Scientists have long theorized the existence of lunar caves — underground passageways formed through volcanic processes that connect to the pits covering the moon’s surface. Now, an international team of researchers has found the first direct evidence that the moon’s deepest known pit connects to a cave that could provide shelter for 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 for 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, this discovery is especially significant.
Uncovering the cave
To find the cave, the team studied archival radar measurements of a pit within Mare Tranquillitatis, also known as the Sea of Tranquility, 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. Analyzing the data, the researchers found empty space at an underground depth of 130 to 170 meters (426 to 558 feet), according to their study published in Nature Astronomy.
The newly revealed cave, measuring at least 45 meters (nearly 148 feet) wide and between 30 and 80 meters (98 and 262 feet) long, could be just the initial part of a longer tube cavity formed by lava. Researchers believe there are likely more caves yet to be discovered.
“Lunar caves have remained a mystery for a long time. Scientists hypothesized their existence for more than 50 years. So it was surprising to be able to finally prove the existence of a subsurface conduit,” said study coauthor Leonardo Carrer, an assistant professor at the University of Trento in Italy.
Future research and exploration of lunar caves could also provide new insights into the evolution of the moon and lunar volcanism. “The caves are a unique environment that preserves the history of the Moon,” said study coauthor Lorenzo Bruzzone, a telecommunications professor at the University of Trento. “The analysis of rocks inside these caves could enable new discoveries related to the lunar evolution … such as the timeline and duration of lunar volcanic activity as well as the actual composition of the lunar mantle.”
Using moon caves as shelter
Near the moon’s equator, temperatures reach an average of 250 degrees Fahrenheit (121 degrees Celsius) in daylight and plummet to minus 208 F (-133 C) at nightfall, according to NASA. The harsh temperatures have even dipped below minus 410 F (-246 C), as recorded by the Lunar Reconnaissance Orbiter.
Researchers of a July 2022 study found that the shaded shelters are much more thermally stable, remaining at around 63 F (17 C).
“Having additional evidence of a cave extending from the floor of Mare Tranquillitatis Pit is really exciting,” said Tyler Horvath, a doctoral student at the University of California, Los Angeles, and lead author of the 2022 study.
“Lunar caves could provide massive benefits for astronauts and rovers as they can seek shelter from the hazards on the lunar surface such as radiation, micrometeorites, and temperature extremes, which could also open the door for long-term, large-scale human presence on the Moon,” Horvath added.
The moon has more than 200 known pits like the one at Mare Tranquillitatis, but it’s likely only a handful extend out to caves, Horvath said. Some of the lava tubes could also be inaccessible due to debris blocking the passageways, Carrer added.
The interior features of the newly discovered cave will remain unclear until a rover or human exploration can provide more insight. The inside may appear similar to a recently formed lava tube on Earth but much bigger, Horvath said. This would likely look like smooth walls except where dripping lava might have formed bumps and ridges known as lavacicles, and the cave would be pitch dark, except where sunlight reaches and reflects from the floor of the pit, added Paul Hayne, an associate professor at the University of Colorado, Boulder.
“We shouldn’t be too surprised if future exploration reveals even larger caves. We are getting a glimpse of the Moon’s internal volcanic plumbing system,” Hayne said.
Future exploration of moon caves
The new study’s authors found the cave to be potentially flat or inclined at a maximum of a 45-degree angle, which suggests it would likely be accessible, Carrer said.
However, because of the cave’s depth, it could take some creativity to get astronauts down to the pit floor — possibly involving the use of rock-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 cave within include a robotic crane to lower an astronaut or rover down to the floor, which is currently being explored by the European Space Agency.
Noah Petro, the chief of NASA’s Planetary Geology, Geophysics and Geochemistry Lab, noted that sky-gazers observing the moon can identify Mare Tranquillitatis by looking for a large, dark plain. He recommended looking up and peering at the site on July 20 to celebrate the 55th anniversary of the Apollo 11 landing, the first mission to put humans on the moon.
“The discovery of the pits and potential caves goes back several years, but this new information from the radar data says ‘Oh, wow. They’re a little bit larger 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.
“LRO has been at the moon now for 15 years, creating some incredible datasets that’s changing our perspective of the moon and being used to make these sorts of discoveries,” Petro said. “The cave is one of those wonderful surprises the moon always has in store for us.”
