The Moon is a Barren and Desolate, but Lunar Caves Could Offer Some Shelter From the Harsh Environment

The dream of building a permanent settlement on the Moon: a place where humans from all walks of life can come together and give rise to a new culture and identity. A place where vital scientific research and experiments can be conducted, lunar industries created, and people can go for a little “adventure tourism.” It’s been the stuff of science fiction and speculative literature for over a century. But in the coming years, it could very well become a reality.

This presents many challenges but also opportunities for creative solutions. For years, astronomers have speculated that the perfect place to create a lunar colony is underground, specifically within pits, caves, and stable lava tubes visible and accessible from the lunar surface. According to new research from CU Boulder, preliminary results show these pits to be remarkably stable compared to conditions on the surface.

The research was led by graduate student Andrew Wilcoski of UC Boulder’s Department of Astrophysical and Planetary Sciences, who presented the group’s initial findings at the 2021 American Geophysical Union (AGU) Fall Meeting in New Orleans. This presentation, titled “Thermal Environments and Volatile-trapping Potential of Lunar Pits and Caves,” offers new 3D thermal models to characterize the temperature environments within lunar pits and caves, with the ultimate goal of assessing the stability of a range of volatile species within these pits.

Thanks to missions like NASA’s Lunar Reconnaissance Orbiter (LRO), the twin Gravity Recovery and Interior Laboratory (GRAIL) satellites, and JAXA’s SELenological and ENgineering Explorer (SELENE) – aka. “Kaguya” orbiter – scientists understand that the Moon has numerous pits and caves located across its surface. In many cases, these consist of stable lava tubes that formed when the Moon was still volcanically active billions of years ago.

In many cases, these tubes have collapsed in one or more sections (mainly due to impacts), creating holes from the surface into the interior (aka. “skylights”). These sites are considered to have great potential for future research missions since they would provide insight into the volcanic and impact history of the Moon. Mission planners at NASA, the ESA, Roscosmos, and the Chinese National Space Agency (CNSA) are also investigating them as possible sites for future human exploration.

These pits and caves could provide resources for future human exploration, not the least of which are volatile elements (like water ice). In the right abundance, this ice could be harvested and used to provide astronauts with everything from drinking water, showers, and even rocket fuel. In addition, these pits might be ideal for providing shelter that would protect astronauts (and maybe even settlers) from the hostile surface conditions – i.e., temperature extremes, micrometeorite bombardment, and radiation on the lunar surface.

“If we’re hoping to send people into these caves in the decades ahead, we want to know what they should expect down there,” said Wilcoski in a recent CU Boulder Today press release. To learn more about their suitability, Wilcoski and planetary scientist Paul Hayne – an assistant professor in the Laboratory for Atmospheric and Space Physics at CU Boulder and a co-author on the research – conducted a series of computer simulations to recreate what conditions are like below the Moon’s surface. 

Their initial findings indicate that the lunar pits and caves boast stable temperature conditions that would help astronauts weather some of the Moon’s most extremes. However, those same conditions would make them less than ideal for finding abundant supplies of water ice. In fact, most of the team’s simulated caves hosted temperatures of about -120 to 70 °C (-184 to 94 ° F) throughout an entire lunar day. 

Previous research by Hayne and other scientists has shown that hidden troves of water ice may have accumulated in certain lunar “cold traps” over billions of years. But, based on these new simulations, many lunar pits and caves are probably too warm to harbor similar troves. Ironically, this problem is similar to the situation on the lunar surface, where water ice cannot exist for long due to extreme temperature swings.

“As you get close to the equator, temperatures can reach more than 100 degrees Celsius during the day on the surface, and it will get down to 170 degrees Celsius below zero at night,” said Wilcoski.

Hence, most mission planners are currently looking at the cratered polar regions for potential sites to build habitats. The crater floors in these permanently-shadowed areas act as “cold sinks” that maintain consistently freezing temperatures – hence why abundant supplies of water ice have been observed there. In the same vein, the key to finding pits and caves that could have ice comes down to geographical location and orientation.

Whereas low-to-mid latitude pits are too warm to trap volatiles, pits at higher latitudes may have the right geometry and temperatures for water ice to remain stable over time. In addition, the simulations showed that orientation also played an important role. For example, if a cave’s mouth pointed directly at the rising Sun, it would experience scorching temperatures throughout the day, then plunge to frigid lows at night (compared to others that would remain frigid).

Another positive takeaway from all this, according to Hayne, is that no one knows how many pits and caves might be on the lunar surface. According to research based on LRO data (Wagner and Robinson, 2014), there may be more than 200 ranging between 5 meters (~5 yards) and 900 meters (~984 yards) in diameter. In the end, this latest research highlights the need for knowledge of the thermal environments in lunar pits and presents some exciting possibilities for possible lunar habitats.

“They’re attractive options for establishing a long-term human presence on the moon. One intriguing possibility would be to establish a protected base station inside a lunar pit or cave near one of the polar craters containing water ice. Astronauts could then venture out when conditions were right in order to collect ice-rich soil.”

As they indicated in their presentation, the next iteration of their thermal simulations will include a statistical “coupled Monte Carlo ballistic ‘hopping'” model that will evaluate the vapor pressure how long water can remain in these pits. The ballistic model will allow scientists to measure the role pit geometry and latitude play in trapping different types of volatile elements and predict the compositions of concentrated volatiles that may exist within lunar pits.

Habitats grouped together on the rim of a lunar crate, known as the Lunar Village. Credit: ESA

In the coming years, multiple space agencies intend to build lunar bases in the South Pole-Aitken Basin, with possible base sites including the Shackleton and Shoemaker craters. These bases would be able to harvest ice from the crater floor to fulfill their water needs and see to their power supply by positioning solar panels around the rim of the crater (with the option for nuclear reactors and fuel cells as well). But who says all bases will be established in these environments?

Others may still reside in the polar regions but be located underground in stable lava tubes with large caches of water ice. Who knows? If and when real estate becomes more sought after, settlers and commercial interests may find there’s little room left for surface habitats, and they’ll have to build habitats in the pits and caves around the poles. There’s even the possibility of establishing settlements in lava tubes large enough to house entire cities.

We’re going back to the Moon, alright. But this time, we plan to stay – maybe indefinitely!

Further Reading: CU Boulder, AGU