Scientists have long theorized the existence of lunar caves — underground passageways formed through volcanic processes that are connected to the pits covering the moon’s surface. Now, a team of international researchers has found the first direct evidence that the moon’s deepest known pit connects to a cave that could provide shelter to 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 such as 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 found within 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 by using wavelengths to detect what was below 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 Monday in the journal 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 — and there are likely more caves yet to be found, researchers say.
“Lunar caves have remained a mystery for a long time. Scientists hypothesized their existence since 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 in the department of information engineering and computer science at the University of Trento in Italy, in an email.
Future research and exploration of lunar caves could also provide new insight into the evolution of the moon and lunar volcanism, the authors note in the study.
“The caves are a unique environment that preserves the history of the Moon,” said study coauthor Lorenzo Bruzzone, a telecommunications professor and the director of the Remote Sensing Laboratory at the University of Trento, via email. “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.
“Moreover, (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 moon’s equator, temperatures reach a blistering 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 that looked at a number of pits on the moon — some that were hypothesized to lead into caves by NASA upon discovery — found that the shaded shelters are much more thermally stable and steadily remain 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 of planetary science at the University of California, Los Angeles, who was the lead author of the 2022 study, in an email.
“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. He was not involved with the new study.
The moon has more than 200 known pits such as 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 in the department of astrophysical and planetary sciences at the University of Colorado, Boulder.
The large size of the cave is not surprising, due to the moon’s lower gravity and the effects this can have on the lava tubes’ stability, with larger caves being able to form with less risk of collapsing, said Hayne, who is a coauthor of the 2022 study but was not involved with the new research. Most of Earth’s lava tubes are found in Hawaii, the Canary Islands, Australia and Iceland, and range from 33 to 98 feet in diameter. An August 2020 study had suggested that lava tubes on the moon could reach 1 kilometer (3280.84 feet) in diameter.
“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 in an email.
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.
But because of the depth of this cave (and potentially other lunar caves), 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 that could be used 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, 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 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. Petro was not involved in the new study.
“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.”