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Tilman Spohn Structure and Evolution of Terrestrial Planets Slide 2 2 Chemical Components: Gas (H, He), Ice (NH 3, CH 4, H 2 O), Rock/Iron Mars Ganymede Jupiter Slide 3 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 Slide 4 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 Slide 5 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 Peales experiment Galilean Satellites: C 22 and, in some cases, C 20 Slide 6 6 Planetary Data Slide 7 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. Slide 8 8 New Mars Model Sohl, Schubert and Spohn, 2005 Larger Cores, Thicker Crusts (both a few 10s of km) Slightly Lighter Mantle Slide 9 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 Slide 10 10 Breadboard model Slide 11 11 Interior Structure Slide 12 12 Structures Form Early Kleine et al, 2002 Breuer and Spohn, 2003 Slide 13 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 Slide 14 14 Slide 15 15 Liquid Cores? Solid Inner Cores? Slide 16 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. Slide 17 17 Magnetic Field, the Environment and Life Protects life against cosmic radiation Protects the atmosphere against erosion (Not all forms of erosion, of course) Slide 18 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` Slide 19 19 From 400 km height Slide 20 20 Second Short Episode of Dynamo Action? Lillis et al. 2005 Slide 21 21 Dynamos Necessary conditions for existence An electrically conducting fluid Motion in that fluid Cowlings Theorem requires some helicity in the fluid motion Slide 22 22 Dynamos Hydromagnetic dynamos Driven by thermal bouyancy Driven by chemical bouyancy Thermoelectric dynamo G. Glatzmeiers Dynamo model for Earth Slide 23 23 Thermal Dynamo Fluid motion in the liquid iron core due to thermal buoyancy (=> cooling from above) Critical heat flow out of the core Slide 24 24 Chemical Dynamo Existence of light alloying elements in the core like S, O, Si Core temperature between solidus and liquidus Slide 25 25 Eutectic Slide 26 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. Slide 27 27 Thermal Evolution of the Core Breuer and Spohn, 2003 Slide 28 28 Stevenson et al., 1983 Evolution of the Earths Magnetic Field Thermal Chemical Slide 29 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