Dynamical Consequences of a Chemical Layering in the Martian

Mantle

Sylvaine FerrachatDoris Breuer

Klaus GottschaldtLouise 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 ~ 5.106

Temp. and heat flux at CMB respect the energy balance of the core:

dTCMB/dt = - qCMB SCMB / (VCore Core CpCore)

Heat fluxes are scaled to take into account sphericity

Results

Results

Results

Results (new density profile)

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 (100-150 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