Tilman Spohn Structure and Evolution of Terrestrial Planets

2 Chemical Components: Gas (H, He), Ice (NH 3, CH 4, H 2 O), Rock/Iron Mars Ganymede Jupiter

3 Interior Structure Interior Structure models aim at 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 ( , P, T) Mars

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

5 Interior Structure: The Data Set Relevant data with satisfying accuracy are available only for Earth, Moon, and Mars Moon and Mars: Mass, MoI-factor, Samples, Surface Chemistry, Lunar seimology 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: C 22 and, in some cases, C 20

6 Planetary Data

7 Moment-of-Inertia factor constraint MoI factor constrains mantle density if similar to bulk density and a high- density core exists (e.g., The Moon). core density if similar to bulk density and low-density outer shell exists (e.g., Mercury). The mantle density of Mars is relatively well determined by the planet's MoI factor.

8 New Mars Model Sohl, Schubert and Spohn, 2005 Larger Cores, Thicker Crusts (both a few 10s of km) Slightly Lighter Mantle

9 Seismology, the method of choice With the help of seismology the ambiguity of the models can be removed and the state of the core can be determined

10 Breadboard model

11 Interior Structure

12 Structures Form Early Kleine et al, 2002 Breuer and Spohn, 2003

13 Internal Oceans The icy satellites Europa, Ganymede, Callisto, Titan, Triton,... May have internal oceans Competition between heat transfer and heating rates Melting point gradient

14

15 Liquid Cores? Solid Inner Cores?

16 Magnetism Of the terrestrial planets and major satellites, Earth, Mercury, and Ganymede are known to have self-generated magnetic fields Mars, Venus, Moon, Io, Europa, and Callisto lack self-generated magnetic fields Magnetic fields are generally thought to be enigmatic to planetary evolution during which thermal (and potential energy) is converted into mechanical work and magnetic field energy.

17 Magnetic Field, the Environment and Life Protects life against cosmic radiation Protects the atmosphere against erosion (Not all forms of erosion, of course)

18 Magnetic Field History of Mars  No present-day dynamo  Strong magnetisation of oldest parts of the Martian crust  No magnetisation of large impact basins  Dynamo action before the large impacts ~4 Ga `The Great Nothing`

19 From 400 km height…

20 Second Short Episode of Dynamo Action? Lillis et al. 2005

21 Dynamos Necessary conditions for existence An electrically conducting fluid Motion in that fluid Cowling‘s Theorem requires some helicity in the fluid motion

22 Dynamos Hydromagnetic dynamos Driven by thermal bouyancy Driven by chemical bouyancy Thermoelectric dynamo G. Glatzmeier‘s Dynamo model for Earth

23 Thermal Dynamo  Fluid motion in the liquid iron core due to thermal buoyancy (=> cooling from above)  ‘Critical‘ heat flow out of the core

24 Chemical Dynamo  Existence of light alloying elements in the core like S, O, Si  Core temperature between solidus and liquidus

25 Eutectic

26 Style of Convection Plate Tectonics (PT) Lithosphere Delamination (LD) Stagnant Lid (SL) Differ in „efficiency at cooling“, with PT being the most efficient, SL the least.

27 Thermal Evolution of the Core Breuer and Spohn, 2003

28 Stevenson et al., 1983 Evolution of the Earth‘s Magnetic Field Thermal Chemical

29 Planetary Magnetism Earth: Plate Tectonics cools core efficiently. Dynamo driven by chemical convection Mars, Moon, Venus: Single Plate Tectonics allows early thermally driven dynamo Mercury: Thin mantle cools core effciently. Dynamo driven by chemical convection Ganymede: This is a puzzling case. Core may be young