The Case Against an Early Lunar Dynamo Powered by Core Convection

Abstract

Paleomagnetic analyses of lunar samples indicate that the Moon had a dynamo‐generated magnetic field with ~50 μT surface field intensities between 3.85 and 3.56 Ga followed by a period of much lower (≤ ~5 μT) intensities that persisted beyond 2.5 Ga. However, we determine herein that there is insufficient energy associated with core convection—the process commonly recognized to generate long‐lived magnetic fields in planetary bodies—to sustain a lunar dynamo for the duration and intensities indicated. We find that a lunar surface field of ≤1.9 μT could have persisted until 200 Ma, but the ~50 μT paleointensities recorded by lunar samples between 3.85 and 3.56 Ga could not have been sustained by a convective dynamo for more than 28 Myr. Thus, for a continuously operating, convective dynamo to be consistent with the early lunar paleomagnetic record, either an exotic mechanism or unknown energy source must be primarily responsible for the ancient lunar magnetic field.

Lunar Cumulate Mantle Overturn: A Model Constrained by Ilmenite Rheology

“…The magnetic intensity estimations scaled from our calculated CMB heat flux (e.g., Christensen et al, 2009) can be compared to the observed paleomagnetic record ( Figure S11; supporting information). The predicted paleointensities cannot reach the measured paleointensities of 20-40 μT (Evans et al, 2018;Garrick-Bethell et al, 2016) even with a large core size (D150v1e-4V1e21E100Core410 in Figure S11). However, the timing of the stimulated core dynamo we predict does coincide with an inferred periods of lunar paleomagnetism at 4.25 Ga .…”

The Lunar Paleo‐Magnetosphere: Implications for the Accumulation of Polar Volatile Deposits

“…The frequency and efficiency of such compression events may have been larger if the ancient solar wind density was higher than it is today (Airapetian & Usmanov, ) and if the sun produced more frequent coronal mass ejections (Airapetian et al, ). Such compressions could also enhance the surface field strength (Garrick‐Bethell et al, ), possibly helping to explain the high inferred paleointensities of some Apollo samples beyond what can be explained by dynamo theory (Evans et al, ).…”

Probing space to understand Earth

“…Unresolved questions about the lunar dynamo also prompted research into dynamos generated within basal magma oceans 37 (layers of molten silicates immediately above the core-mantle boundary), as has been proposed for the [38][39][40][41] . For example, core convection in Earth's Moon may provide insufficient energy to sustain a lunar dynamo for the duration and at the intensities indicated by palaeomagnetic data 42 . Instead, the Moon's dynamo could have been generated for much of its lifetime in a basal magma ocean, where high titanium and iron contents could have bolstered its electrical conductivity 43 to values three orders of magnitude higher than that of olivine melts at lunar mantle conditions 37 .…”