Horizon-Glow and the Motion of Lunar Dust

Abstract

Surveyor-7 photographed a bright glow along the western lunar horizon one hour after local sunset. This horizon-glow (HG) must result from the forward scattering of sunlight by electrically charged dust grains [a (grain radius) = 5 × 10⁻⁴ cm] which are electrostatically levitated 3 to 30 cm above rocks or surface irregularities located in the lunar terminator zone. The levitation condition is E(V cm⁻¹) ≥ 270 × [a(µ)]^1/2.Column densities the order of 50 grains cm⁻² are produced. The electrostatic field is generated in the following manner. High energy (500–1500 eV) photoelectrons are ejected from directly illuminated surfaces in the terminator zone. A positive monopole charge is produced which forces the return of subsequently ejected photoelectrons to the vicinity of the illuminated surface. Computer modeling indicates that 1% to 5% of the returned flux will accrete on adjacent completely dark areas. A stable, multipole charge distribution is generated between the light and dark areas. The associated intense multipole electric field (E ≳ 10³ V cm⁻¹) can levitate micron size soil grains located in the charged regions. Approximately 10⁻² gr cm⁻² yr⁻¹ of surface material are ‘churned’ by this process. The photoelectron work function of the lunar material, flux level of solar X-rays (λ ~ 25 Å), and the attenuation of solar ultraviolet photons by multiple scattering (as controlled by local surface geometry and reflectivity) are the physically significant factors.