Dynamical Consequences of a Chemical Layering in the Martian Mantle Sylvaine Ferrachat Doris Breuer Klaus Gottschaldt Louise Kellogg Inst. für Planetologie Westf. Wilhelm-Univ. Münster / DLR Berlin / Geology Dept. UC Davis MArs Geophysical European Network

Fractional crystallization from a deep magma ocean L. Elkins-Tanton et al (Met. Plan. Sci. 2003): –2000km-thick martian magma ocean –Bertka & Fei (JGR 97) bulk composition –Fractional crystallization After Elkins-Tanton et al 03

Fractional crystallization from a deep magma ocean After Elkins-Tanton et al 03 Prone to overturn!

Consequences of a major overturn? Would this phenomenon be able to start a dynamo and reproduce the Martian magnetic history? Let’s investigate this idea with convective models… …sudden cooling of the CMB?

Model Finite-differences double-diffusive convective model (ConMan, King et al 90) 2D cartesian box of aspect ratio 3 Rayleigh number ~ Temp. and heat flux at CMB respect the energy balance of the core: dT CMB /dt = - q CMB S CMB / (V Core  Core Cp Core ) Heat fluxes are scaled to take into account sphericity

Model Finite-differences double-diffusive convective model (ConMan, King et al 90) 2D cartesian box of aspect ratio 3 Rayleigh number ~ Temp. and heat flux at CMB respect the energy balance of the core: dT CMB /dt = - q CMB S CMB / (V Core  Core Cp Core ) Heat fluxes are scaled to take into account sphericity

Results

Results (new density profile)

What about radiogenic heat sources? Radiogenic elements are very incompatible during an upward crystallization process, they should concentrate in the uppermost part

What about radiogenic heat sources? T ~ 48 Ma T ~ 270 Ma

What about radiogenic heat sources?

Discussion / Conclusions (1/2) What comes out from this simple, preliminary convective modeling: –A chemical stratification, due to fractional crystallization of a deep magma ocean, can yield a both intense and brief ( Ma) magnetic field –In the same conditions, pure thermal convection also yields a magnetic field, but over a much longer time-scale –Radiogenic initial distrib. and internal heating: –no effect at short time-scale (~ 300 Ma) –prevents core cooling at longer time-scale (~ 1Ga) –also yields heat enrichment in mid-mantle

Discussion / Conclusions (2/2) What comes out from other models: –With temp-dependent viscosity, parameterized models (Breuer & Spohn 93) show that stagnant-lid convection can also produce an intense and time-limited magnetic field One possible advantage of model shown here: No need to suppose initially super-heated core. Sudden core-cooling appears self-consistently.

Future directions An improved model will take into account: –Phase transitions –Viscosity variations –Partial melting This model will be tested against its effects on: –Magnetic field history –Volcanism –Gravity signal This work is supported by European Community. MArs Geophysical European Network