Science and Exploration

Mimicking the Moon – Here’s the (Simulant) Dirt

By Leonard David
June 28, 2023
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Mimicking the Moon – Here’s the (Simulant) Dirt
Lunar simulant is used to test a rover at Michigan Tech’s Planetary Surface Technology Development Lab.
Image credit: Michigan Technological University.

There is a growing business in selling fake Moon dirt. It is labeled as “simulant:” customized concoctions of lunar topside that are celestial stand-ins for different areas on the Moon.

And simulant sales are up. But why?

Government and private groups are keen on figuring out what wheels spin best, how drills drill, and what excavation methods work on the Moon, without having to guess in advance. Multiple nations are scripting plans to further explore Earth’s nearby escort, even to set up permanent encampments there to sustain lengthy stays by humans.

But as sci-fi novelist Robert Heinlein pointed out, the Moon is a harsh mistress.

The airless body is pounded by solar wind, radiation, and micrometeorites. The Moon is blanketed by grayish, sharp-edged particles and rocky debris termed the lunar regolith. At one-sixth the gravity of Earth, that world is a Disneyland of dust that can stick to space suits, gum up equipment, and jam mechanical components.

The know-how to wrestle with the woes of operating on the Moon took center stage at the 23rd meeting of the Space Resources Roundtable, held in Golden, Colorado on June 6 through 9 at the Colorado School of Mines.

Geotechnical testing

Regolith simulants are very important for testing lunar mechanisms and equipment. Regolith, and therefore realistic simulants, behave very differently than soils here on Earth, Michigan Technological University mechanical engineer Paul van Susante told SpaceRef.

“Many aspects are important but some aspects are more important to be matched than others, depending on the type of testing to be performed,” van Susante said.

For “geotechnical” testing — applying geological science in civil engineering, say for Moon mining purposes — van Susante said particle size and shape are the most vital. So too is the mineral composition of simulant, which often includes the anorthosite and basaltic glasses that coat the Moon. For other tests, properties like electrical conductivity, magnetic susceptibility, or thermal emissivity of lunar regolith must also be carefully considered and precisely imitated, he said.

Wanted: relevant results

“Simulants used in the tests need to meet certain figures of merit before they can be used as an analog material for lunar environment testing,” van Susante added. “For certain testing, it is also required to test under vacuum and cold or hot conditions to create the behavior of lunar regolith simulant that is important for the test results.”

Many simulants exist and can be purchased, van Susante said, but it is important to choose the right simulant and the correct test environment to get relevant results.

Serving up simulant. Different types of lunar regolith can be made on Earth. Image credit: Exolith Lab.

NASA Artemis expedition plans are now focused on investigating permanently shadowed regions or PSRs. Near the north and south poles of the Moon, these regions don’t receive direct sunlight; they are super-cold vaults of trapped water ice and other volatiles, including ammonia and methane. By tapping into these “pools” of compounds, that icy elixir can be converted into oxygen, drinkable water, or rocket fuel — all priceless commodities for any nation establishing base camps.

The recipe for a specialized simulant, van Susante said — one that allows testing of an icy regolith — can be had by mixing water and then freezing or mixing shaved ice into the regolith. “Cryogenically frozen regolith mixed with other volatiles can be made, but this is very challenging and costly, but is needed for certain tests.”

Set of standards

There’s need to maintain a common set of standards for lunar simulant materials, Frederick Slane, executive director of the Space Infrastructure Foundation in Colorado Springs, Colorado, told SpaceRef. To this point, he has already seen the use of sawdust serving as an unrealistic simulant in a test meant to work out the bugs for equipment operating on the Moon — a notoriously sawdust-free environment!

Slane said that a leading lunar sourcebook scoped out a large number of varying characteristics of Moon samples returned to Earth by robotic mooncraft and Apollo human landing missions. These, he explained, can be used to inform future simulant design.

What’s now known about lunar regolith is that the Moon consists of a diversity of rock types that differ widely in both their chemistry and mineral makeup.

“So trying to reduce that to something small for engineers to handle,” is important; a way of “measuring the goodness” of simulant characteristics in terms of chemical composition, density, shape, and size, Slane told SpaceRef.

Level playing field

With private groups on the rise offering lunar services for fee — not free — it seems likely that companies and space agencies will keep what they know about what works and doesn’t work on the Moon close to their chest. But what would help all is a level playing field, where agencies have access to the basic blend of distant lunar regions.

“Depending on where you’re going on the Moon, some things are unique to each of those regions. It would be good to know those things ahead of time, before you show up and find out, well, that didn’t work,” Slane advised.

In developing a standard, you are basically balancing cooperation for the general good and competition. “Competition is always on the table,” Slane told SpaceRef. “You can use a standard as an interface between what really is proprietary and what is general knowledge.”

Stick to the mineralogy

Exolith Lab produces Moon, Mars, and asteroid regolith simulants. It is based at the University of Central Florida’s Center for Lunar and Asteroid Surface Science.

The easiest way to run Moon-bound equipment ahead of time using simulants is “stick to the mineralogy as closely as you can,” University of Central Florida astronomer and planetary scientist Dan Britt, who founded Exolith Lab, told SpaceRef — no sawdust or other poor imitations to be found.

It’s a booming business for Exolith Lab, Britt said. “I think the largest order was for something like 18 tons. We sell quite a bit,” he said, and last year they sold 60 tons of simulant, sending it out all over the world.

Synthesized from terrestrial materials sourced from a number of places including Greenland, Montana, and Idaho, Exolith Lab cranks out simulants that mimic the chemical, mechanical, or engineering properties of the off-Earth materials that one would deal with on the Moon.

“The whole point is to provide something that is a reasonably close, reasonably reliable, and reasonably repeatable to support scientific research and engineering testing,” said Britt, adding that the company usually advises customers on what type of simulant to use.

Intellectual infrastructure

“People are starting to understand the mineralogy and environment of the Moon a lot better and beginning to work smarter,” Britt said. He admitted, however, that there are those getting into the field not knowing the first thing about the Moon.

“They look up at the full Moon,” Britt said, “and they see light stuff … they see dark stuff. We ask them which will it be?” These customers tend to get puzzled throughout the process, Britt said, before eventually saying “Send me some light stuff.”

“Okay,” Britt said, “we can work with that.” The lighter-colored areas, the highlands of the Moon, show the earliest lunar crust, dominated by a brand of rock called anorthosite. The dark regions are the lunar maria, mainly composed of silicates and oxides including olivine, ilmenite, and basalt, AKA lava rock.

Looking to the future, Britt excitedly awaits the output from a spate of robotic landers and rovers, innovative science tools, and also having new lunar material in labs here on Earth. That means that our “intellectual infrastructure” will grow, he said, “to ask better questions with better instruments.”

Leonard David

Leonard is author of Moon Rush: The New Space Race, Mars – Our Future on the Red Planet, and co-authored with Apollo 11’s Buzz Aldrin of Mission to Mars – My Vision for Space Exploration - all published by the National Geographic Society.