Until recently, the Moon had been thought to have a relatively low volatile content due to a combination of its low gravity and hot impact origin. Accordingly, past studies to investigate the Moon’s formation and its chemical and thermal evolution have focused on a bulk Moon with minimal volatile content and absent of water. However, recent geochemical analyses of very-low-Ti glasses and lunar melt inclusions present compelling evidence that water concentrations of at least 260 ppm and up to 6000 ppm were present in the deep lunar interior prior to 3 Gya. Deep reservoirs of water near the core-mantle boundary may have had a profound effect on lunar dynamo generation, enabling a prolonged, and more intense magnetic field. Though the size and properties of the lunar core are still a matter of investigation, re-cent analysis of lunar samples reinforce the Moon had a dynamo-generated magnetic field from at least 4.2-3.56 billion years ago (Ga), consistent with the presence of an iron-rich core. Given bounds on the lunar core size, it may be implausible for thermochemical convection from secular cooling to generate such a long lasting dynamo without mediation by a thermal blanket or, possibly, a water-rich layer near the core-mantle boundary.
We address the influence of water on the early core dynamo evolution on the Moon by incorporating an attenuating strain rate (i.e. decreased viscosity) for potential wet regions in the deep lunar interior. This could potentially sustain a longer-lived thermochemical convective dynamo.
Acknowledgements: Thanks to L. Elkins-Tanton for temperature and density data provided for a postoverturn lunar magma ocean and S. Zhong for assistance in the Citcom2D thermochemical evolution model. Also, thanks to J.C. Andrews-Hanna, T.L. Grove, and J.T. Perron for their assistance and comments. This work was supported by the NASA Lunar Science Institute and a NASA Lunar Advanced Science and Exploration Research grant to BPW.
Affiliation: This work was completed while A. J. Evans was affiliated with the Massachusetts Institute of Technology (MIT).