Shown here is a skylight in the Moon’s Marius Hills.
Credit: NASA/GSFC/Arizona State University

Earth’s Moon is peppered with potential ready and waiting subsurface habitats in the form of lava tubes.

These features are good candidates for permanent human shelters. Moreover, these underground niches could reduce hazards to crews from lunar temperature swings, radiation, seismic activity, and meteorite impacts.

Evidence for their existence under the surface of the Moon has been provided by NASA’s GRAIL mission, Japan’s SELENE spacecraft, and NASA’s Lunar Reconnaissance Orbiter (LRO). Data from GRAIL suggests that lava tubes can be more than 3,000 feet to over 6,500 feet (1–2 kilometers) in width.

NASA’s twin Gravity Recovery and Interior Laboratory (GRAIL) probes.
Credit: NASA

Stability and safety

Still, the safety and resilience of those habitats are main concerns.

The size and geometry configurations under which the lava tubes are stable are not well-defined and there are no well-established criteria for their steadiness. A cave in would mean a bad day for Moon underground crews.

New research provides analytical and numerical solutions to estimate the size and stability of lunar lava tubes.

Deep diving into the Moon’s underground lava tubes. Entrances or “skylights” to lava tubes might provide future explorers a comfy safe haven.
Credit: Pascal Lee/Mars Institute/SETI Institute

Roof work

In work led by Audai Theinat of the Lyles School of Civil Engineering at Purdue University, a research team has used the flow characteristics of molten lava and the limit equilibrium analysis of the collapsed pits to estimate the size of the lunar lava tubes. Different sizes were considered, from 985 feet (300 meters) up to 13,000 feet (4,000 meters) in width.

According to the investigation by Theinat and team, the results show that the key parameters for the stability of the tubes are the roof thicknesses and the strength of the lava, in particular, its tensile strength. As the lava tube gets wider, a larger tensile strength and roof thicknesses are needed for the tube to remain stable, they report in the online journal, Icarus.

High Sun view of the Mare Tranquillitatis pit crater revealing boulders on an otherwise smooth floor.
NASA/GSFC/Arizona State University

The study indicates that 1 kilometer wide lava tubes are likely to exist and remain stable. For larger widths, however, minimum roof thicknesses of few hundred meters might be necessary to preserve their stability.

Skylight

Meanwhile, how best to “ground truth” lunar lava tubes earned the attention of NASA’s Innovation Advanced Concepts (NIAC) program. Recently receiving a NIAC Phase III award is robotic pioneer, William “Red” Whittaker, a professor at Carnegie Mellon University’s Robotics Institute.

Skylight is a fast, autonomous micro-roving mission concept to explore and model these features that might be gateways to caves on the Moon.

Skylights on the Moon are collapses that come about over subsurface voids. Here on Earth, skylights occur in many lava tubes, providing access to underground caves.

Skylight is a fast, autonomous micro-roving mission concept to explore and model these features that might be gateways to caves on the Moon.
Credit: William Whittaker

Smart and speedy robots

Whittaker’s NIAC-supported work is evaluating use of one or more “smart and speedy” robots to inspect and quickly generate on-the-spot, computer-generated models of craters on the Moon.

These craters, up to now only spotted by Moon-orbiting spacecraft, may be sites of caves that could offer future explorers underground shelter, as well as access to minerals, ice and other resources.

It is essential to develop small autonomous robots, Whittaker says, self-thinking machinery that’s fast and can achieve lunar crater investigations in just a week. After that period of time, he points out that the solar-energized equipment would be exposed to the ultra-chilly lunar night that would permanently disable them.

Rim reasoning

“The idea here is that a rover coming onto a pit is akin to seeing the Grand Canyon for the first time,” Whittaker suggests. “The robot will be constantly reasoning to occupy the vantage points at the pit’s rim, then game the risk. Acquiring thousands of images, the robot would generate high-fidelity scientific models that are the purpose of this exploration.”

“The game is on,” underscores Whittaker in terms of renewed human exploration of the Moon.

For more information on the research paper – “Lunar lava tubes: Morphology to structural stability” – go to:

https://www.sciencedirect.com/science/article/abs/pii/S0019103518307826#ab0005

Additional information on NIAC and the Whittaker work can be found at:

https://www.nasa.gov/directorates/spacetech/niac/2019_Phase_I_Phase_II/robotic_technologies-enabling-the-exploration-of-lunar-pits/

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