Density variations between the southern and northern hemisphere of Mars: Implications from gravimetric inversion

Kurzfassung

One remarkable feature of Mars is the crustal dichotomy which divides the surface into an old southern highland hemisphere rising several kilometers above the zero level and a superficially younger northern lowlands hemisphere well below the datum. Whether this crustal dichotomy is also reflected in composition, density, and thickness is not known -- although, a crustal thickness variation is generally suggested based on the assumption of a constant crustal density. We used gravimetric methods to place constraints on the maximum crustal density of the southern highlands. Gravimetric methods are ambiguous with a noted trade- off between the crustal thickness and density. However, combining two different methods, the geoid- topography ratio and Bouguer inversion, helps to constrain a maximum density of the crust for regions that show a homogeneous unit with respect to lateral density variations and compensation state. For the Martian Noachian southern highlands a combination of these methods suggests a maximum crustal density of 3020 ± 70 kgm^-3, assuming a single-layer crustal structure. Two-layer crustal structures show similar results: the upper crustal density is also limited to ~ 3000 kgm^-3, but a denser uniformly thick lower crust is possible. The obtained results together with the findings on crustal densities (and composition) of other regions on Mars are consistent with various scenarios of crustal evolution: 1) A 'temporal' evolution in the densities with low densities of the ancient crust and comparatively higher densities of the young (Amazonian-era) volcanic material. Such a 'temporal' increase may result from different formation mechanisms or possibly from a change in composition of the basaltic magmas over time. 2) The density variation is already manifested in the early evolution during the formation of the crustal dichotomy, i.e. the Noachian crust of the northern lowlands has a different density than the Noachian southern highland crust. The ancient northern hemisphere might have a higher density than the crust of the ancient southern hemisphere, assuming the high density crust of the Elysium region being representative for the entire Northern lowlands. If correct, this also suggests a much lower curst-mantle undulation as generally assumed. 3) As in case 2, the density variation is already manifested in the early evolution during the formation of the crustal dichotomy. However, in contrast to case 2, the ancient northern hemisphere has a lower density than the crust of the ancient southern hemisphere as suggested by TES data. The spectra can be interpreted as basalt in the southern hemisphere and andesite in the northern hemisphere. A consequence of that density variation is a stronger crust-mantle undulation than assumed by Bouguer inversion with constant crust density. In the subsequent evolution of dichotomy formation and bulk crust formation, the volcanism in Elysium and Tharsis becomes more enriched in iron and therefore shows an increasing density.