Could fiber optic cable help scientists probe the deep layers of the moon?

An rising variety of seismologists are utilizing fiber optic cables to detect seismic waves on Earth—however how would this expertise fare on the moon, and what would it not inform us concerning the deep layers of our nearest neighbor in space?

In Seismological Analysis Letters, Wenbo Wu of Woods Gap Oceanographic Institute and colleagues explore the idea of deploying a fiber seismic network on the moon, discussing a number of the challenges to beat.

In addition they take a look at this hypothetical community utilizing synthetic seismograms created from knowledge collected by seismometers positioned on the moon’s floor by the Apollo missions. Based mostly on their outcomes, Wu and colleagues say a fiber seismic community may establish the type of seismic waves that would supply extra details about the moon’s deep core construction.

The 4 seismometers positioned on the moon between 1969 and 1976 by the Apollo missions detected hundreds of seismic occasions over seven years on the close to facet of the moon. These occasions included shallow and deep moonquakes, in addition to meteorite impacts.

The Apollo seismic data got here with some unanswered questions, nevertheless: What explains the mysterious lack of moonquakes detected on the far facet of the moon? And why did the Apollo seismometers detect moonquakes occurring 700 to 1,100 kilometers beneath the floor, at a depth on Earth the place warmth and strain would result in plastic deformation as an alternative of the brittle break of an earthquake?

Answering these questions would require many extra seismometers deployed in a harsh environment to gather extra seismic occasions, a activity for which fiber seismic networks are well-suited, the researchers counsel.

Wu and colleagues suggest utilizing Distributed Acoustic Sensing, or DAS, for a new moon community. DAS makes use of the tiny inner flaws in an extended optical fiber as seismic sensors. An instrument known as an interrogator at one finish of the fiber sends laser pulses down the cable which might be mirrored off the fiber flaws and bounced again to the instrument. When the fiber is disturbed by seismic exercise, researchers can study adjustments within the mirrored pulses to be taught extra concerning the ensuing seismic waves.

“It is a very dense seismic array,” mentioned Wu. “One cable can get you hundreds of particular person sensors.”

One of many largest challenges for lunar seismology is the porous and fractured blanket of rubble known as regolith that covers the floor of the moon. Among the first seismic waves detected after a moonquake are scattered by this layer, and the scatter obscures later-arriving waves that might present extra details about the depths of the moon.

The information collected by the hundreds of sensors in a DAS array may be in contrast in a sign processing method known as array stacking, Wu and colleagues show. This system helps to separate “deep indicators hidden within the scattered waves” and different sources of extraneous seismic noise, Wu defined.

When the crew used the method on the synthetic seismograms, they have been in a position to retrieve a seismic wave phase known as ScS, which is a shear or S-wave that travels from the earthquake origin towards the moon’s core earlier than being mirrored as much as the floor.

Wu mentioned it is vital to run these sorts of experiments earlier than deploying an precise fiber array on the moon. “Earlier than a launch there have to be sturdy numerical simulations of wave propagation,” he mentioned. “We do the homework to search out out if we will get the info, and what sorts of issues we will do with the info.”

If researchers can discover methods to supply energy and repairs to a lunar fiber seismic community, the array may function for years, Wu famous. “On Earth if the ability is okay, we will maintain it operating for many years.”

Within the paper, the researchers counsel it could be attainable to mix DAS with different proposed lunar packages similar to putting a radio telescope, which might already want fiber optic cables to hook up with an antenna, on the far facet of the moon.

“If we will mix these initiatives collectively to avoid wasting the fee, that may actually enhance the prospect to make it occur and have most scientific impression,” mentioned Wu.