Dynamical Consequences of a Chemical Layering in the Martian Mantle Sylvaine Ferrachat Doris Breuer...
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Transcript of Dynamical Consequences of a Chemical Layering in the Martian Mantle Sylvaine Ferrachat Doris Breuer...
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