Charged Ejecta Clouds from Granular Beds after Slow Impacts

Abstract Low-velocity impacts are a significant process on all planetary surfaces, from rebounding particles on asteroids to saltating grains on Earth. Due to triboelectric effects, the material involved can be charged. We carried out experiments to study the net charge on ejecta emitted from a basalt sample with a particle size of 0.64 ± 0.24 mm during impacts with a steel sphere of 4.5 mm diameter and a mass of 0.36 g, and during impacts with basalt pebbles of 13–20 mm diameter and a mass of 0.5–2.6 g in air. The impact velocities were set to 2.2 ± 0.3 m s −1 and 3.7 ± 0.5 m s −1 for the steel impactors and to 3.6 ± 0.3 m s −1 and 2.5 ± 0.7 m s −1 for the basalt pebbles. The masses of the ejecta range between 10 and 600 mg. We find that the ejecta cloud charges regularly to tens of pC. There is a tendency toward negative polarity of the ejecta cloud. The impactor tends to charge positively. We do not see a significant dependence of the charge on the ejected mass. This suggests that the net charge is generated in direct contact between the impactor and the particles of the bed. The net cloud charge divided among the ejected grains would be small compared to the absolute charges on individual grains from grain–grain interactions found in other works. However, charges might add up in multiple collisions and influence the dynamics of grains, especially under low gravity.