Recent lunar magnetism

Abstract

The magnetization of young lunar samples (<1.5 Ga) is a mystery because common sources of magnetic fields (e.g. core dynamo and long-lived impact plasma fields) have not been present within the last 1.5 Ga. To better characterize the source of magnetization in young lunar samples, we conducted paleomagnetic measurements on several subsamples of lunar glass 12017, which has a formation age of -9 ka, as well as on the underlying basalt from the same sample. Various methods of magnetization were tested, including possible contamination from Earth's magnetic field, exposure to transient magnetic fields such as on the Apollo module, and interaction fields from underlying magnetized rocks. The magnetic field emanated by the rock underlying the 12017 glass was determined to be -150 nT, comparable with the noise associated with paleomagnetic techniques. This opens up a new possible source of magnetization for samples- the laboratory. It is likely that other young lunar samples' paleointensities are overestimates, and that the strengths of magnetic fields on the Moon in the last 1.5 Ga are more similar to the strengths we observed from the 12017 glass. Underlying rocks and laboratory techniques are viable sources of magnetic fields, therefore, young lunar sample magnetization can no longer be used as evidence against an ancient core dynamo. Furthermore, the basalt portion of the 12017 is unidirectionally magnetized, suggesting an ancient core dynamo on the Moon when it formed at 3.2 Ga.