Airless body surface weathering by dielectric breakdown

Abstract

A number of studies have suggested that dielectric breakdown weathering (“sparking”) may occur on airless bodies in the Solar System. To experience dielectric breakdown, a regolith must be exposed to a sufficiently high fluence of energetic charged particles (>1010 cm-2), and this fluence must be deposited on a timescale less than the regolith’s discharging timescale, which increases with decreasing temperature. Consequently, dielectric breakdown occurs in regolith that is both cold and exposed to high fluxes of solar energetic particles (SEPs) or radiation belt particles. If breakdown occurs, then it causes space weathering by melting and vaporizing microscopic channels through regolith near the surface. We describe our recent experimental, observational, and theoretical work investigating where dielectric breakdown may be an important space weathering process. In the inner Solar System, airless bodies are exposed sporadically to SEP events with high fluences. At 1 AU, the flux of SEPs is nearly isotropic, and thus they may cause dielectric breakdown over the coldest (<120 K) regions of the Moon’s nightside. We present observational evidence for this nightside process and the results of preliminary experiments investigating its microscopic effects. In addition, we briefly discuss the possibility that dielectric breakdown weathering also occurs on Mercury, the moons of Mars, and some asteroids.In the outer Solar System, where the fluxes of SEP events are significantly reduced, dielectric breakdown is more likely to be caused by radiation belts. In particular, moons in Jupiter’s radiation belts are exposed to the highest continuous fluxes of energetic charged particles in the Solar System. Furthermore, Jupiter’s radiation belts have caused dielectric breakdown in spacecraft dielectrics. We describe the range of evidence showing that dielectric breakdown may occur on some of the Galilean moons (Io, Europa, and Ganymede) and Jupiter’s four innermost moons (Amalthea, Thebe, Adrastea, and Metis).