Remnant magnetization

Rocks containing magnetic minerals can retain a remnant magnetization indicative of the strength and polarity of the magnetic field that was present when the rock solidified. Like a compass needle, magnetic minerals align with the direction of the magnetic field lines. As long as the material remains molten, the minerals are free to realign with any changes in the magnetic field direction. The motion of the magnetic minerals is reduced as the temperature drops and the magma begins to solidify. Once the temperature has dropped below a specific value, called the Curie temperature (TC), the magnetic minerals can no longer move and the direction of the magnetic field at that time is "frozen into" the rock, giving rise to thermal remnant magnetization (TRM). Minerals that retain the ability to exhibit TRM over geologically long time periods are called ferromagnetic minerals. Ferromagnetism results from the spin and orbital angular momenta of the electron, which give rise to a magnetic dipole for the electron. In atoms with filled electron shells, the spins of the electrons are in up/down pairs resulting in a zero net dipole moment. However, ferromagnetic minerals are composed of atoms with unfilled electron shells. In this case the electrons can align along the direction of the external (planetary) magnetic field and retain that direction after the temperature drops below TC. The Curie temperature varies with mineral composition: TC of metallic iron is ~ 1040 K while that of magnetite (Fe3O4) is ~850 K.

Crustal remnance also can be produced at low temperatures through chemical remnant magnetization (CRM) of paramagnetic minerals. Paramagnetic minerals display magnetic properties when subjected to an external magnetic field, but do not retain this magnetization when the field is removed. Paramagnetic minerals must grow to large grain sizes to obtain CRM, but have difficulty retaining a strong magnetization after the magnetizing field is removed (Connerney et al., 2004).

Titanomagnetite and pyrrhotite in the martian meteorites retain small amounts of remnance, but the long-term stability of this remnance is questioned (McSween, 2002). The best evidence of an ancient magnetic dynamo comes from the MGS MAG/ER discovery of remnant magnetization in some regions of the martian crust (Acuña et al., 1999; Connerney et al., 1999) (Figure 3.8). The strongest regions of remnant magnetization are in the Terra Cimmeria and Terra Sirenum region of the ancient southern highlands (30°-90°S 130°-240°E), with inferred crustal magnetizations of ~10-30 A m- (Connerney et al., 1999; Langlais et al., 2004). This is an order of magnitude higher than the strongest terrestrial magnetizations. Weaker magnetizations exist throughout most of the southern highlands and in some places under the northern plains (Acuña et al., 1999; Connerney et al., 2005). Regions surrounding large impact basins are weakly magnetized or show no magnetic remnance, probably resulting from demagnetization of rocks containing pyrrhotite

West Longitude

Figure 3.8 MGS's MAG/ER experiment revealed remnant magnetization within ancient rocks across the martian surface. The areas of the strongest magnetizations (red/blue colors) correspond to ancient rocks in the Terra Cimmeria and Terra Sirenum regions. Open circles indicate impact craters and filled circles are volcanoes. The dichotomy boundary is indicated by the solid line. (Image PIA02059, NASA/GSFC/MGS MAG/ER team.) See also color plate.

(Fe7S8), magnetite (Fe3O4), hematite (Fe2Ü3), and/or titanohematite (Fe2-xTixO3) which are exposed to shock pressures >1GPa (Rochette et al., 2003; Kletetschka et al., 2004). If these basins had formed while the magnetic dynamo was active, these areas would have been remagnetized. The lack of such remagnetization suggests that the dynamo ceased operation before the formation of these large basins ~4 Ga ago (Acuña et al., 1999). Alternately Schubert et al. (2000) suggest that the basins formed prior to the onset of the dynamo, with the magnetization resulting from localized heating and cooling events after basin formation. However, most of the evidence from both geochemical and geophysical arguments supports the idea of an early dynamo (Connerney et al., 2004).

Figure 3.8 shows that crustal remnance appears in lineated patterns with alternating polarities. Connerney et al. (1999) noted the similarities in magnetic pattern to terrestrial seafloor spreading and proposed that the martian lineated magnetic anomalies recorded ancient plate tectonic activity in a reversing dipolar field. Faults running parallel to the directions of the magnetic anomalies have been interpreted as transform faults that were active early in martian history (Connerney et al., 2005). Alternately, the linear magnetic anomalies have been explained by magnetite-rich dike intrusions (Nimmo, 2000), accretion of terrains along a convergent boundary (Fairén et al., 2002), and thermal decomposition of iron-rich carbonates into magnetite (Scott and Fuller, 2004). Reconstruction of paleomagnetic pole positions from the magnetic anomalies suggests some clustering and evidence of a reversing dipolar field (Arkani-Hamed, 2001). However, the positions of these poles vary depending on the magnetic anomalies selected. Sprenke and Baker (2000) primarily utilized the Terra Cimmeria and Sirenum anomalies to place the south magnetic pole near 15°S 45°E. Arkani-Hamed (2001) modeled ten isolated magnetic anomalies and found a south magnetic pole position near 25°N 230°E. Hood and Zakharian (2001) used magnetic anomalies in the northern hemisphere to locate the south magnetic pole near 38°N 219°E. All of these pole locations are >50° from the current rotational pole. Most magnetic axes are offset <15° from the planet's rotation axis (Uranus and Neptune are exceptions), leading Sprenke and Baker (2000) and Hood and Zakharian (2001) to propose that either plate motions or reorientation of the planet due to the Tharsis uplift ("polar wander") has moved the paleomagnetic pole positions far from the current geographic poles.

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