Interior structure of planets

1
Interior structure of planets

• Isostatic equilibrium
• Floating objects displace their own weight on the
  substance on which it floats
• A large mass (mountain) in isostatic equilibrium
  is compensated by a deficiency of mass
  underneath, with total mass of displaced matter
  equal to the mass of the mountain
• A small mass (ocean, impact basin with thin
  crusts) in isostatic equilibrium have extra mass
  at deeper layers
• The degree to which surface topography and
  gravity are correlated is interpreted in terms of
  how much isostatic compensation is present
  strong correlation for thick lithospheres and
  upper mantles (Moon, Mars) relative to the high
  plasticity of Earth

2
Heat balance

• Heat sources
• Gravitational
• Radioactive decay
• Tidal dissipation
• Heat losses
• Conduction
• Radiation
• Convection
• Plate tectonics, volcanism

3
Interior of the Earth

• Seismic wave velocity and density profiles define
• The Moho discontinuity between the crust and
  mantle
• Varying depth 5-10 km below ocean floor, 35 km
  below continents, lt60 km below mountain ranges
• 200-300 m thick
• P wave velocity increases
• Discontinuities in upper mantle
• Depths below 670 km
• Stepwise increasing P and S wave velocities
• Phase and density changes of material, e.g.,
  olivine ? spinel ? perovskite
• The interface between the solid mantle and liquid
  outer core
• 3000 km depth
• P wave velocity decreases, S wave disappear
• Solidification of core material liberates energy
  which drives convection
• The interface between the liquid outer core and
  the solid inner core
• 5200 km depth
• P wave velocity increases
• S waves reappear

4
Seismic wave propagation and the internal
structure of the Earth
5
Interior of the Moon

• Moments of inertia measurements and seismic data
  indicate
• crust
• depth lt5 km under maria, gt100 km under highlands
• on average thicker on the far side,
  center-of-mass to center-of-geometry diplacement
  of 1.7 km towards the Earth in the Earth-Moon
  direction
• S and P wave velocities increase
• fractured basalts and gabbro lt25 km, anorthositic
  gabbro lt50 km
• mantle
• upper mantle at depths lt500 km, olivine
• middle mantle at depths lt1000 km, olivine and
  pyroxene
• lower mantle at depths lt1400 km, S wave velocity
  decreases, partially molten
• core
• P wave velocity increases, solid
• radius lt220-450 km
• iron-rich
• seismically active zones
• near surface
• at depths 700-1200 km

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Mercury

• Lithosphere
• depth 200 km
• Mantle
• depth 600 km
• rocky, silicate material
• abnormally thin
• Core
• size 75 of the planetary radius
• iron, or FeS
• outer liquid core, probably convective

7
Venus

• Similar interior structure to Earth, as suggested
  from size and mean density
• Surface heat flow probably less than on Earth
• interior may be heating up
• with radioactive heating most effective in
  mantle, the core may be relatively cooler
• Lithosphere
• high surface temperature suggests it is
  relatively thin, depth about 20 km
• absence of planet-wide tectonic plate activity
• crust may be too boyant to subduct
• heat loss less than on Earth hot-spot and
  volcanic activity possibly more important
• may lead to crust subduction every few 108 years
• Mantle
• Lower-viscosity asthenosphere may be lacking due
  to volatile depletion
• Core
• probably less FeS than Earths core
• possible state
• frozen solid
• liquid but without material phase change that
  drives convection

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Mars

• Surface and mantle high in FeO relative to Earth,
  as well as in volatiles
• Lithosphere
• low surface temperature suggests relatively thick
  lithosphere
• elevation difference 5 km between S and N
  hemispheres
• absence of plate tectonics
• significant early heat loss due to low-viscosity
  felsic magmas
• topographic features are not fully isostatically
  compensated, consistent with thick lithosphere
• Mantle
• silicate composition
• volcanic magma sources at a depth of 200 km
• Core
• solid Fe-Ni, or liquid Fe-FeS with larger radius

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Interior structure of the terrestrial planets
10
Jupiter

• Core
• Probably relatively small, 5-10 Earth masses
• Probably an inner rocky/iron and an outer
  ice-rich component, but could also be homogeneous
• Much mass was present after solid-body accretion,
  but some may have been added later by
  gravitational settling
• Core envelope
• High-Z (Zgt2) elements constitute 15-30 Earth
  masses distributed within core and its
  surrounding envelope
• Enriched in high-Z material by a factor 3-5
  compared to chondritic composition
• Mantle
• Large amount of ices of H2O, NH3, CH4 and
  S-bearing materials
• The strong magnetic field is probably generated
  by electromagnetic currents in a metallic
  hydrogen region
• Most internally generated heat is attributed to
  gravitational contraction/accretion in the past

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Saturn

• Similar interior structure to Jupiter
• Like Jupiter, the total mass of high-Z elements
  is about 15-30 Earth masses
• As Saturn is smaller and less massive than
  Jupiter, the relative abundance of high-Z
  elements is greater
• Atmospheric C, N, and S appears enhanced by a
  factor 2-3 relative to Jupiter
• Like Jupiter, the magnetic field is generated in
  a metallic hydrogen region
• The field is weaker than Jupiters due to a
  metallic envelope of smaller extent
• Internal excess heat is generated about equally
  by
• Gravitational contraction/accretion
• Release of gravitational energy due to He
  rainout onto the core
• As the interior cooled after formation, He became
  immiscible (insoluble) in metallic H and started
  to drain from the outer envelope
• Explains both excess energy and low He abundance
  in Saturns atmosphere

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Uranus and Neptune

• Neptune is 3 smaller and 15 more massive than
  Uranus
• H and He constitute only a few Earth masses of
  material
• High-Z element mass is similar to Jupiter and
  Saturn
• Consistent with atmospheric C and S abundances
  enhanced about 50 times relative to Jupiter and
  Saturn
• Core
• Possibly about 1 Earth mass, but may be absent
• Possibly differentiated iron in solid phase,
  rock may be liquid
• Mantle
• Constitutes 80 of the mass
• Icy composition hot, dense, ionic oceans of H2O
  and minor NH3, CH4, N2, H2S
• Atmosphere
• Outer 5-15 of the radius
• H- and He-rich
• Internal magnetic fields
• indicate electrically conductive and convective
  icy interiors (rather than metallic H)
• generated at 0.5-0.8 radii from the core

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Interior structure of the giant planets