Charge fluctuations on the sunlit surface of airless bodies and their role in dust levitation

Abstract

The most likely cause of levitation of dust particles above the illuminated surface of airless bodies is the electrostatic mechanism. However, there are two essential problems to explain this effect. The first one is how a large electric charge (several units or even dozens of elementary charges e) which is required for levitation in the field of the electrical double layer, can be accumulated on a dust particle lying on the surface. The second problem is to find out the nature of the force that detaches dust particle from the surface overcoming the adhesive van der Waals force. The paper shows that the density of charge, which arises on a lit surface due to the photoeffect, is very small only when averaged over regions of a macroscopic size. On a submicron scale, the surface appears to be a collection of chaotic “spots” having positive or negative charges. These spots arise due to fluctuations in the fluxes of the photoelectrons taking off and falling back onto the surface, and the modulus of charge density within these spots is millions of times larger than the value of the charge density averaged over macroscopic regions. The number of positively charged spots is only slightly larger than the number of the negatively charged spots, which explains the smallness of the average charge density. The local charge density inside the spots is sufficient to allow the particles about 0.1 μm or less to get a charge sufficient to levitate them within the electric double layer at an altitude about 1 μm or more, where the field strength of the double layer is on the order of 10 Vm−1. The electrostatic field directly above the center of a charge spot is proportional to the local rather than average charge density and appears to be several orders of magnitude greater. Therefore, the force acting on a dust particle lying interior to a charge spot directly on the surface is proportional to the square of the local charge density and its magnitude can reach a tenth of pico-newton, which is probably sufficient for a particle to overcome the adhesive van der Waals force and take off.