New paleointensity measurements of Apollo samples and implications for the lunar dynamo

Spacecraft magnetic field measurements and paleomagnetic studies on Apollo lunar samples indicate that the Moon once sustained a core dynamo. However, strength and  duration of the dynamo field are key unknowns that may constrain the mechanism that powered it. Shedding light on these questions can improve our understanding about the generation of dynamos on small planetary bodies. Here, I will present new paleointensity measurements of the lunar magnetic field, based on alternating field and controlled atmosphere thermal demagnetization. In particular, we measured mare basalts and regolith breccias from the Apollo 16 and Apollo 17 missions, with ages ranging from 1.7 to 3.75 Gy old. I will discuss the results in the context of two issues surrounding lunar paleomagnetism. Firstly, I will show, through the example of an Apollo 17 mare basalt specimen carrying magnetizations acquired at two different epochs, that the magnetic record of these rocks is of lunar origin, as opposed to spacecraft or terrestrial contamination. Secondly, I will show results from the Apollo 16 regolith breccias suggesting that the lunar dynamo was fluctuating in intensity at least since 3.4 Ga. A fluctuating dynamo has been proposed as a possible resolution to the energy conundrum of the early phase of the lunar dynamo. Over a period of several hundred million years, extending up to 3.5 Gy ago, various paleomagnetic studies have inferred paleointensities that require an energy budget in excess of what numerical simulations, assuming a dynamo powered by thermochemical convection, estimate to have been available, given the Moon’s small metallic core. While our results do not directly address the energy budget conundrum during that time period, the fact that magnetic field fluctuations have occurred at least since 3.4 Gy ago, hints at the possibility that they could have occurred also at earlier times. If these fluctuations were large enough, they could allow for a reconciliation between paleomagnetic studies and numerical simulations, without the need to evoke alternative dynamo mechanisms.