Evidence for Dielectric Breakdown Weathering on the Moon

Soil on the Moon's surface is darkened by space weathering, and most studies have assumed that this darkening is dominated by micrometeoroid impacts and/or the solar wind. The lunar maria brighten with increasing latitude, causing some to argue that the solar wind plays the main role, since its flux follows the cosine of latitude. Recent work, however, suggests that another process contributes to weathering: deep dielectric charging during large solar energetic particle events may cause dielectric breakdown, or "sparking" [Jordan et al., 2019]. This process is predicted to melt and vaporize soil at a rate comparable to that of micrometeoroid impacts. But, unlike weathering caused by impacts or the solar wind, dielectric breakdown weathering increases with increasing latitude.

A simple combination of micrometeoroid and dielectric breakdown weathering can explain how the reflectance of the lunar maria varies as a function of latitude. This combination captures two distinctive features of the reflectance data that are not captured by a function of the cosine of latitude: the relatively constant reflectance of the maria from the equator to ~+40 degrees latitude and the rapid brightening of the maria at higher latitudes. I also show that a reasonably slight (~2-3%) reduction in breakdown weathering can explain the difference in brightness between the prominent swirl Reiner Gamma and its surroundings. This reduction appears to be consistent with shielding caused by the swirl's magnetic anomaly. Consequently, it seems that micrometeoroid impacts and dielectric breakdown dominate space weathering in the lunar maria.

Reference

Jordan, A. P., et al. (2019), How dielectric breakdown may contribute to the global weathering of regolith on the Moon, Icarus, 319, 785-794, doi:10.1016/j.icarus.2018.10.025