Lunar magnetism

Abstract

The principal magnetic results of the Apollo program are the demonstration of the natural remanent magnetization (NRM) of returned lunar samples, the discovery of local remanent fields at the Apollo 12, 14, 15, and 16 sites, and the detection with the 15 and 16 subsatellites of magnetic anomalies whose size suggests sources of homogeneous remanent magnetization with a scale size of at least tens of kilometers. The predominant ferromagnetic constituent of the Apollo sample is metallic iron, alloyed sometimes with a few percent of nickel and cobalt. The soils and breccias contain a few tenths to 1% by weight of iron, whereas mare basalts contain about one‐tenth as much. The iron in the mare basalts is typically fine and single domain. Excess iron in the form of superparamagnetic and single‐domain spherules in impact‐generated glass is found in the soil and unannealed breccias. In contrast, in well‐annealed breccias the iron is somewhat coarser and multidomain. A NRM of between 10−3 and 10−7 gauss cm³ g−1 has been reported in the samples. Certain breccias carry the strongest and most stable NRM. The NRM has three major sources: contamination, secondary magnetization acquired on the lunar surface, and primary magnetization associated with the origin of the rock. Some rocks appear to carry primary NRM. Two Thellier‐Thellier intensity determinations have yielded values of 2100±80 and 1.6 Oe for the ancient lunar fields in which NRM was acquired. There are other indications of a large spread in field values, but more determinations are badly needed. The fields observed with the Apollo surface magnetometers have ranged from a few gammas to hundreds of gammas. They are remanent fields due to near‐surface magnetized material. Highland sites exhibit stronger fields than do the mare sites. The sources of the fields have been interpreted as homogeneously magnetized plates that have been disturbed by later impact demagnetization. The anomalies detected by the subsatellites are relatively localized and strong on the farside of the moon, having dimensions comparable with those of the larger craters. The most spectacular feature is near the Van de Graaff crater. On the nearside, subdued magnetic relief predominates. The origin of lunar magnetism remains a puzzle. Numerous suggestions have been made. Although some of them can already be eliminated as being unfeasible, there remain an embarrassingly large number of possibilities.