Optical Fibers Could Illuminate the Moon’s Core

Distributed acoustic sensing can detect earthquakes—why not moonquakes, too?

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In addition to flags, spacecraft parts, and poop, Apollo astronauts left behind a quintet of seismic stations on the moon’s surface. In the 1970s, those stations gave selenologists (scientists who study the moon) early glimpses into what lurked under the lunar surface. In fact, that Apollo-era equipment helped establish that the moon is tectonically active. Decades later, their successors are still crunching the data from the observations of half-century-old moonquakes and meteor impacts.

Today, as NASA prepares to spearhead a permanent return to the moon with the Artemis program, seismologists are contemplating how to follow up on Apollo’s seismic stations. One group of researchers has plotted a very different type of seismometer: a network of optical fibers. Their work suggests fibers may be sensitive to seismic waves from deep inside the moon—a murky place that Apollo’s seismometers could not easily probe.

The idea has precedent. Optical fibers are becoming a staple of seismology on Earth. “I think it’s a natural idea: Why not try this...on the Moon?” says Wenbo Wu, a geophysicist at the Woods Hole Oceanographic Institution, in Massachusetts.

That’s because optical fibers make versatile sensors. When light is sent down a fiber, impurities in the fiber itself will scatter some of the light back in the direction of the light’s source. It’s possible to process the backscattered light waves and reconstruct whatever interfered with the light en route. Backscattering happens all along a fiber’s length, and this can turn a fiber into a very long sensor array. When a portion of the fiber stretches, or contracts, or twists because of an earthquake, the backscattered signal changes, which gives evidence of where the earthquake has happened along the fiber’s length in close to real time.

“On the moon, it’s very hard to deploy hundreds of seismometers globally.” —Wenbo Wu, Woods Hole Oceanographic Institution

This technique is known as distributed acoustic sensing (DAS). Although it has only been around for a bit over a decade, DAS has already found a smorgasbord of applications, including monitoring fossil-fuel wells, catching perimeter intruders, and eavesdropping on whale song.

Geophysicists too have embraced fiber. DAS is a great tool for detecting seismic waves on Earth, especially where it is difficult to set up traditional seismometers, such as inaccessible and inhospitable places like Antarctica and the deep seafloor. DAS allows scientists to detect quakes far from civilization or use those quakes’ seismic waves to study what lies, for example, beneath an ice sheet.

Wu and his colleagues at Woods Hole and NASA’s Jet Propulsion Laboratory (JPL) now want to take DAS to the moon. They conducted a numerical simulation, testing how fibers would fare on the seismic waves from mock moonquakes. Their results suggest that DAS would be especially good at picking up ScS waves, a type of seismic wave that travels through a world’s core.

Selenologists may find that a tantalizing prospect because we still do not know very much about the moon’s deep interior. The loose and meteorite-battered regolith that forms the Moon’s outer layer tends to muffle seismic waves from underneath. Apollo’s seismometers were too few in number to let scientists peer through all the noise.

“It’s going to take weeks—many, many, many weeks—to deploy that kind of long fiber.” —Nate Lindsey, FiberSense

Planting more seismometers is one option, but Wu believes it isn’t an ideal one. “On the moon, it’s very hard to deploy hundreds of seismometers globally,” he says. “[DAS] would have the advantage over traditional seismometers.”

Of course, laying lunar fibers still won’t be simple. If building cables on Earth is any example, a lunar cable layer will need to dig a trench, lay the cable inside, and then close the trench with backfill. A rover could do the task, but today’s rovers are not exactly speedsters: Over on Mars, NASA’s Perseverance set a record by traversing a whole 348 meters in one day. A useful DAS sensor may need to be tens of kilometers long. Rover technology will keep improving, but a construction rover’s demands will slow it down.

“It’s going to take weeks—many, many, many weeks—to deploy that kind of long fiber,” says Nate Lindsey, a geophysicist at FiberSense, a startup that provides DAS as a service. Lindsey wasn’t involved in the Woods Hole and JPL work.

Additionally, DAS comes with formidable data demands. Lindsey says that applying DAS to 50 kilometers of fiber—processing each 10-meter-long segment as a separate channel, sampled at 1 kilohertz—produces about a terabyte of data per day. On Earth, even if a DAS-enabled fiber is at the bottom of the ocean, its operators can place their processing equipment and its power source safely on land. Lunar DAS operators will have no such luxury.

But Lindsey is optimistic. A key advantage of DAS on Earth is that the technique can avail itself of infrastructure that already exists. A traditional seismic station isn’t an obvious inclusion for a new lunar settlement, but that base’s architects are far more likely to plan a fiber link to give their habitat a high-bandwidth connection.

“If you needed to put a fiber out anyway, this would be a natural thing to piggyback on,” says Lindsey. “I think this will be done at some point.”

Indeed, Wu and his colleagues suggest that their seismic network could piggyback on the infrastructure needed to support other science experiments on the moon—for example, a radio telescope on the moon’s far side.

The researchers published their work on 28 February in the journal Seismological Research Letters.

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