Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13d1696f&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P13D-1696
Other
[1595] Geomagnetism And Paleomagnetism / Planetary Magnetism: All Frequencies And Wavelengths, [5440] Planetary Sciences: Solid Surface Planets / Magnetic Fields And Magnetism
Scientific paper
The Apollo era lunar paleomagnetism suffered from the lack of modern instrumentation and data analysis techniques. However, paleomagnetic data for nearly 100 samples were reported. We have completed a reanalysis of these old Apollo paleomagnetic data using modern techniques of analysis. The principal result from the mare basalts is that many samples such as 10020, 10017, 10049, 12022, and 70215 appear to be carrying primary natural remanent magnetization (NRM) acquired on the Moon as they initially cooled on the lunar surface, but in almost every case alternating field (AF) demagnetization was not carried out to strong enough fields to isolate this primary magnetization properly. When modern measurements are available, the agreement between old Apollo era data and new data is strikingly good. It also appears that the fields recorded by the basalts of Apollo 11 and Apollo 17 may be stronger than those recorded by Apollo 12 basalts, but the small number of high fidelity magnetic recorders among the latter group make this tentative at present. The histories of breccias are more complicated than those of mare basalts and their NRM is harder to interpret. The regolith and fragmental breccias have NRM, which is either a combination of shock remanent magnetization (SRM) acquired during shock lithification and partial to total thermal remanent magnetization (TRM) depending upon the residual temperature after the shock event. For regolith breccias, interpretations are complicated because of their strong superparamagnetic components and their complex, polymict lithologies. It would be unwise to use these samples for paleointensity estimates unless one can be sure that the NRM was entirely acquired as TRM during cooling after the shock event, such as may be the case for 15498. In contrast, the melt rock and melt breccias, which are formed at high temperatures far above the Curie point of any magnetic carriers, have an excellent chance of recording lunar fields faithfully when they cool. This cooling may have taken place in a melt pool in a simple crater, or in a melt layer in a complex crater. Such samples would then have been excavated and deposited in the regolith. Samples 14310, 68416, 77017 and 77135 may have had such simple histories and some appear to have recorded strong fields, but more work needs to be done to test this suggestion. Other melt rocks and melt breccias have had more complicated histories and appear to have been deposited in ejecta blankets, where final cooling took place. The samples from the Apollo 17 layered boulder 1 at station 2 provide an example of this history. If a pTRM can be related to this secondary cooling, then we may recover a record of the field during this cooling. Samples such as 62235 and 72215 may provide just such a record, with Apollo-era and modern estimates of fields of the order of around 100 microT. Explaining such high paleointensities so late in lunar history is a major challenge to dynamo models given the small size of the lunar core.
Fuller Michael M.
Weiss Benjamin P.
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