Detection of spark discharges in an agitated Mars dust simulant isolated from foreign surfaces

Numerous laboratory experiments, starting in the Viking Lander era, have reported that frictional interactions between Martian analog dust grains can catalyze electrostatic processes (i.e. triboelectrification). Such findings have been cited to suggest that Martian dust devils and dust storms may sustain lightning storms, glow discharges, and other complex electrostatic phenomena. However, in many cases (if not most), these experiments allowed Martian dust simulant grains to contact foreign surfaces (for instance, the wall of an environmental chamber or other chemically-dissimilar particles). A number of authors have noted that such interactions could produce charging that is not representative of processes occurring near the surface of Mars. More recently, experiments that have identified and corrected for collisions between dust simulants and chemically dissimilar laboratory materials have either failed to replicate near-surface Martian conditions or directly measure discharging. In this work, we experimentally characterize the triboelectrification of a Martian dust simulant resulting from both isolated particle–particle collisions and particle–wall collisions under a simulated Martian environment. We report the direct detection of spark discharges in a flow composed solely of natural basalt grains and isolated from artificial surfaces. The charge densities acquired by the fluidized grains are found to be of order 10-6 Cm-2 (in excess of the theoretical maximum charge density for the near-surface Martian environment). Additionally, we demonstrate that the interaction of simulant particles with experimental walls can modulate the polarity of spark discharges. Our work supports the idea that small-scale spark discharges may indeed be present in Martian granular flows and may be qualitatively similar to small-scale discharges in terrestrial volcanic vents.