Measurement of bipolar charge distribution of lunar dust simulant under VUV irradiation

Upcoming missions to the Moon represent new science opportunities and challenges. The electrostatic nature of the regolith combined with the solar wind makes it loft and adhere to almost any surface, which represents a threat for future manned and robotic missions. Understanding the charge state of the lunar soil under a representative environment is a key step towards ensuring safe lunar missions. While the global first order effect of exposure to the Sun's UV is to charge the soil positively, past experiments suggested that the transported dusts could be charged negatively. This counter-intuitive behavior was then supported by modeling, which explained the existence of negative charges but also predicted that of positively charged ones. To investigate the charging behavior of dust under a representative environment, we developed an experimental protocol based on a polarized sensitive sensor dedicated to the charge measurement of single dust grains with an accuracy of about 1 fC. The first set of measurements obtained with JSC-1A lunar dust simulants in high vacuum reveals the bipolar nature of lunar dust net charge in the regolith when exposed to UVs. Indeed, both positive and negative dusts were detected, supporting the complexity of the regolith charging processes suggested by the models.