Scientists have long hypothesized the existence of lunar caves, underground passages formed by volcanic processes and connected to the pits that cover the Moon’s surface. Now, a team of international researchers has found the first direct evidence that the deepest known pit on the Moon is connected to a cave that could house astronauts.
Identifying areas on the moon that can shield humans and robotic explorers from the harsh lunar environment — including powerful radiation and extreme temperatures — could be crucial to future space exploration as countries like the United States and China race to establish a long-term human presence on Earth’s closest natural satellite.
To find the cave, the team studied archival radar measurements of a pit discovered in a vast plain called Mare Tranquillitatis, or the Sea of Tranquility, which was the landing site of the Apollo 11 mission in 1969.
NASA’s Lunar Reconnaissance Orbiter collected radar data in 2010 using wavelengths to detect what was beneath the surface. By analyzing the data, researchers discovered an empty space at a depth of 130 to 170 meters (426 to 558 feet) underground, according to their study published Monday in the journal Nature Astronomy.
The newly discovered cave, measuring at least 45 meters (nearly 148 feet) wide and between 30 and 80 meters (98 and 262 feet) long, may be just the beginning of a longer tubular cavity formed by the lava – and there are likely more caves to be discovered, the researchers say.
“Lunar caves have remained a mystery for a long time. Scientists have been hypothesizing their existence for over 50 years, so it was surprising to finally be able to prove the existence of an underground conduit,” Leonardo Carrer, a co-author of the study and an assistant professor in the Department of Information Engineering and Computer Science at the University of Trento in Italy, said in an email.
Future research and exploration of lunar caves could also provide new insights into the evolution of the Moon and lunar volcanism, the study authors note.
“The caves are a unique environment that preserves the history of the Moon,” study co-author Lorenzo Bruzzone, a professor of telecommunications and director of the Remote Sensing Laboratory at the University of Trento, said via email. “Analyzing the rocks inside these caves could provide new insights into lunar evolution … such as the timing and duration of lunar volcanic activity and the actual composition of the lunar mantle.”
“In addition, (lunar caves) can be an alternative or an integration to a base on the surface of the Moon,” he added.
Using Moon Caves as Shelter
Near the lunar equator, temperatures reach a scorching average of 121 degrees Celsius during the day and drop to -133 degrees Celsius at nightfall, according to NASA. These extreme temperatures have even dropped below -246 degrees Celsius, as recorded by the Lunar Reconnaissance Orbiter.
Researchers in a July 2022 study that examined a number of pits on the moon — some that NASA had hypothesized would lead to caves when they were discovered — found that shaded shelters are much more thermally stable, consistently staying around 63°F (17°C).
“Having additional evidence of a cave extending from the floor of the Mare Tranquillitatis trench is really exciting,” Tyler Horvath, a doctoral student in planetary sciences at the University of California, Los Angeles, who was the lead author of the 2022 study, said in an email.
“Lunar caves could provide significant benefits to astronauts and rovers by providing shelter from hazards on the lunar surface, such as radiation, micrometeorites and extreme temperatures, which could also pave the way for a long-term, large-scale human presence on the Moon,” Horvath added. He was not involved in the new study.
The moon has more than 200 known lava tubes, like the one at Mare Tranquillitatis, but only a handful of them likely extend into caves, Horvath said. Some of the lava tubes may also be inaccessible because of debris blocking the passages, Carrer added.
The interior features of the newly discovered cave will remain obscure until a rover or human exploration can provide more information. The interior might resemble a recently formed lava tube on Earth, only much larger, Horvath said. It would likely look like smooth walls, except where flowing lava might have formed bumps and ridges called lavacicula, and the cave would be dark except where sunlight reaches and reflects off the pit floor, added Paul Hayne, an associate professor in the department of astrophysical and planetary sciences at the University of Colorado Boulder.
The cave’s massive size isn’t surprising, given the moon’s weaker gravity and the effects that can have on the stability of lava tubes, with larger caves able to form with less risk of collapse, said Hayne, who co-authored the 2022 study but was not involved in the new research. Most of Earth’s lava tubes are found in Hawaii, the Canary Islands, Australia and Iceland, and are between 10 and 30 meters (33 and 100 feet) in diameter. An August 2020 study suggested that lava tubes on the moon could be as large as 1 kilometer (3,280.84 feet) in diameter.
“We shouldn’t be too surprised if future explorations reveal even larger caves. We’re getting a glimpse into the Moon’s internal volcanic plumbing system,” Hayne said in an email.
Future exploration of lunar caves
The authors of the new study found that the cave was potentially flat or tilted at a maximum angle of 45 degrees, suggesting it would likely be accessible, Carrer said.
But because of the depth of this cave (and potentially other lunar caves), getting astronauts down to the bottom of the pit may require some creativity, possibly involving the use of climbing gear like ropes and belay devices and larger cables for equipment and building materials, Hayne said. Eventually, the process could involve a more permanent structure like a freight elevator, he added.
Other ideas for exploring the deep trench of Mare Tranquillitatis and the recently discovered cave inside include a robotic crane that could be used to lower an astronaut or rover to the ground, which is currently being explored by the European Space Agency.
Noah Petro, chief of NASA’s Planetary Geology, Geophysics and Geochemistry Laboratory, noted that skywatchers looking at the moon can identify Mare Tranquillitatis, where the cave was discovered, by looking for a large dark plain. He recommended looking up and observing the site on July 20 to mark the 55th anniversary of the Apollo 11 landing, the first mission to send humans to the moon.
“The discovery of (potential pits and caves) goes back several years (but) this new information from the radar data tells us, ‘Oh, wow. These are a little bit bigger than we thought,'” said Petro, who is the project scientist for the Lunar Reconnaissance Orbiter and Artemis III, a mission that aims to land humans on the moon for the first time since Apollo 17 in 1972. Petro was not involved in the new study.
“The LRO team has been on the Moon for 15 years now, creating incredible datasets that are changing our view of the Moon and are being used to make these kinds of discoveries,” Petro said. “(The cave) is one of those wonderful surprises that the Moon always has in store for us.”
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