we can reduce the contaminating signals from the scattering” through lunar regolith. Stacking should cancel out the random noise while amplifying the signal.

Artemis and Lunar Seismology

In the team’s lunar synthetic seismograms, the DAS model didn’t perform as well as an equally dense array of 40 traditional seismometers. But practically speaking, said Schmerr, “you’re not going to get 40 seismometers on the Moon.” DAS, on the other hand, would be cost-effective and simpler to deploy.

“This is a really great example of a paper that is setting up the case for a future mission,” continued Schmerr.

Future missions are on their way. NASA’s Artemis missions will return astronauts to the Moon for the first time in 50 years in 2026, with the goal of eventually establishing a permanent human presence there. Fiber optics will almost certainly be part of the infrastructure future astronauts and rovers establish on the Moon. And fiber-optic networks used for communications and other purposes can be used for DAS, too—as they already are on Earth.

Still, noted Wu, Schmerr, and Garcia, the technical challenge of deploying fiber-optic cables on the Moon could be considerable. “We would like to get a cable with a length of tens of kilometers. That’s a big challenge” in and of itself, said Wu.

Securing cables to the ground well enough to act as seismometers might be more of a challenge on the Moon than on Earth, for example. On Earth, the usual solution is to bury the cables in trenches. But that might not work so well on the Moon, with its dry, fluffy regolith and microgravity. Solving the problem, however, would have practical as well as scientific benefits for future missions: Seismometers can be used to prospect for useful materials like ice or metals and to learn about hazards posed by micrometeorites and moonquakes.

“There are a lot of questions for science,” said Garcia, “but also for exploration.”

—Elise Cutts (@elisecutts), Science Writer

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Citation: Cutts, E. (2024), Fiber-optic networks could reveal the Moon’s inner structure, Eos, 105, https://doi.org/10.1029/2024EO240159. Published on 3 April 2023.

Text © 2024. The authors. CC BY-NC-ND 3.0
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