The regolith is the layer rock or blanket of unconsolidated debris of any thickness that overlies bedrock and forms the surface of the land. On the Moon, the regolith depth is of several tens of kilometers of an accumulation of impact crater ejecta (blocks, rocks, dust) and is named unconsolidated megaregolith. It was assumed that Mars regolith will be similar. One of the first great discoveries of the Mars Global Surveyor (MGS) missions was to show, through the Mars Orbiter Camera (MOC) camera images that the subsurface of Mars was layered mostly everywhere we looked on the planet. Layering means that the geologic history has been much more active and complex than we thought before, and that layers of material from various origins (aqueous, aeolian, volcanic) have accumulated on the top of each other for aeons.
Source: NASA JPL, Malin Space Science Systems