Micrometeoroid

A micrometeoroid is a tiny meteoroid; a small particle of rock in space, usually weighing less than a gram. A micrometeorite is such a particle that survives passage through the Earth's atmosphere and reaches the Earth's surface.

Micrometeoroids are very small pieces of rock or metal broken off from larger chunks of rock and debris often dating back to the birth of the Solar System. Micrometeoroids are extremely common in space. Tiny particles are a major contributor to space weathering processes. When they hit the surface of the Moon, or any airless body (Mercury, the asteroids, etc.), the resulting melting and vaporization causes darkening and other optical changes in the regolith. In order to understand the micrometeoroid population better, a number of spacecraft (including Lunar Orbiter 1, Luna 3, Mars 1 and Pioneer 5) have carried micrometeoroid detectors.

In 1957 Hans Peterson conducted one of the first direct measurements of the fall of space dust on the Earth, estimating it to be 14,300,000 tons per year. If this were true, then the Moon would be covered to a very great depth as there are limited forms of erosion to remove this material. In 1961 Arthur C. Clarke popularized this possibility in his novel A Fall of Moondust. This was cause for some concern among the groups attempting to land on the Moon, so a series of new studies followed to better characterize the issue. This included the launch of several spacecraft designed to directly measure the micrometeorite flux (Pegasus satellite program) or directly measure the dust on the surface of the Moon (Surveyor Program). These showed that the flux was much lower than earlier estimates, around 10,000 to 20,000 tons per year, and that the surface of the Moon is relatively rocky. Most lunar samples returned during the Apollo Program have micrometeorite impacts marks, typically called "zap pits", on their upper surfaces.

...
Wikipedia