Persistence and origin of the lunar core dynamo

Abstract

The lifetime of the ancient lunar core dynamo has implications for its power source and the mechanism of field generation. Here, we report analyses of two 3.56-Gy-old mare basalts demonstrating that they were magnetized in a stable and surprisingly intense dynamo magnetic field of at least ∼13 μT. These data extend the known lifetime of the lunar dynamo by ∼160 My and indicate that the field was likely continuously active until well after the final large basin-forming impact. This likely excludes impact-driven changes in rotation rate as the source of the dynamo at this time in lunar history. Rather, our results require a persistent power source like precession of the lunar mantle or a compositional convection dynamo.high-K mare basalts | paleomagnetism

Iron snow, crystal floats, and inner-core growth: modes of core solidification and implications for dynamos in terrestrial planets and moons

“…Lunar paleomagnetic data suggest a magnetizing field of up to~100 μT between at least 4.25 and 3.56 Ga b.p., a field similar in strength to the present terrestrial magnetic field (Cisowski and Fuller 1986;Garrick-Bethell et al 2009;Shea et al 2012;Suavet et al 2013). The field strength decreased by an order of magnitude between 3.56 and~3.3 Ga b.p.…”

A long-lived lunar dynamo powered by core crystallization

“…Nevertheless, the current view suggests that a magnetic field of several tens of μT was present at the surface of the Moon between 4.2 and 3.56 Ga ago (Garrick-Bethell et al, 2009;Shea et al, 2012;Tikoo et al, 2012a;Suavet et al, 2013). The lack of data before 4.2 Ga ago implies that the dynamo could be older, and there is also no definitive proof that the dynamo shut down 3.56 billion years ago, only that the surface magnetic field beyond that time was weaker (Tikoo et al, 2012b).…”

“…These low estimates are consistent with paleomagnetic studies of the sample younger than about 3.3 Ga (Tikoo et al, 2014). Nevertheless, the much larger magnetic fields (several tens of microteslas) that are inferred between 4.2 and 3.56 billion years ago (Garrick-Bethell et al, 2009;Shea et al, 2012;Tikoo et al, 2012a;Suavet et al, 2013) can not be accounted for by any current model.…”

Magnetic anomalies concentrated near and within Mercury's impact basins: Early mapping and interpretation

“…Thermal diffusion calculations show that relatively thin basaltic lava flows cool efficiently by radiation and do not heat subsurface materials to temperatures sufficient to thermally demagnetize plausible anomaly sources at depth [see, e.g., Rumpf et al, 2013]. In contrast to Caloris, the lunar Imbrium and Orientale basins show no internal anomalies even though a core dynamo apparently existed when they formed [e.g., Shea et al, 2012;Suavet et al, 2013]. This may imply that the latter impacts did not produce impact melt containing sufficient magnetic carriers to produce detectable anomalies or that most impact melt was not retained in the basin interior due to the weak lunar gravity field.…”