Boffins propose fiber-optic network for the Moon

To detect seismic waves, silly

Researchers at the Woods Hole Oceanographic Institute in the US are proposing the deployment of a fiber-optic network on the Moon to probe our natural satellite's interior by detecting seismic waves.

In the research journal Seismological Research Letters, the boffins note that understanding the composition of the Moon and its internal structures could be key to unlocking the origins and evolution of the Earth-Moon system, as well as other planets.

As seismologists are already using fiber-optic cables to detect seismic waves here on Earth, the authors of "Fiber Seismic Network on the Moon" reckon the technology could be deployed to Earth's satellite for the same purpose.

Seismometers placed on the Moon by Apollo astronauts between 1969 and 1976 are said to have detected thousands of seismic events on the side of the Moon facing Earth. These included shallow and deep moonquakes, as well as meteorite impacts.

Yet many critical questions such as the state and composition of the Moon's interior remain unsolved, owing to the fact that few Apollo seismic monitors were deployed as well as the strong scattering of seismic waves in the top layer, or regolith, of the Moon. Hence, while the Apollo data provides some hints about the Moon's core, details on its size and properties are still hazy.

The authors argue that an emerging technique called distributed acoustic sensing (DAS) can provide a more cost‐efficient solution to getting the necessary data. It enables continuous, real-time measurements along the entire length of a fiber-optic cable thanks to Rayleigh scattering from small variations in the refractive index of the fiber.

This works best over distances of 100km or more, meaning that the proposal would require the deployment of a considerable amount of optical fiber across the surface of the Moon – which is a challenging environment at the best of times.

The authors propose that a fiber seismic network using "tens of kilometers" of cable could dramatically increase the chances of observing clear seismic waves, providing opportunities for future lunar seismic surveys. The boffins think "more efforts and further evaluations are required to develop a space‐proof DAS."

However, the scientists speculate on the possibility of combining DAS with other cable-based lunar programs to maximize scientific returns – not to mention getting it deployed in the first place.

One proposal is to place a radio telescope on the far side of the Moon, so that it will be able to avoid radio interference from Earth-based sources and detect signals that might otherwise be absorbed by the Earth's atmosphere.

According to the authors, current concepts involve a cable-linked antenna in a natural crater on the far side of the Moon. The fiber optic cables laid down for this could enable deployment of both DAS and the telescope with minimum additional cost.

It gets a little unclear when it comes to how this would be accomplished, with the researchers suggesting that astronauts might deploy the cables using rovers as part of NASA Artemis missions or that autonomous rovers could do the job.

If a smaller lander is employed – such as with a Commercial Lunar Payload Services (CLPS) mission – cables may be distributed across the surface using small rockets or mortars. It all sounds a bit speculative – especially as the recent Odysseus and SLIM lunar probes couldn't even manage to land upright.

Given that NASA's Artemis missions have recently been delayed, this is all for the distant future. There is no guarantee that anything like this fiber network will ever actually be implemented. But it's a cool thought. ®

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