Deciphering the Early Lunar Dynamo

Remanent magnetization of the lunar crust and samples indicates the past existence of an ancient, internally generated magnetic field on the Moon induced by the systematic churning of molten metal within its core (i.e., core dynamo). Multiple mechanisms have been proposed to generate the sustained motion in the lunar core required to produce a magnetic field, however, no one model or scenario has been able to reproduce both the paleointensity and minimum longevity suggested by modern high-fidelity paleomagnetic analyses of lunar samples.

Image Credit: LANL

Image Credit: LANL

Of the multiple processes that have been proposed to generate the sustained motion in a core necessary to induce a dynamo, thermochemical convection is the process most commonly recognized to operate within planetary cores. Several models of lunar interior evolution have been successful in reproducing longevities for a convective core dynamo consistent with the early lunar paleomagnetic record. However, these models typically yield magnetic field surface intensities that are a factor of 100 below those required by lunar samples and rely on thermochemical mantle evolution models that are strongly and inexorably dependent on poorly constrained parameters for the lunar mantle (e.g., density, temperature, and viscosity). As a result, such models provide limited insight into the overall capabilities of a lunar core dynamo in sustaining surface field intensities consistent with the paleomagnetic record.

Through recent work, we have found that a 1 Gyr protracted foundering of cool, dense Ti-rich material initially emplaced at shallow depths is able to generate the high-intensity magnetic fields (≥50 μT) preserved within returned lunar samples through temporary episodes of elevated superadiabatic heat flow out of the lunar core. This simultaneously explains both the high magnitude and large-scale variability in field intensities before at least 3.5 Gyr ago, while still permitting lower fields to have persisted thereafter.

Collaborators:  Sonia Tikoo and Jeffrey C. Andrews-Hanna.