Regolith

Regolith is the loose layer of dust, sand, and small rocks that lies on top of solid bedrock. It is found on the Moon, on Mars, and on many asteroids. On Earth, a similar layer is usually called soil, but there life plays a role: plant remains and bacteria turn it into humus. The Moon’s regolith contains none of that. It consists purely of shattered rock that has been ground down over billions of years. The topmost layer is as fine as flour, while beneath it lies coarser debris that on the Moon can be several meters thick.

Building material you don’t have to haul along

Bringing one kilogram of material to the Moon costs several tens of thousands of euros, depending on the rocket. Anyone wanting to build a station there simply cannot bring every brick from Earth. Regolith, on the other hand, lies free right at the doorstep. That’s why space agencies are planning to build using material found on site. In technical language, this principle is called in-situ resource utilization, meaning the use of raw materials at the destination.

Even more important is the oxygen. About 40 to 45 percent of lunar regolith consists of oxygen, though it is chemically bound tightly to metals. Anyone who extracts it gains breathable air and a component of rocket propellant. What remains is iron, aluminum, and titanium — useful metals. A lunar base could thus partially supply itself instead of waiting for every delivery from Earth.

Regolith also provides protection from radiation. Without a dense atmosphere, cosmic radiation hits the surface unimpeded. A few meters of piled-up regolith over a habitat act like a protective blanket. This is one of the reasons why designs for lunar bases often look half-buried.

How impacts turn into dust

On the Moon there is neither wind nor water to wear down rock. Meteorites do the work instead. Large chunks blast craters and shatter the bedrock, while tiny millimeter-sized particles continuously bombard the surface. Over billions of years, this creates an ever-finer layer of debris. Experts call this process space weathering.

During impacts, rock is briefly melted. The melt fuses fragments into irregular clumps called agglutinates. In addition, impacts fling material far and wide, so that everything is constantly mixed together. This is why lunar regolith looks similar in structure even at landing sites very far apart from one another.

But that is exactly what makes it dangerous. Because no water erosion rounds off the edges, the grains are sharp-edged and splintery. At the same time, they are electrically charged and stick to everything. During the Apollo missions, the dust worked its way into seals, scratched visors, and got inside the lunar module. Astronauts reported respiratory irritation. Dust management remains an unsolved problem in lunar spaceflight to this day.

Of 3D printers and simulants

Regolith comes up in the news in connection with Artemis, the American program to return to the Moon. Chinese missions and companies like ispace or Astrobotic are also testing devices that collect and process regolith. One well-known method is sintering: a laser or a solar mirror heats the dust so intensely that the grains fuse together. The result can be used to print landing platforms or components.

Because genuine lunar material is extremely rare, laboratories work with simulants. These are terrestrial volcanic ashes engineered to resemble the original in grain size and chemistry. A kilogram of it costs a few dozen euros, whereas genuine Apollo samples are practically priceless. Without such substitutes, it would be impossible to meaningfully test drills, rover wheels, or furnaces.

A common misconception is that regolith is soil and that plants could simply be planted in it. Nutrients for roots are largely absent, and some components are toxic. Experiments with Apollo samples did show that plants can germinate, but they grew noticeably worse than in control soils. So there is still a long way to go before lunar dust becomes farmland.