Experimental Studies of Charging of Lunar Dust: Levitation, Adhesion, & Transportation

It is well known since the Apollo missions that the lunar surface is covered with a thick layer of micron size dust grains with unusually high adhesive characteristics. The dust grains levitated and transported on the lunar surface are believed to have a hazardous impact on the robotic and human missions to the Moon. A horizon glow and transient dust clouds over the lunar horizon were observed by during the Apollo 17 mission. The observed dust phenomena are attributed to the lunar dust being charged positively during the day by UV photoelectric emissions, and negatively during the night by the solar wind electrons. The dust grains are believed to be levitated by the induced electric fields and transported in the near vacuum environment. The current dust charging and the levitation models, however, do not fully explain the observed phenomena, with the uncertainty of dust charging processes and the equilibrium potentials of the individual dust grains. Since the abundance of dust on the Moon's surface with its observed adhesive characteristics has the potential of
severe impact on human habitat and operations and lifetime of a variety of equipment, it is necessary to investigate the charging properties and the lunar dust phenomena in order to develop appropriate mitigating strategies.

It is well recognized that the charging properties of individual dust grains are substantially different from those determined from measurements made on bulk materials that are currently available. An experimental facility has been developed in the Dusty Plasma Laboratory at NASA Marshall Space Flight Center for investigating the charging and optical properties of individual micron/sub-micron size dust grains by levitating them in an electrodynamic balance in simulated space environments. I will discuss our recent results on laboratory measurements on charging of Apollo 11 and 17 individual lunar dust grains by low energy electron beams and by photoelectric emissions. The measurements are made by levitating dust grains of 0.2 to 10 mm diameters, in an electrodynamic balance and exposing them to mono-energetic electron beams and UV radiation. The charging rates and the equilibrium potentials produced by UV radiation and by electron impact are discussed. Photoelectric emission induced by the solar UV radiation is recognized to be the dominant process for charging of the lunar dust. In a unique laboratory facility at MSFC/NSSTC, the optical and physical properties of individual lunar dust grains are being investigated with the objective to better understand the complex lunar dust phenomena in order to develop and evaluate various mitigating strategies. We will discuss the first laboratory measurements of the photoelectric yields of individual sub-micron/micron size dust grains selected from sample returns of Apollo 17, and Luna 24 missions, as well as similar size dust grains from the JSC-1 simulants. The measured yields of micron-size individual dust grains are determined to be more than an order of magnitude larger than the bulk values reported in the literature, with a size dependence that indicates higher values for larger grains. The current and future experimental and analytical programs for investigations of the lunar dust phenomena will be discussed.

Dr. Mian Abbas
NASA MSFC