Getting humans back to the moon — "this time to stay" — will require the exploitation of lunar resources, NASA officials and exploration advocates say.
The most important resource, at least in the short term, is water ice, which is abundant on the floors of permanently shadowed polar craters. The ice found in these "cold traps" is thought to be stable and accessible.
But there may be other spots on the moon that could yield a mother lode of scientific data — as well as the resources needed to sustain human occupation of Earth's celestial next door neighbor.
That's the pits
Researchers have identified "pits" on the moon, which are likely lava-tube "skylights" — geological doorways to underground tunnels that were once filled with lava.
If they do indeed provide access to lava tubes, skylights could be a game-changer for human lunar exploration, said NASA Chief Scientist Jim Green. Lava tubes are protected from the harsh environment of the lunar surface, which is bombarded by radiation and experiences temperature extremes. One lunar day lasts about 29 Earth days, meaning surface locations endure about two straight weeks of daylight followed by two weeks of darkness.
"There are a number of things on the moon that are going to be surprises," Green said.
"We need to get in there," he added, referring to lunar skylights. "We need to verify. Maybe there's a lot of water in these skylights? We don't know. We're finding them all over the moon."
A lava-tube network would suggest protected corridors, free of temperature swings, bombarding radiation and menacing meteoroids. They also might offer a much larger habitat capability for future moon explorers.
"We could actually build connective roads in them," Green told Space.com. "It could be a whole new world for us. That's another absolute game-changer."
More data needed
We don't have enough information yet to ascertain if skylights on the moon represent an interconnected underground roadway, said Pascal Lee, a planetary scientist at the SETI (Search for Extraterrestrial Intelligence) Institute. He is also chairman of the Mars Institute and director of the NASA Haughton Mars Project at NASA's Ames Research Center in Mountain View, California.
"For starters, not all pits on the moon are necessarily lava tube skylights," Lee told Space.com. He said that some might be associated with isolated underground cavities.
"Secondly, not all lava tubes in a given region should be expected to be interconnected," he added. "Indeed, some might have formed at different times, and might run at different levels or depths underground."
Maze of corridors?
Lee also said that while some lava tubes on Earth have smooth walls and floors, most have very rough surfaces and debris piles on their floors.
"We don't know how rough lava tubes on the moon might be, but the term underground roadway seems optimistic," Lee said. "In any case, in my view, it's not that pits on the moon would lead to a maze of underground corridors that makes them most interesting — although that is fascinating — but the fact that they give access to an environment that's radically different from the surface, whatever shape that underground environment might have."
Any underground cavity on the moon, after all, would provide shielding — from temperature swings, space radiation, micrometeoritic bombardment and sandblasting from the rocket engines of landing or departing spacecraft.
Most intriguing to Lee are candidate pits recently identified inside Philolaus Crater near the north pole of the moon.
"They might be skylights associated with a network of lava tubes formed not in volcanic lava flows, but in an impact melt sheet, the temporary pool of molten rock that ponded inside Philolaus Crater following the large impact that created the crater," he said.
Interestingly enough, Lee said, the candidate pits inside Philolaus are located at such a high latitude that sunlight would never enter the underlying caves.
"These would be in perpetual darkness and so cold that ice could be cold-trapped in them, much like it is in the permanently shadowed regions at the actual poles of the moon," Lee said.
Exploring high-latitude pits on the moon might therefore offer an additional opportunity to harvest water on our lunar neighbor, Lee said.
Meanwhile, researchers have begun assessing the viability of underground lunar habitats.
Anahita Modiriasari, a postdoctoral researcher in Purdue University's Lyles School of Civil Engineering, and her colleagues have been appraising lunar imagery, reconstructed into a 3D model to evaluate lava tubes as a potential habitat for humans on the moon. This is a task that a rover or drone could potentially accomplish on the lunar surface.
The work is part of Purdue's Resilient ExtraTerrestrial Habitats (RETH), a project that investigates the value of future human habitats on the moon or Mars.
"All of this collected data is vital," Modiriasari said. "We are using it to build an advanced model of the size, strength and structural stability of the lava tube," she said. For example, what happens during seismic activity? What would happen if a meteorite strikes?
In another development, the NASA Innovative Advanced Concepts (NIAC) Program recently awarded a Phase 3 contract to researchers developing robotic technologies to enable the exploration of lunar pits.
The "Skylight" concept mission is led by William Whittaker of Carnegie Mellon University. The NIAC award will help Whittaker and his team flesh out ways to explore and model a lunar pit. Doing so will require fast, autonomous micro-roving, which achieves significant exploration in a single lunar daylight period.
According to Whittaker, descent into and exploration of the lunar subsurface will come, but "pit-specific" questions must first be answered from the surface: How navigable are the rims? Are there caves? Are there rappel routes? What is the morphology?
Specifically, a mission of this type would create and downlink the first high-resolution, science-quality, 3D model of a vast planetary pit, Whittaker said.
"This [Skylight] initiative matures and transitions that technology. The technology innovations are exploration autonomy, in-situ 3D modeling, fast, far micro-roving and the aggregate means to achieve mission-in-a-week," Whittaker said.
The unanswered questions of lava-tube exploration aren't just technological. Also looming large, as with all aspects of lunar resource use and settlement, are space-law issues.
"Potentially exciting research areas cannot be claimed by sovereignty, by means of use or occupation, or by any other means," said Joanne Gabrynowicz, professor emerita of space law at the University of Mississippi and editor-in-chief emerita at the Journal of Space Law.
"Doing things like digging corridors and building roads could easily be interpreted as making a claim by use or other means. This is prohibited by the Outer Space Treaty," Gabrynowicz said. "The U.S. and all spacefaring nations are party to it. A location with high scientific value will require an international agreement regarding its use and who can access it."
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Leonard David is author of the recently released book, "Moon Rush: The New Space Race" published by National Geographic in May 2019. A longtime writer for Space.com, David has been reporting on the space industry for more than five decades. Follow us on Twitter @Spacedotcom or Facebook.