The Electrical Environment of the Moon

The discovery of lightning on several gas giants and Venus has captured the imagination of both the public and the scientific community, representing the most dramatic and visible evidence of the ubiquitous nature of atmospheric electrical phenomena in extraterrestrial environments. Small solid bodies with little or no atmosphere are also likely to exhibit active, dynamic electrical phenomena. Here we discuss the Earth's Moon as an example system where electrical effects arise due to the direct action of solar and plasma inputs at the surface. We show data from the Lunar Prospector mission that directly reveal variations in the surface potential of the Moon as a function of solar and plasma conditions, as the solar wind, UV, lunar wake, and the magnetosphere all contribute to the overall current balance at the lunar surface. As with planetary lightning, the extreme events are of special interest, and the Moon is no exception. Rather than resulting from atmospheric disturbances, the Moon's electrical "storms" arise from solar events such as coronal mass ejections, and during interactions with the Earth's geotail. We discuss the implications of lunar surface charging for dust electrification and transport, the dynamics of neutrals and ions, and the potential impact of these processes on the exploration of the surface. As we learn more about the electrical environment at the Moon, we may gain important insights into the detailed physical processes that influence surface charging on other airless bodies as well, such as Mercury, asteroids, and the moons of the outer planets.