Other
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....2305d&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #2305
Other
Scientific paper
Mars' large southern-hemisphere magnetic anomalies require some combination of (1) a large Martian magnetic field at the time the lithosphere acquired thermoremanent magnetization (TRM), (2) large magnetic mineral concentrations compared to Earth's lithosphere, (3) a mineral or minerals whose grain size and resulting domain structure generate intense TRM, and (4) a high Curie temperature and deep Curie-point isotherm. Induced magnetization can safely be ignored because all viable candidate minerals with appropriate grain sizes and domain structures have Koenigsberger Qt ratios >>1 and the present Martian field is in any case small compared to the Earth's field. Based on the mineralogy of Martian meteorites and spectroscopy of the Martian surface, the magnetic minerals likely to be important in Mars' lithosphere are magnetite, hematite and pyrrhotite, all of which are familiar constituents of Earth's continental lithosphere. If the Martian anomalies are the integrated effect of minerals occurring over a considerable depth interval, pyrrhotite, which has a low Curie point (about 300oC) and is found only in specialized settings on Earth, is a less appealing candidate than the ubiquitous magnetite and hematite with their high Curie points (580oC and 675oC, respectively). The TRM of magnetite decreases as the inverse of increasing grain size, whereas the opposite size dependence is observed for hematite. The two minerals have approximately the same TRM intensity around grain sizes of 10-20 micrometers, which is close to both the upper limit for pseudo-single-domain (PSD) behavior in magnetite and the critical single-domain size of hematite. The lack of any substantial internal demagnetizing field in hematite permits a TRM in multidomain hematite that is orders of magnitude larger than the TRM of multidomain magnetite for field strengths like the Earth's. Either single-domain/ PSD magnetite or multidomain hematite could explain strong anomalies, with single-domain magnetite requiring less concentration but having restrictively small (submicron) grain sizes. Recently single-domain hematite has been found to have more intense TRM than previously measured, and it too could be viable. Depending on magmatic conditions, fine-grained magnetite and hematite can occur as segregated phases within titanomagnetite and titanohematite, respectively, the fineness of subdivision required for single-domain behavior being much less for hematite than for magnetite. Finally, thermoviscous magnetization, which is important for Earth's lithosphere, is probably negligible for Mars because of the small present field and the minor enhancement of TRM that is likely to have occurred in ancient fields.
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