A precession and convection driven lunar dynamo
Abstract
Explaining the intensity and timescales of the lunar dynamo from the magnetic record of lunar rocks has been a conundrum. Earth-like dynamo mechanisms such as thermal and thermochemical convection have struggled to simultaneously reproduce an inferred high paleofield (~ 10-100 μT) epoch between 3.56 and 3.85 Ga, a subsequent weaker epoch (~ 1-5 μT) beyond ~ 2.5 Ga, followed by the cessation of the dynamo between ~ 1.92 and ~ 0.80 Ga. In the past, Williams et. al. (2001) and Dwyer et. al. (2011) have proposed precession as a possible mechanism to generate a dynamo that could potentially reproduce the high field epoch, while Tikoo et. al. 2017 suggested a possibility of the dynamo being powered by two separate mechanisms. Motivated by these proposals, we investigated what happens when precession is imposed on a seed field generated by convection. We investigated this problem in two ways. In the first case, we maintained the magnitude and direction of the rotation of the fluid core constant along that of the reference frame as the mantle precessed around it. This scenario is arguably applicable to the lunar core since the viscosity and asphericity of the core mantle boundary (CMB) are too low to enable the core to react to the mantle's motion. Here, the dynamo is thus generated due to the differential fluid motion at the CMB. This was the mechanism proposed by Williams et. al. and Dwyer et. al. and is a case we call "differential precession". In the second case, we allowed our simulations to self-consistently determine the base flow of the fluid core as it viscously reacts to the motion of the mantle. This is a more physically accurate picture at the highly diffusive parameters of our simulations and takes into account the effect of the Poincaré force. The downside is that the base (Poincaré) flow of the fluid core does not resemble that of the outer core of the Moon and makes a high angle with the precession axis. The first suite of simulations showed a marked amplification of the field generated by pure thermal convection, while the second suite showed the same for select parameters. A combination of convection and precession thus has the attractive features of (a) amplifying the convection driven field in the early life of the moon when the precession angle was high and (b) ceasing to do so later on when the precession angles become low.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMGP015..02B
- Keywords:
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- 1594 Instruments and techniques;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1595 Planetary magnetism: all frequencies and wavelengths;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 5734 Magnetic fields and magnetism;
- PLANETARY SCIENCES: FLUID PLANETS;
- 5443 Magnetospheres;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS