Statistics – Computation
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p41a..02k&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P41A-02
Statistics
Computation
[0545] Computational Geophysics / Modeling, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
One of the most striking surface features on Mars is the crustal dichotomy. It is the oldest geological features on Mars and was formed more than 4.1 Ga ago by either exogenic or endogenic processes (e.g. Keller and Tackley, 2009). In order to find an internal origin of the crustal dichotomy, located within a maximum of 400 Ma of planetary differentiation, the thermal state of the planet resulting from core formation needs to be considered. It was suggested that a primordial crust with up to 45 km thickness can be formed already during the Martian core formation (Norman, 1999). Therefore we suggest that the sinking of iron diapirs delivered by pre-differentiated impactors induced impact- and shear heating-related temperature anomalies in the mantle that fostered the formation of early Martian crust. In this study, we examine parameter sets that will likely cause an onset of hemispherical low-degree mantle convection directly after, and coupled to, an already hemispherical core formation. To test this hypothesis we use a numerical model to simulate the formation of the Martian iron core, while peridotite melting is enabled to track melting caused by shear and radioactive heating. We perform 2D simulations using the spherical-Cartesian code I2ELVIS (Gerya and Yuen, 2007). It combines finite differences on a fully staggered rectangular Eulerian grid with Lagrangian marker-in-cell technique. In our model setup, the planet is surrounded by a low viscosity, massless fluid (“sticky air”) to simulate a free surface. We apply a temperature- and stress-dependent viscoplastic rheology inside a Mars-sized planet. Radioactive and shear-heating as well as consumption of latent heat by silicate melting are taken into account. The depth of neutral buoyancy of silicate melt with respect to solid silicates is determined by the difference in compressibility of the liquid and solid phase. To self-consistently simulate the silicate phase changes expected inside a Mars-sized body, we use the thermodynamical database Perple_X (Connolly, 2005). As initial condition, we apply randomly distributed iron diapirs with 75 km radius inside the planet, representing the cores of stochastically distributed impactors. Additionally, we explore the effect of one giant impactor core on the planetary evolution. Results indicate that the presence of a large impactor core induces hemispherically asymmetrical core formation. Furthermore, the amplitude of shear heating anomalies often exceeds the solidus of primitive mantle material. The formation of a considerable amount of silicate melt is observed. Some of the generated melt segregates to the surface to form primordial crust, whereas negatively buoyant melt from deeper sources sinks to the CMB. The hemispherical asymmetry in temperature induced by a giant impactor works in favour of an onset of low-degree mantle convection after core formation. Such a hemispherical convection geometry might subsequently be sustained by phase-dependent viscosity (Keller and Tackley, 2009), and thus harbor an early evolution of a dichotomous crustal thickness distribution. REFERENCES Connolly, J.A.D. 2005. EPSL, 236. Gerya, T.V. & Yuen, D.A. 2007. PEPI, 163. Keller, T. & Tackley, P.J. 2009. Icarus, 202. Norman, M.D. 1999. Meteorit. Planet. Sci., 34.
Connolly James
Gerya Taras
Golabek Gregor J.
Keller Thomas
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