Magnetization, paleomagnetic poles, and polar wander on Mars

Using the geometry of an areocentric dipole field, we have remodeled the anomalies observed in the Terra Sirenum and Terra Cimmeria regions by the Mars Global Surveyor (MGS). For the origin of the anomalies, we used both magnetic stripe and magnetic shell hypotheses. For magnetic stripe models, we assume that sea-floor spreading formed the anomalies in the presence of a reversing axial dipole field. We found that a simple magnetic stripe model with constant magnetization and crustal thickness results in poor agreement with the observed anomalies. However, by allowing the intensity of magnetization to vary freely between and along the stripes, a reasonable fit to the observed data can be found while maintaining the observed polarity along the stripes. Applying the simple magnetic stripe model to the Terra Sirenun and Terra Cimmeria anomalies results in best-fit spreading centers located well to the north of the surviving magnetic stripes. This could possibly explain the apparent lack of symmetry that has been noted in the anomalies. For the magnetic shell hypothesis, we assume that a monopolarity dipole field magnetized the early crust of Mars. The anomalies that persist today would then be the result of partial demagnetization of the original magnetic shell by more recent processes. The magnetic anomalies in Terra Sirenum and Terra Cimmeria are consistent with this model, especially if the magnetization of the original magnetic shell was laterally inhomogeneous. If the monopolarity magnetic shell hypothesis is correct, best-fit magnetic paleopole locations for the original magnetization lie in a sub-equatorial belt centered near the prime meridian. If the ancient magnetic pole of Mars coincided with the spin pole, this result implies that either apparent or true polar wander has occurred on Mars. Apparent polar wander would be consistent with plate tectonics having occurred after the formation of the magnetic anomalies. True polar wander has previously been proposed to explain certain morphologic features found on Mars. (C) 2000 Academic Press.