Evidence for Magnetic Fields on the Early Moon

The surprising discovery of magnetization in returned Apollo samples suggested that substantial magnetic fields once existed on the Moon. Previously reported data have revealed an epoch of apparently intense fields from ~3.9-3.6 Ga that has been thought to signify a core dynamo. However, there are virtually no data before ~4.0 Ga, making it difficult to determine if this high-field epoch was a unique event possibly related to impact processes, or the tail end of a weakening core dynamo that persisted following accretion.

To determine if the high-field epoch was a unique event and if the Moon ever had a core dynamo, we have taken a multidisciplinary approach to studying the magnetic record in the small number of lunar samples older than ~4.0 Ga. We have performed Ar-Ar dating on shocked norite 78235/6, anorthositic breccia 60025, feldspathic breccia 78155, and fifteen 2-4 mm breccias from soil 63503. We have used Ar-Ar thermochronolgy to constrain how any sub-Curie temperature heating events have affected the sample's magnetization. We have also applied petrology, shock magnetization considerations, and cooling timescale calculations to determine the timescale over which each rock acquired its magnetic remanence.

We found Ar-Ar ages that cluster around ~4.2 Ga for samples 78235/6, 60025, 78155, and five breccias from 63503. This clustering of ages is itself interesting in understanding the cratering record before the putative late heavy bombardment at ~3.9 Ga. Most outstanding among our magnetism results is a stable remanence in troctolite 76535, which has a published age of 4.2 Ga. Its lack of shock features rules out shock remanent magnetization. The conductive cooling timescale from the Curie temperature to ambient lunar soil temperatures for a rock the size of 76535 is ~1000 seconds, suggesting that the remanence it acquired was from a long-lived field most compatible with a core dynamo.