Dielectric breakdown weathering of the Moon's polar regolith

Abstract

Galactic cosmic rays and solar energetic particles (SEPs) can charge the Moon's subsurface, a process expected to be particularly important in the polar regions. Experiments have shown that sufficient fluences (i.e., time‐integrated fluxes) of energetic charged particles can cause dielectric breakdown, in which the electric field rapidly vaporizes small, filamentary channels within a dielectric. Lunar regolith has both the characteristics and, in some polar locations, the environment needed to make breakdown likely. We combine the Jet Propulsion Laboratory proton fluence model with temperature measurements from the Lunar Reconnaissance Orbiter's (LRO's) Diviner instrument and related temperature modeling to estimate how often breakdown occurs in the polar regions. We find that all gardened regolith within permanently shadowed regions (PSRs) has likely experienced up to 2×106 SEP events capable of causing breakdown, while the warmest polar regions have experienced about 2 orders of magnitude fewer events. We also use measurements from the Cosmic Ray Telescope for the Effects of Radiation on LRO to show that at least two breakdown‐inducing events may have occurred since LRO arrived at the Moon in 2009. Finally, we discuss how such “breakdown weathering” may increase the percentage of fine and monomineralic grains within PSRs; explain the presence of so‐called “fairy castle” regolith structures; and contribute to other low‐albedo features detected by LRO's Lyman Alpha Mapping Project, possibly establishing a correlation between these features and the average temperatures within craters that are only partly in permanent shadow.

A tale of two poles: Toward understanding the presence, distribution, and origin of volatiles at the polar regions of the Moon and Mercury

“…Because PSRs are so different than other planetary environments, they contain a wide range of fascinating effects, processes, and targets of scientific study. In addition to volatile enhancements, other interesting attributes about PSRs include unique surface charging and space plasma physics effects [ Zimmerman et al ., ; Jordan et al ., ; Farrell et al ., ], potentially distinctive geotechnical properties of the persistently cold and volatile‐rich regolith [ Schultz et al ., ], and the possible organic synthesis that may take place within PSR volatiles due to long‐term cosmic ray bombardment [ Crites et al ., ]. Because of their unique nature, PSRs can be difficult to study, and even now in the early 21st century there are many fundamental aspects of PSRs that are not understood.…”

Effects of Space Weathering and Porosity on the Far‐UV Reflectance of Amundsen Crater

“…If the fragmented grains are small enough that van der Waals forces can overcome gravity, then high‐porosity fairy‐castle structures could be formed in the upper regolith (Hapke & Horn, ). Because the frequency of dielectric breakdown is temperature dependent (higher frequency at lower temperatures; Jordan et al, ), and the lowest albedos observed by LAMP correlate with cold polar PSRs, this process may explain the low albedos seen in Amundsen's PSRs.…”

Particles and Photons as Drivers for Particle Release from the Surfaces of the Moon and Mercury

“…The incident high-energy protons of SEPs (∼MeV) may cause dielectric breakdown of the lunar regolith, in particular in the shadowed regions inside the polar craters (Jordan et al 2015(Jordan et al , 2017Jordan 2021). The dielectric breakdown may alter the porosity of the subsurface (∼1 mm), facilitating vaporization of volatile elements in the regolith.…”