Mathematics – Logic
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmgp41c..06h&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #GP41C-06
Mathematics
Logic
1517 Magnetic Anomalies: Modeling And Interpretation, 1541 Satellite Magnetics: Main Field, Crustal Field, External Field, 1560 Time Variations: Secular And Longer, 1595 Planetary Magnetism: All Frequencies And Wavelengths, 1599 General Or Miscellaneous
Scientific paper
The lunar crustal magnetic field was first discovered during the Apollo program and recent measurements by the magnetometer and electron reflectometer on Lunar Prospector have added to our knowledge of this enigmatic lunar property. Studies of the geologically less complex near side suggest that basin ejecta materials, which contain relatively large amounts of the metallic iron remanence carriers, may be the main sources of magnetic anomalies detected from orbit. The largest concentrations of magnetic anomalies occur on the lunar far side in locations approximately antipodal to the four youngest and largest basins. Therefore, although an early core dynamo may have provided the primary magnetizing field, there is good evidence that basin-forming impacts have played an important role in determining the global distribution of crustal magnetization. Many of the strongest individual anomalies correlate with higher-albedo regions with curvilinear shapes. Recent work has shown that the Reiner Gamma anomaly, in particular, has a "mini-magnetosphere" capable of strongly deflecting the solar wind. This supports earlier suggestions that the higher surficial albedo may be caused, in effect, by the reduced ion flux over the > 3 b.y. since the formation of this anomaly. In the case of the martian crustal field, early evidence from MGS magnetometer data for a predominance of anomalies over the older southern highlands pointed to an early core dynamo origin of the magnetizing field. Unlike the lunar anomaly sources, the martian sources appear to be deep-seated in the crust so that there is little or no correlation of anomalies with surface geologic features. One exception is the dearth of anomalies within the Hellas, Argyre, and Isidis basins, which may be due to impact demagnetization. Because of the more oxidizing early environment at Mars, the main martian ferromagnetic carriers consist of iron oxides and/or iron sulfide (pyrrhotite). Recent quantitative modeling of major martian magnetic anomalies has yielded estimates for the paleomagnetic pole location during the core dynamo epoch. It was apparently centered northwest of what is now Olympus Mons. This paleopole location is consistent with recent estimates calculated from the observed martian geoid under the assumption that the formation of Tharsis was the primary cause of polar reorientation after cessation of the core dynamo. The crustal anomaly field distribution appears to be roughly symmetric about the inferred paleoequator, providing a possible clue to the nature of the source regions.
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