Decline of the lunar dynamo

Abstract

Paleomagnetic investigations of extraterrestrial samples provide an avenue to test for the presence of ancient core dynamos on planetary bodies. If a core dynamo was indeed present, determining the lifetime of the magnetic field informs about the nature of the dynamo generation mechanism, which in turn may provide constraints for the body's thermal (for convection-based dynamos) and dynamical history (for precession-based dynamos). Several recent paleomagnetic studies have demonstrated the existence of a robust core dynamo (generating surface fields of tens of pT) on the Moon between at least 4.2 and 3.56 billion years ago (Ga). An outstanding question is when the lunar core dynamo actually ceased. The presence of unstable alternating field (AF) demagnetization behavior has hindered interpretation of the magnetic remanence present in many young lunar samples. In this thesis, I demonstrate that the lack of stable magnetic remanence in many lunar samples may be attributed to their poor magnetic recording properties. I develop a method to determine the minimum magnetizing field strength that a sample is capable of having accurate paleomagnetic records retrieved from using AF methods. This method is then used to place constraints on the surface field strength of the lunar core dynamo after 3.56 Ga. Paleomagnetic data from several mare basalts indicates that the lunar dynamo surface dynamo field had declined to <4 [mu]T by 3.19 Ga. A lunar breccia may record evidence for a core dynamo field of at least ~770 nT at its formation age (<3.3 Ga). Samples acquire shock remanent magnetization (SRM) when impact shock waves propagate through rocks in the presence of an ambient magnetic field. In this thesis I investigate the magnetic behavior of SRM at pressures <2 GPa to assess its stability over geologic timescales.