At http://phys.org/print353753684.html … apparently, the moon is engulfed in a permanent, but lopsided, cloud of dust that increases in density when annual meteor showers such as the Gemenids, occur.
Hence, scientists have been seeking an explanation. In this offering it is proposed that the cloud is made up of small grains of dust kicked up from the surface by the impact of high speed interplanetary dust particles (such as meteorite dust). A single dust grain from a comet striking the moon is capable of lifting other dust grains and specks into the airless environment. The cloud is maintained by constant bombardment.
The explanation being suggested came about as a result of orbiting astronauts during the Apollo mission reporting a higher elevation of the dust cloud – so what caused the change. They also saw a glow on the surface. Why was the dust cloud thicker back then?