Orbital Evolution of the Galilean Satellites

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Orbital Evolution of theGalilean Satellites

• Kaveh Pahlevan
• Doug Hamilton
• Dept. of Astronomy University of Maryland
• College Park, MD 20742

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Why study the Galilean Satellites?

• Diverse interesting planetary bodies-
  Local analogue to planetary systems- Can
  give insight to general formation processes.

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Observations I. Physical Properties
r (g/cm3) I/MR2
Io 3.53 0.378
Europa 3.01 0.347
Ganymede 1.94 0.310
Callisto 1.83 0.355
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Observations II. Orbital Properties
Eccentricity Inclination (?)
Io 0.0041 0.036
Europa 0.0094 0.469
Ganymede 0.0011 0.170
Callisto 0.0074 0.187
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Resonances I.

• When a small perturbation adds coherently over
  time, the system is in resonance.
• A mean motion resonance can occur when the ratio
  of two orbital frequencies is a small integer.

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Resonances II.

• Splitting When satellites orbit an oblate
  planet, the components of a resonance split into
  several closely spaced resonances.

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The Laplace Resonance

• Tidal Origin The outward migration of the
  satellites due to tides from Jupiter collected
  the inner three satellites into the resonance.

• Primordial Origin Inward migration during the
  formation era collected the satellites into the
  resonance.

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Resonances I.

• When a small perturbation adds coherently over
  time, the system is in resonance.
• A mean motion resonance can occur when the ratio
  of two orbital frequencies is a small integer.

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The 21ecc/42inc Resonance
Rate 10-6 RJ/day
Rate 10-11 RJ/day
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The 21/42 Resonance Summary

• Eccentricity Kicks

• Inclination Kicks

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Resonances I.

• When a small perturbation adds coherently over
  time, the system is in resonance.
• A mean motion resonance can occur when the ratio
  of two orbital frequencies is a small integer.

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The 31 Resonance at 10-9 RJ/day
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The 31 Resonance at 10-12 RJ/day
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The 31 Resonance - Summary

• If the Laplace resonance is primordial, Ganymede
  must have passed through the 31 resonance with
  Callisto.
• Simulations such a passage typically leads to
  inclinations of a degree or more.
• No known process can damp inclinations ? Argues
  for a tidal origin for the assembly of the
  Galilean satellites into the Laplace resonance.

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Resonances I.

• When a small perturbation adds coherently over
  time, the system is in resonance.
• A mean motion resonance can occur when the ratio
  of two orbital frequencies is a small integer.

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The 52 Resonance at 10-11 RJ/day
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The 52 Resonance at 10-12 RJ/day
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So, what have we learned?

• 21 resonance can deliver an early heat pulse
  to both satellites, but preferentially heats
  Ganymede.

• 31 resonance probably did not occur. If we
  can confirm this result, then the primordial
  assembly of the satellites into resonance did not
  take place.

• 52 resonance third order resonance ? much more
  difficult to study. Our preliminary results
  suggest that the present day inclinations might
  be explained by passage through this resonance.

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• Man with all his noble qualities . . . with
  his god-like intellect which has penetrated into
  the movements and constitution of the solar
  systemwith all these exalted powersman still
  bears in his bodily frame the indelible stamp of
  his lowly origin.
• Charles Darwin, The Descent of Man,Closing words
  of book.