Magnetic field direction and lunar swirl morphology: Insights from Airy and Reiner Gamma

Abstract

Many of the Moon's crustal magnetic anomalies are accompanied by high albedo features known as swirls. A leading hypothesis suggests that swirls are formed where crustal magnetic anomalies, acting as mini magnetospheres, shield portions of the surface from the darkening effects of solar wind ion bombardment, thereby leaving patches that appear bright compared with their surroundings. If this hypothesis is correct, then magnetic field direction should influence swirl morphology. Using Lunar Prospector magnetometer data and Clementine reflectance mosaics, we find evidence that bright regions correspond with dominantly horizontal magnetic fields at Reiner Gamma and that vertical magnetic fields are associated with the intraswirl dark lane at Airy. We use a genetic search algorithm to model the distributions of magnetic source material at both anomalies, and we show that source models constrained by the observed albedo pattern (i.e., strongly horizontal surface fields in bright areas, vertical surface fields in dark lanes) produce fields that are consistent with the Lunar Prospector magnetometer measurements. These findings support the solar wind deflection hypothesis and may help to explain not only the general form of swirls, but also the finer aspects of their morphology. Our source models may also be used to make quantitative predictions of the near surface magnetic field, which must ultimately be tested with very low altitude spacecraft measurements. If our predictions are correct, our models could have implications for the structure of the underlying magnetic material and the nature of the magnetizing field.