Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmgp34a..01w&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #GP34A-01
Mathematics
Logic
[1517] Geomagnetism And Paleomagnetism / Magnetic Anomalies: Modeling And Interpretation, [5440] Planetary Sciences: Solid Surface Planets / Magnetic Fields And Magnetism, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
The origin of lunar remanent magnetization has remained an enigmatic topic ever since the first magnetic fields of lunar origin were detected during the Apollo era. Although the Moon does not at present possess a dipolar field of internal origin, it is possible that the lunar core might have once powered a geodynamo in its early history. Such a field, if stable in direction over thousands of years, could have magnetized lavas at the surface and magmatic intrusions deep in the crust. In contrast, some of the strongest magnetic anomalies appear to be correlated with impact structures, such as near the antipodes of large basins, and this observation has led to the hypothesis that transient magnetic fields generated during impact events might be responsible for magnetizing some portions of the lunar crust. Both hypotheses possess theoretical obstacles that may or may not be overcome in the future. One manner by which we might hope to distinguish transient impact-generated fields from those of an internal dynamo would be to constrain the depths of magnetization within the crust. If the sources of the magnetic anomalies are surface basalt flows or the deep crust, this might provide evidence for a thermoremanent magnetization acquired by slow cooling in the presence of geodynamo. In contrast, if the magnetic anomalies are all associated with non-magmatic surficial deposits, then these could perhaps represent impact ejecta magnetized by transient fields. Shock remanent magnetization (SRM) processes could perhaps give rise to magnetizations deep in the crust, but these regions would be expected to be spatially correlated with impact craters. Unfortunately, magnetic fields observed from orbit alone cannot constrain uniquely the depths of magnetization. Nevertheless, when combined with reasonable geologic assumptions and sample constraints, it becomes possible to place bounds on the source geometry. In this work, spatial and spectral approaches will be used to constrain the source depths of lunar magnetizations. Though preliminary, it appears likely that at least some magnetic sources lie deep below the surface and were acquired in the presence of a stable field of internal origin. We first show that the absence of detectable magnetic fields over the mare basalts does not imply that they are not magnetized. Even under Earth-like field strengths and assuming thermoremanent magnetization, most lunar basalts do not contain enough magnetic carriers to give rise to detectable fields at satellite altitudes. We next show that two isolated magnetic anomalies that are confined to the Crisium impact basin cannot easily be explained as being the result of magnetized mare basalts. A possible source is instead a several km thick impact melt sheet that crystallized in the presence of a stable magnetic field of internal origin. Finally, we interpret the global power spectrum of the lunar magnetic field in terms of a statistical model consisting of randomly magnetized magmatic sills. The source depths implied by this model are between 10 and 35 km, and the magnetizations appear inconsistent with an SRM origin; an internal dynamo field is thus implied. In addition to these analyses, a localized spectral analysis will be presented, as well as results concerning an inversion for the directions of isolated magnetic anomalies.
Weiss Benjamin P.
Wieczorek Mark A.
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