A Precession-Driven Lunar Dynamo

Friday, 19 December 2014: 4:45 PM
Bob Yunsheng Tian1, Sabine Stanley1, Sonia M Tikoo2,3 and Benjamin P Weiss4, (1)University of Toronto, Physics, Toronto, ON, Canada, (2)University of California Berkeley, Earth and Planetary Science, Berkeley, CA, United States, (3)Berkeley Geochronology Center, Berkeley, CA, United States, (4)MIT, Earth, Atmospheric and Planetary Sciences, Cambridge, MA, United States
Paleomagnetic studies of Apollo samples suggest that the Moon generated a magnetic field with surface field intensities of several tens of microteslas until at least 3.56 billion years ago (Ga). The field then declined by an order of magnitude from 3.56 - 3.19 Ga. Because of difficulties in reproducing such a long-lived and intense field with convection-driven dynamos, a dynamo driven by precession of the mantle relative to the core was proposed as an alternative. However, there have not been any detailed numerical models demonstrating the feasibility, lifetime, and intensity of such a lunar dynamo. Using fully 3D magnetohydrodynamic simulations, we determined the strength and duration of a mechanically-driven dynamo powered by mantle precession. We found that this mechanism was capable of not only generating the 10-100μT paleomagnetic intensities observed in Apollo samples aged between 4.25 and 3.56 Ga, but also reproducing the precipitous decline in paleointensity beyond 3.56 Ga as the obliquity of the Moon decreased below 15°.