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

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. When calculating the charging currents related to photoelectrons, a single constant work function is assumed in the conventional lunar dust charging theory. 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 its work function, it is necessary to analyze the effect of the work function on dust charging and dynamics near the lunar surface. In this work, we use 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 derive their subsequent charging and dynamic results at different solar zenith angles (SZAs). As SZA varies from 0 degrees to 90 degrees, the work function value of dust decreases into 6 eV (Apollo lunar soil), 5.58 eV (plagioclase), 5.14 eV (pyroxene), and 4.29 eV (ilmenite), correspondingly. With each decrement in work function, the equilibrium charging current of dust particles increases about 0.25 times, the equilibrium charge number increases about 120-170 elemental charges, and the equilibrium height increases about 0.3-2 m. It is found that dust particles cannot levitate stably at a critical SZA, and the critical SZAs for the four types of dust particles are 28 degrees, 76 degrees, 85.8 degrees, and 89.6 degrees, respectively (arranged in decreasing order of work functions). These results indicate that the equilibrium heights, equilibrium currents, and critical SZAs all have an inverse relationship with the work function of dust particles as the SZA varies from 0 degrees to 90 degrees. Furthermore, a higher photoelectron density in areas with lower work functions leads energy losses to decrease, thus causing dust particles to take longer time to reach equilibrium. This means that the equilibrium time follows the pattern similar to that of the work function.