For years, lunar scientists have been intrigued by pits and holes that pock the surface of the moon. Since 2009, the Japan Aerospace Exploration Agency (JAXA) has hypothesized that some of these holes are actually skylights leading to vast underground networks of caves and caverns carved by lava flows in the moon's ancient past, known as lava tubes. If such subterranean realms exist, they could be perfect for constructing habitats and research stations on the moon, safely shielded from the radiation and intense sunlight that bombards the lunar surface.

Recently, new research from NASA, JAXA, as well as the University of Padova and the University of Bologna in Italy has supported the idea that vast underground networks exist on the moon, stretching for dozens of miles, large enough to house city-sized colonies.

Radar data from JAXA's Kaguya spacecraft—which orbited the moon from 2007 to 2009 and is officially named SELENE—was analyzed to locate and measure the size of lava tubes on the moon. The new study, published this week in Geophysical Research Letters, measured an "echo pattern" at one of the sites of a possible lunar cavern among the moon's Marius Hills, a group of volcanic domes on the western edge of the near side of the moon.

The Marius Hills Hole on the moon, thought to be a skylight leading to a vast subterranean cave. NASA/Goddard/Arizona State University

Similar echo patterns were measured at several other locations. American scientists were able to support the readings with data from NASA's Gravity Recovery and Interior Laboratory (GRAIL), which orbited the moon between 2011 and 2012. The echo patterns, which show the radar data tapering off and then spiking in a second echo peak, correspond with locations where GRAIL measured lower gravitational forces on the moon, suggesting hollow areas beneath the surface.

"Intact lunar lava tubes offer a pristine environment to conduct scientific examination of the moon's composition and potentially serve as secure shelters for humans and instruments," reads the study. Using the data from Kaguya's Lunar Radar Sounder (LRS), researchers estimate the underground cave accessed through the Marius Hill skylight to be about 150 feet (46 meters) wide and 30 miles (48 kilometers) long.

These images from NASA's LRO spacecraft show 10 of the known mare pits and highland pits. Each image is 222 meters (about 728 feet) wide. NASA/GSFC/Arizona State University

In addition, the University of Padova and the University of Bologna in Italy recently conducted a study to compare lava tubes on Earth in locations such as Hawaii and Australia to those glimpsed through skylight holes on the moon and Mars. These holes were likely created by impacting asteroids that penetrated the caverns beneath. Lava tubes are created when liquid hot rock near the core of a planet is thrust up through existing cracks and fissures as it jets toward the surface. These can leave vast caverns, but as demonstrated by the Italian study, presented last month at the 2017 European Planetary Science Congress, the size of these caverns varies significantly based on the gravitational forces of the planet.

"The comparison of terrestrial, lunar and martian examples shows that, as you might expect, gravity has a big effect on the size of lava tubes," said Riccardo Pozzobon, of the University of Padova, in a press release from the conference. "On Earth, they can be up to thirty meters across. In the lower gravity environment of Mars, we see evidence for lava tubes that are 250 meters in width. On the Moon, these tunnels could be a kilometer or more across and many hundreds of kilometers in length."

ESA astronauts studying terrestrial lava tubes in Lanzarote, Canary Islands, during the PANGEA 2016 course. ESA/L. Ricci

As Pozzobon points out, the findings have implications not only for building subterranean outposts on the moon, but also for the search for life on Mars. If any microbes can thrive on the Red Planet, they may be hiding in the relative warmth and safety of underground lava tubes.

The new NASA leadership is focused on returning to the moon, as are private industry and international space agencies. When humanity does return to the moon, this time to build permanent infrastructure, the first outposts might just slot into the lava tube caves hidden beneath the lunar surface.

Sources: Geophysical Research Letters and EuroPlanet

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