Can the lunar crust be magnetized by shock: Experimental groundtruth

Abstract

Since the first evidence of magnetized lunar crust, two mechanisms of magnetization have been suggested to account for lunar magnetism: thermoremanent magnetization (TRM), or shock remanent magnetization (SRM). We present here the first experimental acquisition of shock remanence by lunar rocks in the 0.1-2. GPa range, and discuss their implications for the interpretation of the paleomagnetic record of these rocks, as well as for the distribution of magnetic anomalies revealed by orbital data. Laser shock experiments in controlled magnetic fields performed on lunar mare basalts demonstrated that in the presence of an ambient field these rocks can be magnetized significantly starting at low pressure (~ 0.1. GPa). Hydrostatic loading experiments up to 1.8. GPa in controlled magnetic fields were used to impart piezo-remanent magnetization (an analogue for shock remanent magnetization) to mare basalts and highland regolith breccias. These experiments allow quantifying the shock remanence as a function of pressure and ambient field. Regarding the lunar antipodal magnetic anomaly model, our results show that lunar soils, regolith breccia and about 40% of lunar highland rocks (comprising regolith and impact-melt breccia) in the upper crust can be magnetized by low pressure shocks (< 10. GPa) to sufficient levels to account for the observed lunar antipodal anomalies. Therefore, the antipodal magnetization model appears to be plausible based on our experimental results, provided that several km of regolith and/or impact-processed rocks can be found at the antipodes of large impact basins. For typical lunar rocks dominated by multidomain FeNi with low Ni content, the maximum remanent magnetization that can be acquired during a low pressure shock (< 10. GPa) is about a third of what is expected for a TRM acquired in the same ambient field. Some mare basalts have identical coercivity spectra for their natural remanent magnetization and their SRM, leaving open the possibility that the NRM was imparted during an impact at the lunar surface. In that case, magnetizing fields of the order of 40 to 95 μT are required. SRM acquisition experiments appear necessary to ground the interpretation of lunar paleomagnetism, and should become a standard technique in lunar and extraterrestrial paleomagnetism.