Lunar colonization isn’t mere science fiction anymore. Billionaires plan to send tourists on once-in-a-lifetime trips, and politicians say that they hope to colonize the Moon in the next few decades. There may even be ways for human colonists to harvest water from ice that may be permanently shadowed in certain caves.
But where could a human colony actually live? The Moon has no atmosphere or magnetic field to shield it from solar radiation and micrometeorites that constantly rain onto its surface. That’s no environment for our squishy, earthling bodies.
Scientists studying the Moon’s surface may have found the answer: shelter humans in lunar lava tubes. The Moon is covered in huge swaths of ancient basaltic lava flows. Earth’s volcanoes can also erupt similar flows, with basalt sometimes running as molten rivers. In these rivers, the outside cools faster than the inside, creating a hard shell. The remaining lava pours out, leaving a hollow space behind.
Do similar lava tubes exist on the Moon?
In a presentation on Wednesday at the 48th Lunar and Planetary Science Conference (LPSC) in The Woodlands, Texas, Junichi Haruyama, a senior researcher at the Japan Aerospace Exploration Agency (JAXA), discussed one such lava tube that he suspects may be snaking underneath the Moon’s surface.
Searching for Lava Tubes
In 2009, Haruyama and his team spotted evidence of a dark hole in the Moon’s Marius Hills region in data from the Japanese lunar orbiter Kaguya (nicknamed SELENE, which stands for Selenological and Engineering Explorer). What the researchers didn’t know was whether the pit led to something larger below. Two narrow surface depressions called sinuous rilles, which scientists think represent collapsed portions of lava tubes, stretch away from the pit. Could the pit be a skylight opening to an intact lava tube’s long, narrow passage?
Ancient basaltic lava flows called lunar mare cover much of the Moon, similar to the much younger Columbia River basalts in the western United States. But because the Moon’s gravity is one sixth that of Earth’s, gravity doesn’t impede lava flow as much, allowing lava to spread widely across the surface. Nonetheless, lunar lava tubes may have formed in an Earth-like way, Haruyama said.
Last year, another team spotted gravity anomalies that suggested hollow, narrow spaces around the Marius Hills pit. These data came from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, which consists of two spacecraft orbiting the Moon. The orbiters can detect these anomalies by measuring how much the Moon’s gravity tugs on them. Areas of more mass tug on the spacecraft more, whereas hollow areas have less mass and so tug on GRAIL less.
To confirm GRAIL’s findings, Haruyama and his colleagues turned again to SELENE’s data, looking closer at the sinuous rilles. They specifically looked at data from SELENE’s Lunar Radar Sounder (LRS), which imaged the subsurface using low-frequency radio waves.
The LRS data revealed hollow space more than 100 meters deep in some places and tens of kilometers long underneath one of the rilles near the pit. The pit it self looked to be 50 meters deep. These data led researchers to believe that the pit could, indeed, be a collapsed portion of a lava tube roof. These data also match the gravity readings from GRAIL, Haruyama said.
Exploring the Tubes
If humans, via rover or their own two feet, ever got access to the tubes, “the science would be amazing,” said Brent Garry, a geophysicist at NASA Goddard Space Flight Center. The tube’s interior tempts with pristine surfaces, absent of lunar soil or bombardment by micrometeorites, he said. These surfaces could offer answers to questions about the Moon’s origin and formation.
In another LPSC presentation on Wednesday, Garry detailed a way to closely explore these tubes, using light detection and ranging, also known as lidar. On Earth, scientists use lidar scanners to map both land and the ocean floor. More recently, scientists have started employing lidar to map Earth’s vast network of caves.
Over the past 2 years, Garry and his team used a lidar scanner to map the inside of a lava tube at Craters of the Moon National Monument and Preserve in Idaho. The park is named for the otherworldly feel of its basaltic lava flows. Apollo astronauts even studied the geology of the park there before ever stepping on its namesake.
Lidar scanners work by pinging their surroundings with beams of laser light and measuring the time it takes for the light to bounce back. The scanners can take millions of data points every second, allowing for the creation of highly detailed 3-D maps. They also don’t depend on sunlight, which could make them useful in a shadowy lunar pit. Garry suggests that lidar would be extremely useful in mapping centimeter- to millimeter-scale features, helping future explorers determine the structure of a lava tube.
How to get the scanner into a tube is another story, one that would involve transportation using a rover, Garry said.
Future of Lunar Exploration
Haruyama and Garry agree that lava tubes could, in theory, shield humans from the Sun’s unfiltered radiation and the wide surface temperature fluctuations experienced on the Moon: temperatures over one Moon day (27 Earth days) can range between 123°C and −153°C. In contrast, Earth’s average temperature is only about 16°C.
What’s more, lunar lava tubes likely have flat floors like those on Earth, easing the way for vehicles or instruments, Haruyama said.
However, long-term human colonies on the Moon likely won’t happen in the near—or even far—future. Ben Bussey, chief exploration scientist for the Human Exploration and Operations Mission Directorate at NASA, explained during a NASA town hall meeting on Monday that NASA’s deep-space habitability plans are currently focused on reaching Mars and that “plans don’t call for going onto the lunar surface before going to Mars.”
But if those plans ever change, at least we know may have a place to crash—figuratively, at least.
—JoAnna Wendel (@JoAnnaScience), Staff Writer
Editor’s Note, 18 October 2017: An academic paper on the research featured in this news story published this week in Geophysical Research Letters.