Paleomagnetic poles of Mars: Revisited

Abstract

[1] The huge amount of data acquired by Mars Global Surveyor during its mapping period provides a unique opportunity to reassess the paleomagnetic pole positions of Mars previously determined on the basis of the limited low-altitude magnetic data. We identify nine small and isolated magnetic anomalies on the basis of the global magnetic maps and model each anomaly using a vertical prism of elliptical cross section. Both highaltitude (360-430 km) and low-altitude (100-200 km) magnetic data are used simultaneously. We calculate a paleomagnetic pole position assuming that the body is magnetized by a dipole core field. Although the new pole positions do not cluster as closely as the old ones, the new cluster overlaps the older cluster. The clustering suggests that Mars' rotation axis has likely wandered by $50°-60°in the last $4 Gyr. The number of north and south poles in the cluster suggests at least one reversal of the core field during the time the source bodies acquired magnetization.

The cause for the north–south orientation of the crustal dichotomy and the equatorial location of Tharsis on Mars

“…Our proposed TPW that turns an initially east-west dichotomy to its present-day north-south orientation also supports the recent proposed large TPW based on estimates of paleomagnetic pole positions (Arkani-Hamed and Boutin, 2004;Hood et al, 2005). Additionally, our results highlight the importance in understanding why Tharsis was formed near the dichotomy boundary, possibly as a result of lithospheric/crustal thickness variation or edge-driven convection (King and Anderson, 1998;Wenzel et al, 2004).…”

“…5f-5h). Recent estimates of paleomagnetic pole positions northwest of Olympus Mons that suggest an episode of TPW since the rocks were magnetized (Arkani-Hamed and Boutin, 2004;Hood et al, 2005) are also consistent with an initially eastwest dichotomy from our model. Our scenario suggests that the thickened crust (i.e., the present-day southern highlands) may be younger than the thinner crust, but probably not significantly because of the rapid formation of the dichotomy.…”

“…5b), is particular to the scenario described above, the subsequent north-south orientation of the dichotomy is controlled entirely by the crustal thickness variations associated with the dichotomy, regardless of its formation mechanism. Alternative origins for the magnetization of the highland crust may also be consistent with our scenario, provided that the magnetization occurs before the crust-induced TPW event as suggested by the magnetic paleopole studies (Arkani-Hamed and Boutin, 2004;Hood et al, 2005). Future work in this area may be helpful in providing constraints on the timing of the reorientation.…”

Equilibrium rotational stability and figure of Mars

“…[Note that the Gold (1955) scenario includes a physical inconsistency since it assumes that the rotational bulge will relax perfectly to a change in centrifugal potential but that the surface load is never completely compensated isostatically.] It remains a common view, following Gold's influential analysis, that the stability of the martian rotation pole-both at present day and in response to Tharsis loading-is governed by the observed figure of the planet after correction for a hydrostatic form (Melosh, 1980;Bills and James, 1999;Sprenke et al, 2005).…”

“…First, to what extent did the development of Tharsis, which is now located near the equator of the planet (Zuber and Smith, 1997), cause TPW (Melosh, 1980;Sprenke et al, 2005)? Second, how stable is the current rotation axis to changes in the surface and internal mass distribution (Bills and James, 1999)?…”