Scientists search for answers to moon living in south China lava caves

HAIKOU, July 24 (Xinhua) -- How to live on the moon in the future? Scientists are looking for answers to this question in a lava cave in China's southernmost island province of Hainan.

For the past month, a team of scientists has moved through a pitch-black lava cave in northern Hainan, setting off artificial tremors and crisscrossing the darkness via light, radio waves and seismic waves. They are there rehearsing for what is likely to be humanity's next giant leap: building a permanent shelter on the moon.

To solve the biggest problem of how to "see" underground on a distant world without carrying heavy machinery, they're turning to an unlikely source for help, namely, a spacecraft that will be sent to the moon.

The joint team from the Hainan Earthquake Agency, Southern University of Science and Technology (SUSTech), Peking University and the Institute of Geophysics of the China Earthquake Administration, descended into the caves of Hainan's Leiqiong volcanic field with an arsenal of devices -- a distributed fiber-optic sensing system, a vibroseis truck, a broadband seismometer, a ground-penetrating radar, LiDAR, robotic dogs and drone-mounted magnetometers, all to produce a three-dimensional map of the cave's hidden architecture.

These lava tubes, formed by eruptions some 10,000 years ago, are highly similar in shape and formation to those that scientists believe riddle the lunar subsurface.

If humanity is serious about establishing long-term bases on extraterrestrial bodies, where the surface is a hostile place with radiation and extreme temperatures, the answer, scientists believe, lies underground. Volcanic lava tubes offer relatively stable temperatures and protection from cosmic rays, making them ideal natural shelters.

UNDERGROUND CT SCAN

Before the practical human plan and design for the shelters, there is a need to know exactly what is built on. This is where seismology, the science of listening to the Earth's "heartbeat," comes in.

"It's essentially like doing a CT scan of the subsurface," explained Chen Bo, assistant director of the Hainan Earthquake Agency.

Seismic waves travel differently through rock, air and water. Hard rocks speed them up, cavities or fractures slow them down, and make them reflect, refract and scatter. By generating tiny vibrations and recording how they bounce back, the team can reconstruct what lies beneath, including empty spaces, cracks and layered structures.

Initial data from the team's experiment are promising.

Liu Peiyu, a doctoral student from SUSTech and also a member of the research team, said that the team's LiDAR point cloud data, which map the caves' hollow areas from above, match well with the underground findings produced by their combined seismic and electrical surveys.

The team has mapped the lava tubes' orientation, cavity shape, roof thickness and shallow velocity structure of the rock layers.

Equally important, the team has proved that their delicate fiber-optic sensing system can survive the caves' punishing humidity and heat, providing sustained stable operation. The data will serve as a foundational reference for future planetary lava tube exploration, Liu said.

ROCKET-POWERED SOLUTION

However, transplanting this entire technological toolkit to an extraterrestrial body poses a practical problem: some of the heaviest gear simply cannot be carried by spacecraft. So, on the moon, could there be a locally available "artificial seismic source?"

An innovative paradigm -- Rocketquake Seismology Serving Lunar Base Construction -- turns a potential problem into a powerful solution.

As lunar exploration progresses, regular lunar surface launches and landings will generate significant vibrations. "Vibrations from the launch of the spacecraft can be treated as a known source of seismic waves, and one can then apply standard seismological techniques to decode what lies beneath, be it underground structures, planetary geology or even the path of orbiting debris," said Chen.

In practice, he explained, if there's a rocket launch on the moon in the future, seismometers placed on the lunar surface would record the vibrations generated by the rocket. Scientists would then invert that data to map how seismic waves travel through different underground materials, revealing not just underground voids, but also faults, water-ice layers or mineral deposits.

"Using the rocket as the source gives us precise timing, known locations and zero extra deployment cost," Chen said. "It's economical and efficient."

Preliminary studies and field validations by the Hainan Earthquake Agency and SUSTech showed that a medium-sized rocket launch generates energy roughly equivalent to a magnitude 1.0 earthquake, with waveforms clearly recordable by sensitive seismometers dozens of kilometers away.

"Rocketquake seismology is a frontier area of international exploration, and we will continue to advance its theoretical validation, methodological expansion and practical application," Chen said.

With strong support from the Hainan commercial spacecraft launch site, the team is ready to sustain its frontier experiments and application exploration in rocketquake seismology so as to provide solid technological support for China's commercial space sector and the broader strategy of deep-space exploration.