Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmgp23a..08w&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #GP23A-08
Physics
1507 Core Processes (1213, 8115), 5418 Heat Flow, 5430 Interiors (8147), 5440 Magnetic Fields And Magnetism
Scientific paper
Mercury's weak dipole signature observed by the Mariner 10 spacecraft presents an enigma. The field strength is too large to be explained by induction and a field produced by crustal remanent magnetization of a dynamo operating in the past, although can not be ruled out, is difficult to reconcile with the field's strength and dipole magnitude. It is, however, too small to be generated by an Earth-like dynamo. Recent work by Stanley et al. (2005) demonstrated that a thin shell dynamo geometry is capable of producing weak surface fields consistent with that observed at Mercury for a limited range of liquid outer core thickness and Rayleigh number parameter space. This explanation then requires a partially molten core with a large percentage having solidified after 4.5 Gyrs. Solidification of a large percentage of the core however would produce significantly more thermal contraction than is implied by the numerous lobate scarps. Deformation of the planet, apparent in the lobate scarps, implies ~2 km of radial contraction has occurred since the end of the period of heavy impact bombardment. We use a thermal evolution model to explore the conditions under which a dynamo is energetically possible during the history of Mercury and estimate the magnitude of radial contraction that results. We have carried out several preliminary model runs varying: core sulfur concentrations, initial core temperatures, and the amount of potassium in the core. We were able to generate present-day dynamos in only a few cases with a few wt% S and potassium on the order 100 ppm in the core. Several runs produced dynamos in the past, however they occurred after the crust had stabilized and the majority of magmatic activity had ceased and therefore do not support a remanent magnetization explanation. Our preliminary results indicate that the presence of a solidifying inner core is not synonymous with a dynamo. To the contrary, most of the runs produced inner core growth over 4.5 Gyrs, but ipresent-day dynamos were not possible.
Aharonson Oded
Nimmo Francis
Williams Jedediyah
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