HR: 11:35h
AN: GP42A-06    [Abstracts]
TI: An Early Nutation-Driven Lunar Dynamo
AU: * Dwyer, C A
EM: dwyer@ugcs.caltech.edu
AF: Division of Geological and Planetary Sciences, Caltech, 1200 East California Blvd, Pasadena, CA 91125 United States
AU: Stevenson, D J
EM: djs@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, Caltech, 1200 East California Blvd, Pasadena, CA 91125 United States
AB: Paleointensity data have long been argued to support an ancient lunar magnetic dynamo, but a driving mechanism for the dynamo has been hard to find. Lunar laser ranging supports a small (~335 km) liquid core at the present-day, a necessary condition for any dynamo. However, a convectively driven dynamo (with or without an inner core) is difficult to sustain at any epoch and unlikely to provide the inferred paleointensity. Lunar nutation mechanically stirs a liquid core, an effect that would have been much larger earlier in lunar history, when the moon was closer and had a much larger tilt of the spin axis relative to the orbital plane. We consider likely histories of the lunar orbit and spin axis tilt to evaluate the history of mechanical stirring. If the work from nutation equals the Ohmic dissipation in a simple field (one dominated by low harmonics), then the timing and strength of an ancient lunar dynamo might be explained. The epoch at which the dynamo fails is determined by when the stirring fails to overcome the tendency of the core to cool to a subadiabatic state. The model is not able to make precise predictions because of the well-known uncertainties in lunar tidal history. If high harmonics dominate the dissipation, then the predicted field strength at the surface might not be large enough to match the paleointensity data. This analysis suggests the importance of more work on mechanically driven dynamos to assess whether they have different properties and scaling laws from convective dynamos. It seems that the only other alternative to a mechanically driven dynamo is that the Moon never had any dynamo.
DE: 1510 Dynamo: theories and simulations
DE: 1595 Planetary magnetism: all frequencies and wavelengths
DE: 5440 Magnetic fields and magnetism
DE: 6250 Moon (1221)
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005