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

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

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p31c0155r&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #P31C-0155

Physics

1221 Lunar And Planetary Geodesy And Gravity (5417, 5450, 5714, 5744, 6019, 5417 Gravitational Fields (1221), 5450 Orbital And Rotational Dynamics (1221), 5455 Origin And Evolution, 6225 Mars

Scientific paper

The crustal dichotomy and the Tharsis Rise are the most prominent topographic features on Mars. The dichotomy is largely an expression of crustal thickness variations between the northern and southern hemispheres. Tharsis is centered near the equator at the dichotomy boundary. However, the causes for the orientation of the dichotomy and the equatorial location of Tharsis remain poorly understood. Here we show that geoid anomalies associated with the dichotomy may have driven an episode of True Polar Wander (TPW). Based on recent crustal thickness models, we find that such TPW would have placed the dichotomy boundary near the equator. The dichotomy was therefore established in close to its present-day orientation during or before the early Noachian, and before the formation of Tharsis. Because the dichotomy was formed so early, the planet was still very warm and weak. The lithosphere at this time had little long-term strength and could not support a fossil rotational bulge to impede the reorientation. The Tharsis region dominates the present-day long-wavelength geoid, and would reorient the planet to place Tharsis on the equator if it did not form there. However, the largely north-south dichotomy implies that the formation of Tharsis at the dichotomy boundary constrains it to form near the equator initially. Thus, Tharsis need not have driven a large additional episode of TPW in order to bring the planet into its final stable configuration. This may explain why lithospheric fractures expected from a Tharsis-driven reorientation are not observed. We consider the possibility that the dichotomy was formed by long-wavelength convection and have examined the effects of a spherical harmonic degree-1 mantle plume on the reorientation. Our results also have implications for the sharply different remanent magnetizations between the two hemispheres.

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