Magnetism and the interior of the Moon

Abstract

During the time period 1961–1972, 11 magnetometers were sent to the moon. The primary purpose of this paper is to review the results of lunar magnetometer data analysis, with emphasis on the lunar interior. Magnetic fields have been measured on the lunar surface at the Apollo 12, 14, 15, and 16 landing sites. The remanent field values at these sites are 38, 103 (maximum), 3, and 327 γ (maximum), respectively. Simultaneous magnetic field and solar plasma pressure measurements show that the Apollo 12 and 16 remanent fields are compressed during times of high plasma dynamic pressure. Apollo 15 and 16 sub‐satellite magnetometers have mapped in detail the fields above portions of the lunar surface and have placed an upper limit of 4.4 × 1013 G cm³ on the global permanent dipole moment. Satellite and surface measurements show strong evidence that the lunar crust is magnetized over much of the lunar globe. Magnetic fields are stronger in highland regions than in mare regions and stronger on the lunar far side than on the near side. The largest magnetic anomaly measured to date is between the craters Van de Graaff and Aitken on the lunar far side. The origin of the lunar remanent field is not yet satisfactorily understood; several source models are presented. Simultaneous data from the Apollo 12 lunar surface magnetometer and the Explorer 35 Ames magnetometer are used to construct a whole moon hysteresis curve from which the global lunar permeability is determined to be µ=1.012±0.006. The corresponding global induced dipole moment is ∼2 × 1018 G cm³ for typical inducing fields of 10−4 G in the lunar environment. From the permeability measurement, lunar free iron abundance is determined to be 2.5±2.0 wt%. Total iron abundance (sum of iron in the ferromagnetic and paramagnetic states) is calculated for two assumed compositional models of the lunar interior. For a free iron/orthopyroxene lunar composition the total iron content is 12.8±1.0 wt%; for a free iron/olivine composition, total iron content is 5.5±1.2 wt%. Other lunar models with a small iron core and with a shallow iron‐rich layer are also discussed in light of the measured global permeability. Global eddy current fields, induced by changes in the magnetic field external to the moon, have been analyzed to calculate lunar electrical conductivity profiles by using several different analytical techniques. From night side transient data, ranges of conductivity profiles have been calculated. At a depth of 250 km into the moon, the conductivity ranges between 1 × 10−4 and 2 × 10−3 mhos/m. Thereafter, conductivity rises with depth and ranges between 2 × 10−3 and 8 × 10−2 mhos/m at 1000 km depth. Harmonic analyses of day side data are similar to night side results except at the greater lunar depths, where harmonic day side profiles show lower conductivities than the night side results do. Transient response analysis has recently been applied to data measured in the lobes of the geomagnetic tail, and thus calculation is allowed of a conductivity profile that increases with depth from 10−9 mho/m at the lunar surface to 10−4 mho/m at 200 km depth then to 2 × 10−2 mho/m at 1000 km depth. This profile is generally consistent with conductivity results from transient response analysis in the solar wind, in which data measured on the lunar night side are used. A temperature profile is calculated from this conductivity profile by using the data of Duba et al. (1974): temperature rises rapidly with depth to 1100°K at 200 km depth then less rapidly to 1800°K at 1000 km depth.