How strong are lunar crustal magnetic fields at the surface? Considerations from a reexamination of the electron reflectometry technique

Since its surprising discovery in the Apollo era, lunar magnetism has proven enigmatic and difficult to interpret. A crucial piece of information needed to determine the origins of lunar remanent crustal magnetization is its strength and relationship to surface and subsurface geology. However, we cannot easily determine the distribution of surface crustal magnetic field strength, let alone make the connection from magnetic field to subsurface magnetization. Surface magnetometer measurements provide ground truth, but only exist for a few sites. Spacecraft magnetometers have mapped much of the lunar field distribution, but the observation altitude (tens of km) limits the resolvable wavelength - potentially problematic since surface measurements indicate significant or even dominant small-wavelength components. Electron reflectometry (ER) measurements, meanwhile, provide a remote measurement of the surface field strength, but ER sensitivity to crustal fields is also potentially limited by the electron gyroradius (~10 km). The response of a magnetometer to distributions of magnetization with different spatial wavelengths is well understood and easily modeled; however, the analogous response of the electron reflectometry technique has not yet been considered in detail. Therefore, we have simulated the trajectories of millions of electrons through magnetic fields produced by synthetic crustal magnetization distributions, in order to determine the sensitivity of the ER technique to distributions of magnetization with different strengths and spatial wavelengths. We report on the results of these simulations, and consider the implications for lunar magnetism and its origins.