Lunar Magnetic Field Measurements, Electrical Conductivity Calculations and Thermal Profile Inferences

Abstract

Steady magnetic field measurements of magnitude 30 to 100 γ on the lunar surface impose problems of interpretation when coupled with the non-detectability of a lunar field at 0.4 lunar radius altitude and the limb induced perturbations of the solar wind reported by Mihalov et al. at the Explorer orbit. The lunar time varying magnetic field clearly indicates the presence of eddy currents in the lunar interior and allows calculation of an electrical conductivity profile. The problem is complicated by the day-night asymmetry of the Moon’s electromagnetic environment, the possible presence of the TM mode and the variable wave directions of the driving function. The electrical conductivity is calculated to be low near the surface, rising to a peak of 6 × 10⁻³ Ω ⁻¹ m⁻¹ at 250 km, dropping steeply inwards to a value of about 10⁻⁵ Ω ⁻¹ m⁻¹, and then rising toward the interior. A transition at 250 km depth from a high conductivity to a low conductivity material is inferred, suggesting an olivine-like core at approximately 800 °C, although other models are possible.