Williams reported in 2015 that the term regolith refers to any layer of material covering solid rock, which can come in the form of dust, soil or broken rock. The word is derived from the combination of two Greek words – rhegos (which means “blanket”) and lithos (which means “rock).
Earth
On Earth, regolith takes the form of dirt, soil, sand, and other components that are formed as a result of natural weathering and biological processes. Due to a combination of erosion, alluvial deposits (i.e. moving water deposing sand), volcanic eruptions, or tectonic activity, the material is slowly ground down and laid out over solid bedrock.
Picture of Mt Magnet in the Central Yilgarn Craton in Western Australia, which dates to the Precambrian Era. Credit: geomorphologie.revues.org
It can be made up of clays, silicates, various minerals, groundwater, and organic molecules. Regolith on Earth can vary from being essentially absent to being hundreds of meters thick. Its can also be very young (in the form of ash, alluvium, or lava rock that was just deposited) to hundreds of millions of years old (regolith dating to the Precambrian age occurs in parts of Australia).
On Earth, the presence of regolith is one of the important factors for most life, since few plants can grow on or within solid rock and animals would be unable to burrow or build shelter without loose material. Regolith is also important for human beings since it has been used since the dawn of civilization (in the form of mud bricks, concrete and ceramics) to build houses, roads, and other civil works. The difference in terminology between “soil” (aka. dirt, mud, etc.) and “sand” is the presence of organic materials. In the former, it exists in abundance, and is what separates regolith on Earth from most other terrestrial environments in our Solar System.
The Moon
The surface of the Moon is covered with a fine powdery material that scientists refer to it as “lunar regolith”. Nearly the entire lunar surface is covered with regolith, and bedrock is only visible on the walls of very steep craters. Earth viewed from the Moon by the Apollo 11 spacecraft, across a sea of lunar soil.
The Moon regolith was formed over billions of years by constant meteorite impacts on the surface of the Moon. Scientists estimate that the lunar regolith extends down 4-5 meters in some places, and even as deep as 15 meters in the older highland areas. When the plans were put together for the Apollo missions, some scientists were concerned that the lunar regolith would be too light and powdery to support the weight of the lunar lander. Instead of landing on the surface, they were worried that the lander would just sink down into it like a snowbank. However, landings performed by robotic Surveyor spacecraft showed that the lunar soil was firm enough to support a spacecraft, and astronauts later explained that the surface of the Moon felt very firm beneath their feet. During the Apollo landings, the astronauts often found it necessary to use a hammer to drive a core sampling tool into it.
Once astronauts reached the surface, they reported that the fine moon dust stuck to their spacesuits and then dusted the inside of the lunar lander. The astronauts also claimed that it got into their eyes, making them red; and worse, even got into their lungs, giving them coughs. Lunar dust is very abrasive, and has been noted for its ability to wear down spacesuits and electronics.
The reason for this is because lunar regolith is sharp and jagged. This is due to the fact that the Moon has no atmosphere or flowing water on it, and hence no natural weathering process. When the micro-meteoroids slammed into the surface and created all the particles, there was no process for wearing down its sharp edges. The term lunar soil is often used interchangeably with “lunar regolith”, but some have argued that the term “soil” is not correct because it is defined as having organic content. However, standard usage among lunar scientists tends to ignore that distinction. “Lunar dust” is also used, but mainly to refer to even finer materials than lunar soil.
As NASA is working on plans to send humans back to the Moon in the coming years, researchers are working to learn the best ways to work with the lunar regolith. Future colonists could mine minerals, water, and even oxygen out of the lunar soil, and use it to manufacture bases with as well.
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