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Interior structure of planets

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S and P wave velocities increase ... upper mantle at depths 500 km, olivine. middle mantle at depths 1000 km, olivine and pyroxene ... – PowerPoint PPT presentation

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Title: 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

6
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

8
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

9
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

11
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

12
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

13
Interior structure of the giant planets
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