Underground moon cities possible with lunar lava tubes

Chuck Bednar for redOrbit.com – @BednarChuck
Underground cities on the moon could be theoretically possible, according to a new Purdue University-led study suggesting that there may be large, structurally-sound lava tubes. Lava tubes, the authors explained during last month’s Lunar and Planetary Science Conference in Texas, are tunnels formed from the lava flow of volcanic eruptions. As that lava cools, it cools around the edges to form a pipe-like crust, and once the eruption ends, it leaves behind a hollow tunnel. In this case, the tunnels may be large enough to house an entire city on the moon.
What kind of lava tubes would be needed for this to work?
According to Space.com, in the theoretical study, Jay Melosh, a distinguished professor of Earth, atmospheric, and planetary sciences at Purdue, and his colleagues looked at data from the NASA Gravity Recovery And Interior Laboratory (GRAIL) mission. They discovered that lava tubes on the moon could have diameters of more than half a mile (one kilometer).
Provided the lunar lava tubes have strong arched shapes like those found here on Earth, they may be stable at widths of up to several miles (5,000 meters), explained lead investigator David Blair. While structures like this would not be possible on Earth, the reduced gravity on the moon and the fact that lunar rock does not need to withstand the same degree of erosion and weathering makes it theoretically possible.
The researchers concluded that the stability of a lava tube (and its subsequent ability to support an underground lunar colony) would depend upon its width, the thickness of the roof and the stress state of the cooled lava. They created models using a range of these variables, as well as lava tubes with walls created by lava placed by both one thick layer and several thin ones.
Breaking down the research with Professor Melosh
“Lunar sinuous rilles have long been recognized as the feeder channels of enormous lunar lava flows,” Professor Melosh told redOrbit, explaining that these long, narrow depressions are “far larger than any similar channels on Earth” and are often up to five kilometers wide, 500 meters deep, and nearly 100 kilometers long due to the “much larger volume of lunar lava flows” in comparison to similar events that take place here on Earth.
Direct proof of caves beneath the lunar surface were recently detected during the recent Japanese Kaguya mission, Melosh said, and that discovery was both confirmed and extended by NASA’s Lunar Reconnaissance Orbiter mission. These so-called open pits or “skylights” can be up to 100 meters across, he continued, and attest to the presence of larger caverns beneath.
“Our group has been using GRAIL gravity data to search for empty subsurface lava tubes,” the professor told redOrbit. “This search is difficult because the signature of lava tubes is at the limit of what the GRAIL data can tell us. While we do not yet have definitive evidence of the existence of large lava tubes, we wanted to find the maximum size limits of possible lunar lava tubes.”
If confirmed, these caves could be “ready-made” lunar shelters
Melosh said that he and his colleagues were “surprised to find that quite large tubes would be stable under lunar gravity, up to several km across and several hundred meters high. These dimensions are similar to those of observed rilles and so it is possible that underground caves similar to the open rilles might exist below the Moon’s surface.”

“If such caves do exist, they would provide ready-made shelter from cosmic radiation and small meteorite strikes, both serious hazards to astronauts or would-be lunar colonists, and they would greatly decrease the cost of creating safe habitats on the Moon,” he added.
“The next step in confirming the existence of such large open cavities,” the professor concluded, “would be direct sounding below the Moon’s surface, using a ground-penetrating radar (perhaps aboard an orbiting spacecraft), similar to instruments flown on Apollo 17, Kaguya, and the recent Chang’e 3 missions. Radar waves of long wavelength can penetrate several km into the lunar surface and should show up the presence of large subsurface caves, if they exist.”
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