Plasma Sheath Formation at Craters on Airless Bodies

The surfaces of airless planetary bodies directly interact with the solar wind plasma and ultraviolet radiation, resulting in surface charging and formation of a sheath above the surface. These interactions are further complicated by the presence of craters with characteristic sizes spanning over orders of magnitude from centimeters to kilometers. Here we present laboratory results to show that the plasma sheath formed in a crater varies significantly, depending on the radius of the crater compared to the Debye length of ambient plasma. When the Debye length is much smaller than the radius of the crater, the plasma expands into the crater and forms a sheath along the crater wall and floor. When the Debye length is comparable to or larger than the radius of the crater, the potential in the crater becomes more homogenous with a largely reduced electric field. A double layer is formed between the crater and ambient plasma. It is likely that the electrons slowed down by the sheath cannot be efficiently reflected back to the plasma due to the reduced electric field and pile up within the crater, causing an increased electron density that crosses over the density of the ions.