Planetary Interiors. Clues to the innerds. Density 3/20/2013. Density. Structure. Mantle intermediate density, fluid layer

Transcription

1 Planetary Interiors Structure Terrestrial (icy satellites) Core, Mantle, Crust Giants Core, Mantle, gas layer Mantle intermediate density, fluid layer Clues to the innerds Mass Radius Density Rotation Oblateness Gravity field Magnetic field (or not) Energy output Surface layer/likely composition Physics! Density Average r=mass/volume Compressed Actual density Varies with radius Uncompressed Account for weight Ice, Rock, Metal Ave. Unc Planet r r Mercury Earth Planet Mercury Earth Core Pressure 0.4 Mbar 3.6 Mbar 1

2 Calculating the interior Hydrostatic Equilibrium Balance of gravity, pressure dp r( r) GM( r) 2 dr r Simplification (constant density assumed): P c 3GM 4 8 R 2 Equation of State Relationship between T, P, r Composition Phase (gas, solid, liquid, plasma,?) Not always known Eutectic Mixture Behaves different Carbon s phase diagram 2

3 Hydrogen and Helium T range: 50 20,000 K P range: ~0 80 Mbars Ice - water 15 different crystalline forms density varies (920 kg/m kg/m 3 ) Water also ionized Rock/Metal Very complex Changes in minerals with depth Earth Upper Mantle olivine, pyroxene 400 km: olivine wadsleyite 520 km ringwoodite (spinel) ~660 km: spinel magnesium oxide, perovskite Deeper: perovskite stable Core: dominated by iron Must have something else FeS, FeO? 3

4 Other Clues Rotation indirectly Moment of inertia Distribution of material, rotational motion Non-uniform, non-spherical wobble Oblateness Geometric oblateness = (a-b)/a Gravity Field 4

5 Magnetic Field Form, Source dipole, poorly defined? History strength, orientation Current status strength, orientation 5

6 Energy Output Temperature Gradient How hot are they? Why are they hot? Cool down rate? Heat flow mechanism? Convection, Conduction, Radiative Energy Input? Surface composition Remote sensed Limited sampling How accurate? 6

7 Seismology Earth, Moon, Mars Interiors The Earth Core Iron-nickel alloy and other stuff Inner solid, outer liquid Central T~ 6000 K Central P ~ 3.6 Mbar Source of magnetic field Mantle rocky layer Not all the same, mineral discontinuities Siderophile rich rocks (iron loving) Upper mantle=asthenosphere Non-uniform flow 7

8 Crust Continental low density, granitic, thick Oceanic high density, basaltic, thin Average Age < 2 Gyr Plate tectonics 8

9 Moon All mantle, no iron core (maybe) Small iron core/iron rich core Ancient magnetic field Interior Olivine, pyroxene Thick crust Feldspar Anorthosites 9

10 Heating history complex Formation through impact (4.4 Gyr) Very hot magma ocean Cooled Cratering basins Radioactive decay heating Mare formation Lithosphere solidified Magnetic field disappears Core solid or solid+liquid? Venus Similar to Earth, but lesser But closer to the Sun Less volatiles, sulfur Less iron-sulfides in core Higher melting temperature in core not liquid? Slow rotation No magnetic field Solid core? Or no convection? Observations indicate liquid core Lithosphere and Crust Mantle probably similar to Earth Lithosphere is thinner, too thin Subduction not possible? Heat loss? Volcanism Not very efficient bottling up heat! Upper mantle is hotter than lower mantle! Asthenosphere Not likely no water Catastrophic resurfacing 10

11 Mars Lower density more volatiles Not a uniform shape Lithosphere is rigid Cooled quickly - volcanism No plate tectonics Iron present on surface, in mantle Magnetic field? Scattered, weak Rocky compositions- FeO Mars: 17.9 Earth: 7.8 MgO Mars: 30.2 Earth:

12 Mercury Take the Earth, remove most of the mantle Rigid lithosphere Cooled quickly (or has it?) Core contraction (2 km) Faults (scarps) Galilean Satellites Io Core Fe or Fe-FeS? Solid mantle Asthenosphere Crust SO 2, sulfur, silicates Europa Core solid metal Mantle rocky/ice Crust Liquid water? Fluid ice? 12

13 Ganymede Liquid metal core Silicate mantle Icy crust/top layer Liquid Callisto Icy/Rock interior Icy surface Liquid Titan Silicate rich core Icy mantle/surface Liquid layer Enceladus Iron/Rocky core Icy mantle/surface Triton Probably like Titan and Enceladus Giants Jupiter Rocky/metal core Icy layer (transition) Liquid Metallic H (& He) Liquid Molecular H (& He) Atmosphere Magnetic field Protects, preserves Internal heat 13

14 Saturn Just like Jupiter, but less Internal heat sources Formation Helium rain Weaker magnetic field Uranus/Neptune Not well understood Possible Model Rocky core Icy mantle Outer atmosphere No metallic hydrogen Magnetic field Ionized ices/water Heat flow Neptune hot why? Uranus constipated? 14