Lunar Magnetic Anomalies and the Cayley Formation

Abstract

THE observation of the existence of local magnetic fields on the Moon is striking since it is known from the Explorer 35 satellite that the global magnetic dipole moment of the Moon is less than 1020 gauss cm3 implying a mean surface magnetic field of less than 2 gamma (ref. 1). Apollo subsatellite data suggest that the mean dipole moment may be even less than this2. Local magnetic anomalies have been measured, however, by three independent techniques: (1) surface magnetometers showed values of 36 gamma at the Apollo 12 landing site; 43 gamma and 104 gamma at two positions near the Apollo 14 site; less than 5 gamma at the Apollo 15 site; 121, 125, 180, 231 and 313 gamma at various positions near the Apollo 16 site3–8; (2) the magnetometers aboard the Apollo 15 and 16 subsatellites recorded anomalies as large as 1 gamma at a flight height of about 100 km apparently associated with large basins on the back side of the Moon9–12; (3) limb compression effects detected by the Explorer 35 satellite reveal many magnetic sources most of which are located in the lunar highlands; more detailed results were obtained by the Apollo 15 subsatellite due to its lower flight height1,9,13,14. We shall consider here the material which causes these anomalies.