Final answer:
The dark brown surface in an abrasion on the Moon is caused by dark basaltic dust from the lunar maria, resulting from billions of years of impacts that have fragmented the surface layer.
Step-by-step explanation:
The dark brown surface on an abrasion, such as the one experienced by astronauts on the Moon's surface, is usually caused by dark basaltic dust that originates from the lunar maria. As billions of years of impacts have occurred, the surface layer of the Moon has been broken up and reduced to particles about the size of dust or sand. The process of cratering from these impacts scatters fragments and results in the porous, loosely packed dust layer into which astronauts' boots would sink.