Plasma focus discharge on mercury

On January 14, 2008 the NASA space probe Messenger came within 200-km of Mercury, photographing a plasma focus discharge penumbra etched into the surface. The pattern recorded is identical to that etched on steel plates in experimental discharges: 112-rays from a discharge channel terminating at and producing a thick torus, here 40 km wide. The rays emanate from the 2-3 GA termination as shown in the bottom figure, the etched lines differing from radial overlay lines only by surface conductivity irregularities. Previously, we reported the observation of a surface-traveling, infrared-radiating, plasma focus penumbra on the Jovian satellite Io whose effluent concentrated into filaments whose terminus was a narrow, well-defined, concentric annulus