The possible contribution of dielectric breakdown to space weathering on Phobos

Abstract

The origins of Phobos and Deimos are uncertain; both are so space weathered that their surface compositions are difficult to determine using spectral reflectance measurements. We show how the winter temperatures and associated conditions in the polar regions of Phobos could make the regolith susceptible to space weathering from dielectric breakdown caused by solar energetic particles (SEPs). During SEP events, charged particles accumulate throughout the top ∼1 mm of the regolith, which has low conductivity, and create subsurface electric fields that act to dissipate any net buildup of charge. The faster the net charge accumulates, the larger the electric field needed to dissipate it. If the magnitude of the subsurface electric field exceeds ∼106 V m−1, then dielectric breakdown is likely. This process rapidly dissipates the buildup of charge by vaporizing electrically conducting channels through the regolith. Dielectric breakdown is expected to be more prevalent in colder regions, where the electrical conductivity of the regolith is lower and the dissipation of charge is consequently slower. If the regolith on Phobos is made of silicates, or possibly phyllosilicates, we predict that dielectric breakdown weathering has melted or vaporized 5–10% of the impact gardened regolith in the polar regions, although this percentage depends on how long the regolith has been exposed to SEPs. This, in addition to the long exposure time of the regolith to other forms of space weathering, may help explain why both Phobos and Deimos are highly space weathered compared to other airless bodies in the Solar System, such as Earth’s Moon.