Effect of work function on dust charging and dynamics near lunar surface

Abstract

Charged dust on the lunar surface poses a threat to space missions. Research into charged dust is essential for the safety of future space missions. The conventional lunar dust charging theory assumes a single constant work function when calculating the charging currents related to photoelectrons. However, the components of lunar regolith exhibit considerable diversity, including plagioclase, pyroxene, and ilmenite. Because the ability of the lunar surface or lunar dust to emit photoelectrons strongly depends on their work function, it is necessary to analyze the effect of work function on dust charging and dynamics near the lunar surface. In this work, we used a novel method that can predict the photoelectric yield of materials with different work functions to recalculate the surface charging currents of four types of dust particles and derived their subsequent charging and dynamic results at different solar zenith angles (SZAs). When SZA varies from 0°to 90°, the work function of dust decreases incrementally through four values: 6 eV (Apollo lunar soil), 5.58 eV (Plagioclase), 5.14 eV (Pyroxene), and 4.29 eV (Ilmenite). With each decrement in work function, the equilibrium charging currents of dust particles increase by approximately 0.5 times, the equilibrium charge numbers increase by approximately 120-170 elemental charges, and the equilibrium heights increase by approximately 0.3-2 m. We found that dust particles could not levitate stably at a critical SZA, and the critical SZAs for the four types of dust particles are 28°, 76°,85.8°, and 89.6°, respectively (arranged in order of decreasing work function). These results indicated that the equilibrium heights, equilibrium currents, and critical SZAs all have an inverse relationship with the work functions of dust particles as the SZA varies from 0°to 90°. In addition, a higher photoelectron density in areas with lower work functions results in smaller energy losses, causing dust particles to take longer to reach equilibrium, which means the equilibrium time follows the same pattern as that of the work function.