Scientists have long theorized the existence of lunar caves — underground passages formed by volcanic processes and connected to sinkholes that cover the moon’s surface. Now, an international team of researchers has found the first direct evidence that the deepest known sinkhole on the moon is connected to a cave that could provide shelter for astronauts.
Identifying areas of the Moon that can protect humans and robotic explorers from the moon’s harsh environment—including powerful radiation and extreme temperatures—could be vital to future space exploration, as countries like the U.S. and China race to establish a long-term human presence on the closest natural satellite to Earth.
To find the cave, the team examined archival radar measurements of a depression in a large plain called Mare Tranquillitatis, or the Sea of Tranquility, where the Apollo 11 mission landed in 1969.
NASA’s Lunar Reconnaissance Orbiter collected radar data in 2010, using wavelengths to detect what was below the surface. Analyzing the data, researchers found empty space at depths 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, at least 45 meters (about 148 feet) wide and 30 to 80 meters (98 to 262 feet) long, may be just the first part of a longer tube cavity created by lava, and there are likely more caves yet to be discovered, the researchers said.
“Lunar caves have long remained a mystery. Scientists have assumed their existence for more than 50 years. So it was surprising to finally be able to prove the existence of an underground channel,” study co-author Leonardo Carrer, an assistant professor in the department of information engineering and computer science at the University of Trento in Italy, said in an email.
The authors note in the study that future exploration and study of lunar caves could provide new information about lunar evolution and lunar volcanism.
“Caves are a unique environment that preserves the history of the Moon,” said study co-author Lorenzo Bruzzone, director of the Remote Sensing Laboratory at the University of Trento and professor of telecommunications, via email. “Analysis of the rocks inside these caves could allow new (discoveries) regarding lunar evolution… such as the timeline and duration of lunar volcanic activity and the actual composition of the lunar mantle.”
“Moon caves could also be an alternative or integration to a base on the lunar surface,” he added.
Using moon caves as shelter
According to NASA, near the Moon’s equator, temperatures reach 250 degrees Fahrenheit (121 degrees Celsius) during the day and drop to minus 208 F (-133 C) by nightfall. As recorded by the Lunar Reconnaissance Orbiter, harsh temperatures have even dropped below minus 410 F (-246 C).
Researchers of a July 2022 study that examined a series of pits on the Moon (some of which NASA assumed were caves) found that the shaded shelters were much more thermally stable, staying consistently around 63 F (17 C).
“It’s really exciting to have additional evidence of a cave extending from the floor of the Mare Tranquillitatis Trough,” Tyler Horvath, lead author of the 2022 study and a doctoral student in planetary science at the University of California, Los Angeles, said in an email.
“Lunar caves could provide major benefits for astronauts and rovers as they seek refuge from lunar surface hazards such as radiation, micrometeorites and extreme temperatures, which could open the door to a long-term, large-scale human presence on the Moon,” added Horvath, who was not involved in the new study.
There are more than 200 known pits on the moon like the one in Mare Tranquillitatis, but only a handful of them lead to caves, Horvath said. Carrer added that some of the lava tubes may be inaccessible because of debris blocking the passageways.
The interior features of the newly discovered cave will remain unclear until a rover or human explorer provides more information. Horvath said its interior could resemble a freshly formed lava tube on Earth, but much larger. It would likely appear to have smooth walls, except where dripping lava forms protrusions and ridges known as lava stalactites, and the cave would be pitch black except where sunlight reaches the bottom of the pit and is reflected, added Paul Hayne, an associate professor in the department of astrophysics and planetary sciences at the University of Colorado, Boulder.
The cave’s large size isn’t surprising given the moon’s lower gravity and its effects on the stability of lava tubes, which could make larger caves less likely to collapse, said Hayne, who co-authored the 2022 study but was not involved in the new research. Most of Earth’s lava tubes are located in Hawaii, the Canary Islands, Australia and Iceland, and range in diameter from 33 to 98 feet. An August 2020 study suggested that lava tubes on the moon could reach up to 1 kilometer (3,280.84 feet) in diameter.
“We shouldn’t be too surprised if future exploration reveals even larger caves. We are 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 sloped at a maximum angle of 45 degrees, indicating that it was likely accessible, Carrer said.
But because of the depth of this cave (and potentially other lunar caves), lowering astronauts to the pit floor may require some creativity — possibly involving the use of rock-climbing equipment such as ropes and belay devices, and larger cables for equipment and building materials, Hayne said. Eventually, the process may involve a more permanent structure such as a freight elevator, he added.
Other ideas for exploring the deep Mare Tranquillitatis sinkhole and the newly discovered cave within it include a robotic crane, currently being investigated by the European Space Agency, that could be used to lower an astronaut or rover to the ground.
Noah Petro, director of NASA’s Planetary Geology, Geophysics, and Geochemistry Laboratory, suggested that lunar skygazers could spot the Mare Tranquillitatis, where the cave was discovered, by looking for a large, dark plain. He suggested looking up and looking at the area 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 the pits and potential caves) goes back a few years, (but) this new information from the radar data says, ‘Wow. They’re a little bigger than we expected,'” Petro said. Petro is the project scientist for both the Lunar Reconnaissance Orbiter and the Artemis III mission, which 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 and is generating incredible datasets that are changing the way we look at the moon and are 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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