Unraveling the history of the lunar dynamo - Sonia Tikoo - 2021 MagIC Workshop (1/19/21)
Channel: Magnetics Information Consortium (MagIC)
Date: 2021-01-20

"Unraveling the history of the lunar dynamo" by Sonia Tikoo, Department of Geophysics, Stanford University.

Talk presented at the virtual 2021 MagIC Workshop (https://earthref.org/events/MAGIC/2021/) - Rock and Paleomagnetism through Time and Space. Jan 19th-21st, 2021.

Abstract:
Magnetic studies of the lunar crust and Apollo samples suggest that the ancient Moon generated a dynamo field [Weiss and Tikoo, 2014]. The dynamo commenced by 4.25 billion years ago (Ga) [Garrick-Bethell et al., 2009; Garrick-Bethell et al., 2017]. Between ~3.85-3.56 Ga, lunar surface fields reached peak intensities of up to ~40-110 μT that are comparable to the modern Earth's field [Cournède et al., 2012; Shea et al., 2012; Suavet et al., 2013]. The dynamo subsequently declined and remained weak (surface fields less than ~5 μT) until it ultimately ceased sometime between ~1.9-0.9 Ga [Mighani et al., 2020; Strauss et al., 2021, in press; Tikoo et al., 2012; Tikoo et al., 2014; Tikoo et al., 2017]. Due to significant gaps in ages between studied lunar samples, it is not known whether the dynamo operated continuously or intermittently during its lifetime. It is important to establish a high-fidelity lunar paleointensity record because it helps distinguish what mechanismswere responsible for generating the dynamo at different stages in its history.

The high-field epoch prior to 3.56 Ga, in particular, presents a conundrum because lunar thermal evolution models and dynamo scaling laws predict that it would be difficult for a convective core dynamo to produce surface field intensities greater than 10 μT for extended periods of time on the Moon [e.g., Evans et al., 2018]. This problem has motivated investigations into mechanically-driven dynamos [Cébron et al., 2019; Dwyer et al., 2011; Le Bars et al., 2011], thermally-driven dynamos bolstered by precession [Stys and Dumberry, 2020; Barik et al., in prep.], and dynamos produced by convection of a lunar basal magma ocean [Scheinberg et al., 2018]. However, establishing the feasibility of these mechanisms requires further research.

Another intriguing feature of the lunar high-field epoch emerges when viewing the Apollo-era and modern paleointensity datasets in conjunction; specifically, there appears to be an order-of- magnitude dispersion in paleointensity values obtained from groupings of roughly coeval rocks (and occasionally from multiple analyses of individual Apollo samples) (Figure 1). Because a number of modern paleomagnetic studies were selectively conducted on rocks that yielded high paleointensities during the Apollo-era, limited progress has been made thus far in understanding the origin of large paleointensity variations occurring over short (less than 100 Myr) time intervals. The dispersion in Apollo-era paleointensities for samples aged greater than 3.5 Ga and the difficulty in reproducing continuously strong fields during this period via theoretical models jointly raise the following questions: (1) Is it possible that some paleointensities determined for the high-field epoch are either significantly overestimated or underestimated? (2) Did the dynamo field intensity fluctuate over time such that high (greater than 40 μT) surface fields are not required to be continuously sustained for greater than 100 Myr spans of time?

Here we discuss several ongoing research efforts in lunar paleomagnetism, including assessing uncertainties associated with commonly used lunar paleointensity methods and determining the potential effects of impact shock events on paleointensity estimates. We will also summarize recent work aimed at constraining dynamo field properties such as geometry and short-term
intensity fluctuations.