Density profile of an SNC model Martian interior and the moment-of-inertia factor of Mars

Abstract

The density profile of an SNC model Martian interior is calculated from the results of previous experimental work that determined the modal mineralogy of the model mantle up to Martian core-mantle boundary pressures. The moment-of-inertia factor of Mars is calculated as a function of core composition and crustal thickness using the SNC model mantle density profile as a constraint. Two sets of calculations are performed. In the first, the bulk composition of the planet is not constrained to a C1 chondrite composition. Assuming a Martian crust density of 2.7–3.0 g/cm 3, a crust thickness of 25–150 km, and the core composition proposed by Dreibus and Wänke, Fe 14 wt% S, the calculated moment-of-inertia factor ranges from 0.367 to 0.357. These models include a perovskite-bearing zone in the Martian interior. Considering core compositions ranging from pure Fe to pure FeS changes the moment-of-inertia factor by only ±0.001, but at higher S abundances, core size increases, such that the depth of the core-mantle boundary is shallower than the depth of perovskite stability. In the second set of calculations, the bulk composition of the planet is constrained to a C1 composition requiring a crust thickness of 180–320 km, assuming a crust density of 2.7–3.0 g/cm 3. The calculated moment-of-inertia factor is 0.354 and a perovskite-bearing layer is absent from the Martian interior. As it is unlikely that the thickness of the Martian crust is greater than 100 km, the bulk composition of Mars cannot be constrained to a C1 chondrite composition as proposed by the Dreibus and Wänke model (G. Dreibus, H. Wänke, Meteoritics 20 (1985) 367–382). In order to determine if the Martian mantle is more iron-rich than the Earth's mantle, we may need not only an improved estimate of the moment-of-inertia factor of Mars, but also tighter constraints on Martian crust thickness and density. The absolute degree of iron-enrichment, however, cannot be specified without also knowing the size of the Martian core. A moment-of-inertia factor of less than 0.342 is not geochemically feasible, because it requires that the mantle of Mars contains no iron.