Mars Internal Structure, Activity, and Composition. Tilman Spohn DLR Institute of Planetary Research, Berlin

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1 Mars Internal Structure, Activity, and Composition Tilman Spohn DLR Institute of Planetary Research, Berlin

2 Interior Structure Interior Structure models aim at 2 the bulk chemistry of the planet the masses of major chemical reservoirs the depths to chemical discontinuities and phase transition boundaries the variation with depth of thermodynamic state variables (ρ, P, T) Mars

3 Dynamic Planets The evolution of the surface is governed by processes in the interior (endogenic dynamics): Tectonics, volcanism Magnetic field Evolution of atmosphere, hydrosphere Relate observations to interior dynamics models 3

4 Interior Structure 4 Constraints Mass Moment of inertia factor Gravity field Surface rock chemistry/ mineralogy/geology Cosmochemical constraints Laboratory data Future: Seismology Heat flow Rotation, Nutation Samples MGS Gravity Field of Mars

5 Interior Structure: The Data Set Mars: Mass, MoI-factor, Samples, Surface Chemistry, Geology Venus: Small rotation rate does not allow to calculate MoI-factor from J 2 under the assumption of hydrostatic equilibrium Mercury: MoI from Peale s experiment Galilean Satellites: J 2 and C 22 5

6 Mars: The MoI Debate 6 C Pathfinder : = ± MR Yoder et al Sohl et al I MR 2 P C 2003: MR = ± 2 P 2 P C = MR 2 2 P P 2 3 I 2005 : MR = ± J 2

7 SNC Meteorites Samples from Mars? SNC chemistry provides a model for planetary composition about 15 weight-% S in core 7

8 Surface Rock 8

9 Laboratory Data Bertka and Fei,

10 Temperature Temperature requires some additional considerations Heat transfer model Model of interior chemistry, EOS Model of initial temperature 10

11 Heat Transfer: Stagnant Lid Convection Convection in a strongly temperature dependent viscosity fluid tolerates an order of magnitude viscosity increase in convecting layer Therefore a stagnant lid forms on top through which the viscosity increases dramatically 11

12 Stagnant Lid Convection The figure shows a lunar thermal evolution model The planet cools by thickening its lithosphere while the deep interior stays warm Characteristic of SL convection 12

13 Thickness of the lid T ν = ν 0 exp A m 1 T dtm Tm( P) = Tm0 + P dp z stag T Q m0 A ( A + ln10) dt T A m 2 4 π Rk P dz ( A+ ln10) S 13

14 Mars: Two-layer Model Just the MoI factor and mass Three unknowns (core, mantle density, core radius) two constraints Mantle density quite well constrained; trade off between core density and radius 14

15 z stag T Q m0 A T ( A + ln10) dt A m 2 4 π Rk P dz ( A+ ln10) S Fe 15 weight-% S FeS Perovskite 15

16 Improved MoI: Core Size 16 Rc / RP Rc / RP R 50km c

17 Improved MoI: Fe/Si 17 Fe / Si 1.68 Fe / Si 1.58 chondritic : Fe / Si 1.70 ± 0.1

18 Effects of smaller MoI Bigger core: less chance for perovskite mantle layer Larger Fe/Si: more chondritic Smaller mantle density (by about 100kg/m³): more Earth like mantle 18

19 The Crust: MGS Average thickness ~ 50 km, but ambiguous Crust thickness variations from gravity Minimum underneath hellas Maximum underneath Tharsis Crust production rate peaked in the Noachian and extended into the Hesperian 19 Period ends Gyr b.p. Noachian Hesperian Amazonian today

20 Garnet Granulite Eclogite Lower Crust? 0.2 GPa Kinetically inhibited 40 km 25 km 0.1GPa 10 km 20

21 Crust Growth Growth models have the crust form early (e.g., Breuer and Spohn, 2003;2005) 21

22 Thermal Evolution of the Core Breuer und Spohn,

23 Stagnant Lid (Thermal Lithosphere 23

24 Chemical Reservoirs The chemistry of the SNC meteorites suggests that there are 3 reservoirs in the silicate portion of Mars: one enriched, one depleted, and one primordial These reservoirs formed early, together with the core (within 10 Ma since formation) and remained basically unmixed The lid may help to keep the reservoirs seperate 24

25 Recent volcanism How can we reconcile recent volcanism with present interior structure and evolution models? A thicklidmayfrustrate volcanic activity! 25

26 Either. A bigmantleplume Requires substantial core superheat Difficult to explain Thin lid (locally?) Efficient cooling mechanism for the deep interior or radiogenics in the core T mantle T c core 26 z

27 Or Find a way to transport the magma through the thick lithosphere that is not necessarily too big a problem, viz. terrestrial volcanism 27