The Mare Tranquillitatis skylight spans 65 meters (210 feet) and clearly shows the debris-covered ground 36 meters (120 feet) below the surface. Credit: Arizona State University.
The Moon's surface presents an extremely harsh environment. Because it lacks a magnetic field, it is exposed to dangerous radiation levels 150 times higher than those on Earth. Without an atmosphere, it is totally vulnerable to deadly coronal mass ejections from the Sun, as well as a perpetual shower of meteorite impacts. And its temperatures range from 127 degrees Celsius (260 degrees Fahrenheit) during the day to -173 C (-280 F) during the lunar night.
For all these reasons, future lunar explorers are increasingly considering an alternative to living on the surface of the Moon: living low In the underground caves, images taken from satellites orbiting the Moon over the past 20 years have shown hundreds of large pits lined with overhangs that appear to jut out of the ground. It was hoped that these might lead to even more cavernous spaces, such as lava tubes that could be remnants of volcanic eruptions billions of years ago. But until now no evidence of these more extensive tunnels had been found, leading to decades of debate.
Now, a new analysis of radar observations provides some of the best evidence yet that at least one of those shafts, located in the Mare Tranquillitatis (Sea of Tranquility), leads to an underground tunnel that extends at least a hundred feet (“tens of meters,” the authors write) and possibly much further. The paper was published July 15 in Nature.
The study adds to a growing body of evidence about lunar caves, which are quickly emerging as a viable blueprint for hosting future permanent lunar outposts.
Lunar pits discovered
The existence of lunar lava tubes was first proposed half a century ago by planetary geologist Ronald Greeley, using data collected by the Apollo missions. Lava tubes are hollow channels beneath the lunar surface through which molten lava flowed. When lava flows cease, the remaining channel may be structurally supported as an underground chamber.
On Earth, lava tubes are no more than 30 metres wide. But engineering studies have shown that under the weaker lunar gravity, lava tubes up to 30 metres wide can reach 100 metres wide. 1,260 feet (385 meters)) wide could exist as much as 210 feet (65 meters) below the surface, assuming the density of the rock is similar to that of the samples recovered by Apollo. These supposed lava tubes remain hidden beneath the surface, but some reveal their presence by means of “skylights,” or collapsed pits where a section of the tube’s roof has collapsed, opening an access door to the underground tube.
Scientists first recognized lunar pits 15 years ago in images from Kaguya of Japan and NASA's Lunar Reconnaissance OrbiterSince then, with additional help from India's Chandrayan and China's Chang'e orbiters, more than 200 skylight-shaped pits, ranging from five to 300 meters wide, have been discovered.
A pit on the surface of the Moon is not necessarily proof that an underground lava tube extends from there. But more intriguing evidence for the existence of lunar lava tubes has emerged in the past decade. In 2011, NASA sent the Gravity Recovery and Interior Laboratory (GRAIL) mission to the Moon, which mapped the lunar gravity field at very high resolution. GRAIL data showed areas of the Moon with above-average gravity, as well as areas with a “gravity deficit,” indicating a corresponding mass deficit.
Of the pits observed so far, about a dozen have proven to be skylights leading to underground chambers. The first was discovered in Marius Hills, an ancient volcanic hotspot in Oceanus Procellarum, by Japan's Kaguya orbiter in 2008. 2017 Study Combining gravity data from the Kaguya radar and GRAIL revealed a match between areas showing a double radar echo and the locations of gravity deficits, a strong indication of underground voids in the so-called Marius Hills well.
A new Tranquility Base?
In 2011, NASA's Lunar Reconnaissance Orbiter located other promising skylights, including one in Mare Tranquillitatis that spans 210 feet (65 meters) and features a rock-covered floor 120 feet (36 meters) below the surface.
The new work uses advanced signal analysis techniques to study the radio signal reflected back from Mare Tranquillitatis using LRO's Miniature Radio Frequency instrument in 2010. The radio return signal is consistent with an initial bounce off the Moon's surface and a second bounce off the floor of a subsurface chamber.
3D modeling of the radar signal, which takes into account the satellite's viewing angle, indicates that an underground conduit extends west of the pit and the pile of collapsed rocks at its center. Further observations could determine whether the conduit continues in the opposite direction, toward the east, the team says.
The result is “the first direct evidence of an accessible lava tube beneath the surface of the Moon,” said Lorenzo Bruzzone, a remote sensing expert at the University of Trento. said in a press release. And “it suggests that (the Mare Tranquillitatis pit) is a promising site for a lunar base, as it offers refuge from the harsh surface environment and could support long-term human exploration of the Moon,” the authors write.
How to populate a moon cave
In the near term, NASA's plans for the Artemis program include landing and establishing a base near the lunar south pole. This location offers access to ice within permanently shadowed polar craters, which will be a valuable water resource for humans and could be used to create hydrogen and oxygen for rocket fuel. In addition, permanent solar power is available, since the Sun never sets on the high rims of polar craters.
But in the medium term, when a permanent, protected and secure base on the Moon is needed, underground caves and lava tubes are the natural next step.
Scientists still have much to investigate regarding the long-term stability of lunar underground voids for future habitability. Seismic activity and meteorite impacts have the potential to collapse thin-roofed lava tubes, with disastrous results for habitation either within them or on the surface above them. We know that these thin-roofed lava tubes can form when a surface layer of lava solidifies over a molten flow; a subsequent collapse reveals a sinuous, winding groove. One example is Rima Hadley in eastern Mare Imbrium which was explored by Apollo 15 in 1971.
Any potential lava tube would need to be explored, either by humans or equivalent robots, to confirm its actual habitability. The European Space Agency (ESA) is studying several concepts, including A tethered microrover who would descend into a skylight to explore what lies below.
But if those problems are overcome and a suitable lava tube is identified, a habitat solution may already be at hand. An inflatable module called Bigelow Expandable Activity Module (BEAM) has been docked to the International Space Station since 2016 for long-term testing. A similar inflatable habitat architecture could be used inside lunar lava tubes.
Studies have investigated sealing and pressurize a lava tube The project has proposed using it as a lunar habitat, but that plan is fraught with unknowns. A more practical initiative might be to deploy an autonomous construction robot, such as a large Roomba, that would clean and flatten the floor of a lava tube. Inflatable modules placed in the shelter of a lava tube could quickly establish a permanent base on the Moon. Additional data from LRO’s Diviner lunar radiometer experiment show that the temperature inside Tranquillitatis’ skylight remains a benign 63 F (17 C), simplifying the design of an inflatable habitat.
It would be ironic if humanity evolved over 30 millennia and achieved routine interplanetary spaceflight, only to return to another planet and return to living in caves. But lunar lava tubes hold real promise for housing future astronauts on the Moon.
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