Scientists have long theorized that lunar caves exist: underground passages formed by volcanic processes that connect to the pits that cover the moon’s surface. Now, a team of international researchers has found the first direct evidence that the moon’s deepest known pit is connected to a cave that could house astronauts.
Identifying areas on the moon that can protect humans and robotic explorers from the lunar’s harsh conditions, including intense radiation and extreme temperatures, could be critical to future space exploration as countries including the United States and China race to establish a long-term human presence on Earth’s nearest natural satellite.
To find the cave, the team studied archival radar measurements of a depression in a vast plain called Mare Tranquillitatis, or the Sea of Tranquility, which was the landing site for the Apollo 11 mission in 1969.
The NASA Lunar Reconnaissance Orbiter collected the radar data in 2010, using wavelengths to detect what lay beneath the surface. When the data was analyzed, researchers found empty space at a subsurface depth of 130 to 170 meters (426 to 558 feet), according to their study published Monday in the journal Nature Astronomy.
The newly discovered cave, which is 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 tube-shaped cavity formed by lava. Researchers say more caves are likely to be found.
“Moon caves have long been a mystery. Scientists have hypothesized their existence for more than 50 years. So it was surprising to finally be able to prove the existence of an underground conduit,” study co-author Leonardo Carrer, an assistant professor in the department of information technology 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 authors say in the study.
“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. “The analysis of rocks in these caves could allow new[discoveries]regarding the evolution of the Moon … such as the timeline and duration of volcanic activity on the Moon, as well as the actual composition of the lunar mantle.
“In addition, (moon caves) could be an alternative or an integration to a base on the surface of the moon,” he added.
Using moon caves as shelter
Near the moon’s equator, temperatures reach a blistering average of 250 degrees Fahrenheit (121 degrees Celsius) during the day and drop to -208 F (-133 C) at sunset, NASA says. The extreme temperatures have even dropped below -410 F (-246 C), as measured by the Lunar Reconnaissance Orbiter.
Researchers in a July 2022 study that examined a number of pits on the moon — some of which NASA said led to caves when discovered — found that the shaded shelters are much more temperature stable, holding steady at about 63 F (17 C).
“It’s really exciting to have additional evidence of a cave extending from the bottom of the Mare Tranquillitatis pit,” Tyler Horvath, a doctoral student in planetary sciences at the University of California, Los Angeles, who was lead author of the 2022 study, said in an email.
“Moon caves could provide tremendous benefits to astronauts and rovers, providing shelter from lunar surface hazards such as radiation, micrometeorites, and extreme temperatures, which could also open the door to a long-term, large-scale human presence on the Moon,” added Horvath, who was not involved in the new study.
The moon has more than 200 known pits like the one at Mare Tranquillitatis, but only a handful likely extend into caves, Horvath said. Some of the lava tubes could also be inaccessible because of debris blocking the passages, Carrer added.
The interior of the newly discovered cave remains unclear until a rover or human exploration can provide more insight. The inside may resemble a recently formed lava tube on Earth, only much larger, Horvath said. It would likely appear smooth-walled except where dripping lava would have formed hummocks and ridges known as lavacicles, and the cave would be pitch black except where sunlight reaches the bottom of the pit and bounces off it, added Paul Hayne, an associate professor in the department of astrophysical and planetary sciences at the University of Colorado, Boulder.
The cave’s large size isn’t surprising, given the moon’s lower gravity and the effects that could have on the stability of lava tubes, allowing larger caves to form with less risk of collapse, said Hayne, a co-author of the 2022 study but not involved in the new research. Most lava tubes on Earth are located in Hawaii, the Canary Islands, Australia and Iceland, and range in diameter from 33 to 98 feet. An August 2020 study had suggested that lava tubes on the moon could reach 1 kilometer (3,280.84 feet) in diameter.
“We shouldn’t be too surprised if future explorations reveal even larger caves, giving us a glimpse into the moon’s internal volcanic plumbing system,” Hayne said in an email.
Future exploration of lunar caves
According to Carrer, the authors of the new study found that the cave is potentially flat or has a slope of up to 45 degrees, suggesting that the cave is likely accessible.
But because of the depth of this cave (and possibly other lunar caves), it could take some creativity to get astronauts to the pit floor — possibly using 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, such as a freight elevator, he added.
Other ideas for exploring the deep Mare Tranquillitatis pit and the newly discovered cavern within it include a robotic crane that could be used to lower an astronaut or rover to the bottom. This crane is currently being investigated by the European Space Agency.
Noah Petro, director of NASA’s Planetary Geology, Geophysics and Geochemistry Lab, noted that skywatchers observing the moon can identify Mare Tranquillitatis, where the cave was discovered, by looking for a large, dark plain. He recommended looking up and peering at the site on July 20 to mark the 55th anniversary of the Apollo 11 landing, the first mission to put humans on the moon.
“The discovery of (the pits and possible caves) goes back a number of years (but) this new information from the radar data says, ‘Oh, wow. They’re a little bit bigger than we expected,'” said Petro, who is the project scientist for both 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.
“LRO has been on the moon for 15 years now and is creating some incredible data sets that are changing our perspective on the moon and are being used to make these kinds of discoveries,” Petro said. “(The cave) is one of those amazing surprises that the moon always has in store for us.”
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