The Inside Story on Planets

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The Inside Story on Planets

• Dave Stevenson, Caltech
• COMPRES, Snowbird, June 22, 2006

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How to think about a Planet?

• Could discuss provenance- the properties of an
  apple depend on the environment in which the tree
  grows
• Or could discuss it as a machine (cf. Heron,
  1st century AD)
• Need to do both

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Why do Planetary Scientists Care about the
Interiors of Planets?

• Composition and structure may tell us about how
  planets formA central question
• Internal structure provides the framework in
  which to understand heat loss, convective
  dynamics, volcanism, magnetic field
• Internal structure defines or modifies some
  external properties (e.g., atmosphere climate
  change, habitability)

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Why should you Care? How can you (COMPRES) help?

• Fundamental unsolved problems in the behavior of
  high pressure properties
• Crucial support for NASA (and now international
  space) priorities (even though you may have
  trouble getting money out of NASA!)
• Science as a playground where you can expand your
  horizons (different P, T , composition)..fundament
  al condensed matter physics

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Outline

• Cosmochemical Context general properties of
  planets
• Giant (gas Ice) planets including extrasolar
  planets icy bodies
• Terrestrial planets (but not Earth)
• What do we need?

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Cosmic (Solar) Abundances
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Classes of Planetary Materials
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Fluid Planets

• Gas Giants (primarily hydrogen and helium)-
  Jupiter and Saturn
• Ice Giants (everything, but including large
  amounts of H2O at high P,T) Uranus and Neptune

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Solid Planets

• Terrestrial (silicates, oxides and iron
  alloy)-Mercury, Venus, Earth, Moon, Mars, Io
• Large icy satellites (terrestrial ice)
• Europa, Ganymede, Callisto, Titan, Triton, Pluto

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Gas (H2,He)
Jupiter,Saturn
Line of cosmic ice rock condensate (variable
gas)
Uranus, Neptune
Earth
Ice (mainly H2O)
Large Icy Satellites
Rock (silicates, oxides, met. Fe)
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Gas
subJupiters
Not represented in our solar system
Superganymedes
sJ
J
Ice
Rock
SuperJupiters (most Extrasolars so far)
LI
Gas
M/MJ10-4
E
SG
M/MJ0.01
Increasing mass
M/MJ1
Rock
Ice
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Temp (K)
Planetary upper limit
106
105
10 Jupiter mass
104
Jupiter
Earth cores
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Uranus
Large Icy satellites
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Pressure(Gpa)
1
10
102
103
104
105
106
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Temp (K)
Planetary upper limit
106
105
150 of these have been found
10 Jupiter mass
104
Jupiter
Earth cores
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Uranus
Now being found
Large Icy satellites
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Pressure(Gpa)
1
10
102
103
104
105
106
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T P Conditions
Icy satellites
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Jupiter

• Approach to metallic conduction achieved in
  hydrogen at 0.85 Jupiter radii.
• Factor of three enrichment of heavy elements
• Presence of core not certain, but up to 10 Earth
  masses

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Saturn

• Very similar to Jupiter except metallic core is
  much deeper.
• Heavy element enrichment
• Presence of core almost certain 10 Earth masses

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Uranus Neptune

• Eight to ten earth masses of ice and rock a few
  earth masses of gas.
• Ambiguity of structure, but gas component appears
  to be a nearly hydrostatic nebula add -on.
• Good ionic conduction even at 80 radius.

Neptune
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• This shows the heavy element abundance in the
  four major planets and estimated uncertainties
• A major source of uncertainty is in the equations
  of state.

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The Hydrogen Phase diagram

• Jupiter Saturn are in the fluid region,
  possibly crossing a PPT phase transition.
• Relevant conditions encountered in reverberation
  shock experiments
• Helium immiscibility suggested by observation
  theory but not well understood.

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D2 Hugoniots

• Discrepancy is large and important to giant
  planet models
• Latest (Sandia soviet) data support less severe
  compression

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EOS for other Ingredients

• Not that important for Jupiter Saturn
• Very important for Uranus Neptune -especially
  oxygen (H2O) but also carbon (CH4)
• Chemistry (mixing properties) needed

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Latest transit data
Charbonneau et al, preprint
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PK?2 with K2x 1012 cgs
20 dense core (or 40 larger K)
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PK?2 with K2x 1012 cgs
Cosmic 20 Earth mass core
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Metallization of Hydrogen

• Shock wave experiments show a rapid increase in
  conductivity at high P and T -approaching
  metallic values
• This pressure is reached at 0.85RJupiter or
  0.6RSaturn
• But conductivity at lower P is also very
  interesting

Nellis et al
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Nature, May 4, 2006
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Planetary Magnetism

• Dynamo is responsible for large fields (Earth,
  giant planets, maybe Mercury)
• Energy source is still imperfectly understood
• Only limited information on the field from
  external measurements

Supercomputer simulation by Glatzmaier Roberts
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Giant Planet Condensed Matter Issues
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Possible effect of deep-seated zonal flows
Accuracy required for pole precession
Juno (launch in 2011)
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Ganymede

• Metallic core, plausibly liquid if sulfur rich. K
  in core?
• High sulfur content lowers electrical
  conductivity- this may be essential!
• 40K in mantle or core would also help.

Model interior based on gravity B field
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Imaging from Huygens probe descent into Titans
atmosphere

• Note evidence for dendritic channels. Plausibly
  made by liquid methane. Age not known.

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Clathrates

• Several recent discoveries for clathrates
  (CH4,H2)
• May be important in large icy satellites
  (especially Titan)
• Not important for giant planets (too hot at
  appropriate P)

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Mercury

• 1. Large core.
• Radar data on libration indicate a liquid outer
  core. (Margot, 2004)
• Thermal evolution models predict it is mostly
  frozen by now.

Solid Fe
Liquid Fe
Mantle
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Venus

• 1. Earthlike structure
• Liquid outer core likely (inferred from solar
  tidal gravity)
• No inner core because of higher T lower P than
  Earth. Or
• Planet currently heating up as it transitions
  from mobile to stagnant surface.

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Moon

• Small, partly liquid core suggested by
  response to 18.6 yr nutation.
• Also consistent with moment of inertia, EM
  induction and geochemistry.

mantle
core
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Mars Structure

• Earthlike core mantle
• Liquid outer core now confirmed, presence of an
  inner core not known.

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NASA's Fuse Finds Infant Solar System Awash in
CarbonScientists using NASA's Far Ultraviolet
Spectroscopic Explorer, or FUSE, have discovered
abundant amounts of carbon gas in a dusty disk
surrounding a young star named Beta Pictoris.
The star and its emerging solar system are less
than 20 million years old, and planets may have
already formed. The abundance of carbon gas in
the remaining debris disk indicates that Beta
Pictoris' planets could be carbon-rich worlds of
graphite and methane, or the star's environs
might resemble our own solar system in its early
days.
Nature, June 8, 2006
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Conclusions

• Hydrogen H-He mixtures continue to challenge
  our theoretical models experimental capability.
  Shock techniques most useful, but diamond cell
  work can also be diagnostic.
• Oxygen (H2O) and carbon (CH4) important for ice
  -rich bodies . But there are also many chemistry
  questions that involve other elements and
  mixtures. (Water-rock, gas-ice, etc.) Both shock
  wave diamond cell techniques useful.
• Important Mars Mercury core mantle issues
  readily addressable with existing techniques.
• Extrasolar planets open up exciting new issues
  compositions?