Formation of Magnetized Regions Antipodal to Lunar Impact Basins: Recent Model Calculations and Implications

Details Formation of Magnetized Regions Antipodal to Lunar Impact Basins: Recent Model Calculations and Implications Formation of Magnetized Regions Antipodal to Lunar Impact Basins: Recent Model Calculations and Implications

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmgp23a..04h&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #GP23A-04

Physics

1517 Magnetic Anomalies: Modeling And Interpretation, 1595 Planetary Magnetism: All Frequencies And Wavelengths, 5420 Impact Phenomena, Cratering (6022, 8136), 5440 Magnetic Fields And Magnetism

Scientific paper

The largest concentrations of strong lunar crustal magnetization occur in regions antipodal (diametrically opposite) to the youngest large basins including Imbrium, Orientale, Serenitatis, and Crisium. Previous work has suggested that transient magnetic field generation peripheral to the expanding (partially ionized) vapor cloud could have led to a temporary increase in field intensity in the antipodal region. In addition, statistical studies of crustal field amplitudes versus surface geology in these regions have demonstrated positive correlations with unusual terrain antipodal to Imbrium and Orientale. The latter terrain has been attributed either to convergence of seismic compressional waves or to antipodal deposition of basin ejecta. On the near side, correlative evidence as well as returned sample data suggest that basin ejecta materials are likely sources of orbital anomalies. However, the actual mechanism(s) for producing magnetization sources in basin antipode regions and the need for a former lunar dynamo field to explain the observed magnetization are unresolved issues. Here we report recent numerical simulations carried out with a 3D hydrocode to investigate more accurately (a) the time scale for convergence at the antipode of thermally expanding basin ejecta vapor; and (b) the global distribution and arrival times of ejecta in the antipodal zone. In addition, using the numerical results, we calculate analytically the minimum initial ambient field intensity at the lunar surface required to ensure a stand-off of the converging plasma in the antipodal zone as needed to allow magnetization acquisition. Results and implications for likely anomaly source materials and for the existence of a former core dynamo will be presented.

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