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.
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.
Yet in our evaluation of the the most commonly recognized natural mechanism capable of producing a dynamo within a planetary body, that of core thermochemical convection, we find that the energy sources commonly attributed to core convection can sustain a dynamo from 4.5 to 0.2 Ga with an average magnetic field surface intensity of at most 13.5 µT, with a more likely maximum value of around 1.9 µT. These magnitudes are compatible with the observed values of ≤ 5 µT.
Conversely, we find that the lunar core has insufficient energy to sustain the surface field suggested by the high-intensity (~50 microteslas) paleomagnetic signatures recorded by lunar samples between 3.85 and 3.56 Ga. Our results indicate that conventional energy sources (thermal, latent, gravitational) for driving a core dynamo by thermochemical convection provide less than 1% of the total energy necessary to produce the high-intensity paleomagnetic signatures. Furthermore, we find that the energy deficit cannot reasonably be furnished by radioactivity or a superheated core, as either implausibly high lunar core temperatures in excess of 100,000 K would be required at the start of the high-intensity magnetic field era (3.85 Ga) or implausibly high abundances of heat-producing elements, at least 12 times that expected to be in the total bulk Moon, would need to be sequestered in or above the lunar core.
An active convective dynamo on the early Moon, at the high surface field intensities indicated by the lunar paleorecord, would also require a core heat flux of ~10 W/m2, a factor of 300 greater than those previously predicted by lunar interior evolution models between 3.85 and 3.56 Ga. Undoubtedly such a large transfer of heat into the lower mantle would have resulted in either large-scale melting or vigorous convection within the lunar mantle and may have had an enduring effect on the magmatic history of the early Moon.
The inability of interior evolution models to reproduce a surface magnetic field consistent with the lunar paleomagnetic record could have previously been attributed to poorly constrained properties related to the thermochemical state of the early Moon. However, our results conclusively demonstrate that the core does not possess sufficient energy to sustain a convective core dynamo capable of generating a 50-µT surface field for ~300 Myr. Additionally, the non-convective mechanisms that have been proposed to date are also insufficiently powered to generate the required high-intensity magnetic field for ~300 Myr. Based on the present paleomagnetic record for the Moon, our results suggest that one of the following must be true: (1) paleointensities on the early Moon are significantly overestimated; (2) the scaling laws for internally generated magnetic fields are not applicable for the lunar core; or, (3) an exotic mechanism or unknown energy source that was active between 3.85 and 3.56 Ga is primarily responsible for the generation of the high-intensity paleomagnetic signatures. The answer to the conundrum of both the ancient, high-intensity magnetic field and the later, low-intensity magnetic field may entail a combination of dynamo mechanisms such as convection and mechanical stirring of the core operating during different periods in lunar history. Additional high-fidelity paleomagnetic analyses of lunar samples may provide greater insight into the possibility of an intermittent dynamo. Ultimately, the resolution to this discrepancy may be at the intersection of improved models of interior geodynamics, paleomagnetic analyses, and core dynamo models.
Affiliation: This work was completed while A. J. Evans was affiliated with the Southwest Research Institute and the Lunar and Planetary Laboratory at the University of Arizona.