THE WOODLANDS, Texas—A system of buried, empty lava tubes hides beneath the moon’s surface, remnants of a bygone age when the volcanically active moon launched fountains of fire into space.
At least, that’s what scientists think.
For years, teams have hunted for these elusive sublunar tunnels, which can be large and sturdy enough to house entire cities. In fact, lunar lava tubes could be ideal locations to establish a moon base, as their thick roofs would shield humans from harmful radiation and small meteorite impacts. But until now, the strongest observational hints of the tubes’ existence came from a smattering of detectable surface features, including skylights and rilles, channel-like depressions thought to form when tubes collapse.
This week, scientists announced that the signatures of at least ten buried lava tubes could be written into a map of the moon’s gravitational field.
It’s “the strongest evidence yet that shows signals consistent with that of buried, empty lava tubes on the moon,” said Purdue University’s Rohan Sood, who presented the observations at the Lunar and Planetary Science Conference.
Sood and his colleagues began their search for lava tubes in the Marius Hills region, where scientists suspect that a skylight has opened into one of the buried tunnels. That portal, discovered by Japan’s moon-orbiting Kaguya spacecraft and reported in 2009, is approximately 65 meters wide and 80 meters deep. It also sits by two rilles boringly known as A and B. In other words, there are multiple lines of evidence suggesting that lava once oozed and flowed beneath the Marius Hills.
“We see a skylight that is along this rille, but we don’t know if that is an access point into a lava tube or not,” Sood says. “Can we pick that up using gravity data?”
Sood and his colleagues searched for the Marius Hills tube using data from NASA’s twin GRAIL spacecraft, which flew in tandem above the moon’s surface as they measured and mapped its gravitational field (in 2012, the spacecraft were purposely crashed into a site now named after astronaut Sally Ride). The moon’s gravitational field is affected by masses below the surface, as is Earth’s. Put simply, large chunks of mass will produce an increase in gravity, Sood says, but “if you fly over a lava tube, there’s going to be a dip in gravity.”
The team spotted a gravitational signature that could be a lava tube near the skylight, and then wondered if it would be possible to detect similar signatures in areas with no obvious rilles or skylights. Turns out, the GRAIL data contain at least ten telltale anomalies resembling lava tubes, slithering and twisting beneath the moon’s surface. They’re all located on the moon’s near side, near the dark stains left by ancient volcanic seas, and some of the candidate tubes are more than 100 kilometers long and several kilometers wide—large enough to swallow a small city.
Of course, it’s not certain that the tubes are actually there. The GRAIL data provide the strongest evidence for their presence, but definitive proof would require a moon-orbiting spacecraft that uses ground-penetrating radar to peer beneath the moon’s surface. Sood and his colleagues have proposed just such a space robot, called LAROSS.
“The proposed radar will not only help confirm our findings but will also give us an opportunity to find smaller lava tubes, ones that were beyond the resolution of GRAIL gravity data,” Sood says.
Maybe someday, after looking for lava in all the right places, space-faring humans will not only solve the mysteries of Earth’s closest celestial companion but use it as a giant shield against the dangers of space.
Full citation: Detection of buried empty lunar lava tubes using GRAIL gravity data. R. Sood, L. Chappaz, H. J. Melosh, K. C. Howell, and C. Milbury. LPSC abstract here.