Global Lunar Magnetic Field

Magnetic field measurements were made by small satellites left in lunar orbit by the Apollo 15 and 16 missions. Although localized regions of magnetized rock were detected by the subsatellites, no global lunar magnetic field could be measured. A lunar magnetic dipole moment can be no larger than 1016 A m2. This is nearly seven orders of magnitude smaller than the Earth’s dipole moment. The absence of a present-day global lunar magnetic field is presumably due to the absence of an active dynamo in the Moon. This may indicate that the Moon has no core; on the other hand, a small lunar core could have cooled, or solidified, sufficiently so that convective motions in it are no longer possible. It has been suggested that the localized areas of remanent lunar magnetism were magnetized in the ambient field of an ancient lunar dynamo. The Moon is the only body other than the Earth for which we have in situdeterminations of the surface heat flux. Two lunar heat flow measurements have been made, one on the Apollo 15 mission and the other on Apollo 17. The measured heat flow values are 20 mW m−2 and 16 mW m−2. Although these two determinations may not be representative of the average lunar heat flow, the values are consistent with the Earth’s surface heat loss if the differences in the sizes of the planets are accounted for. The lunar gravity field is known quite well from the radio tracking of 96 Plate Tectonics the many spacecraft that have been placed in lunar orbit. A map of the Moon’s gravity field is shown in a later chapter. The lunar maria are sites of positive gravity anomalies, or excess concentrations of mass known as mascons. These surface loads appear to be supported by the lunar lithosphere, an observation that implies that the Moon’s lithosphere is thicker and therefore stronger than the Earth’s. The Earth’s lithosphere is not thick enough to support large excess loads – mountains, for example – with the consequence that such loads tend to depress the lithosphere and subside. Since the maria were formed by 3 Ga, the Moon’s lithosphere must have thickened sufficiently by then to support the mascons. The Moon’s motion about the Earth is prograde; that is, it rotates in the same sense as the rotation of the planets about the sun. In its present prograde orbit the tidal interactions between the Earth and the Moon cause the separation between the bodies to increase; in the past the Moon was closer to the Earth. Extrapolation of the present rate of tidal dissipation back in time would bring the Moon to within a few Earth radii of the Earth between 1.5 and 3 Ga. Since there is little evidence to support a close approach of the two bodies during this period, it is presumed that the rate of tidal dissipation in the past has been lower than at present. Nevertheless, it is highly likely that the Moon has been considerably closer to the Earth than it is today.