Possible evidence for dielectric breakdown weathering on the Moon

The lunar surface is altered by space weathering processes, such as micrometeoroid impacts and solar wind bombardment, but their relative roles are unknown. In addition, another process has been recently proposed: dielectric breakdown caused by deep dielectric charging by large solar energetic particle events. Two recent studies concluded that space weathering varies as a function of longitude, and that this is evidence that the magnetotail inhibits solar wind weathering. In particular, Sim et al. (2017) found that crater walls show an east-west asymmetry that peaks near the terminator when the Moon is entering/leaving the magnetotail. Their solar wind model, however, predicts the peak asymmetries to be at ±45° longitude, i.e., on the dayside when the Moon enters/leaves the magnetotail. We show, however, that the data are more consistent with the asymmetries peaking near ±75° longitude, which would be on the nightside, where dielectric breakdown weathering is predicted to occur. Thus, the space weathering asymmetry can be explained if the magnetotail's magnetic field prevents some solar energetic particles from crossing the tail. This would create an asymmetry in breakdown weathering that matches the observations. We discuss how data from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO) can be used to test this hypothesis. Reference Sim, C. K. et al. (2017), Asymmetric space weathering on lunar crater walls, GRL, 44, 11,273-11,281, doi:10.1002/2017GL075338