Regolith

Regolith is the outer layer of rock in many planets. When the rock on the surface of a planet is exposed to air, water, sunlight, and life, it starts to wear out and crumble. It stops being one solid piece of rock, and instead becomes a collection of smaller rocks, dust, sand, ash, minerals, and organic material that came from the living things that live on it. 

On planet Earth, the uppermost layer of the regolith is what we call the soil.

On Earth, the regolith isn't the same everywhere. In some places, it's very deep. In others, it's very shallow. There are some places where there is no regolith at all, and you can see the bare rock underneath: the bedrock.

Planets and other celestial bodies that are mostly made of rock (instead of gas) have a regolith.

The Moon is almost completely covered in regolith, because a lot of meteoroids hit it and crumbled into smaller rocks.  The first two centimeters of the Moon's regolith is dusty, but it gets firmer underneath that.

The regolith on Mars is made of a lot of sand and dust.

References

Category:Soil