1. Orbital Evolution of the Galilean Satellites Kaveh Pahlevan Doug Hamilton Dept. of Astronomy University of Maryland College Park, MD 20742

2. Why study the Galilean Satellites? - Diverse & interesting planetary bodies - Local analogue to planetary systems - Can give insight to general formation processes.

3. Observations: I. Physical Properties  (g/cm 3 ) I/MR 2 Io3.530.378 Europa3.010.347 Ganymede1.940.310 Callisto1.830.355

4. Observations: II. Orbital Properties Eccentricity Inclination (  ) Io0.00410.036 Europa0.00940.469 Ganymede0.00110.170 Callisto0.00740.187

5. 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.

6. Resonances: II. Splitting – When satellites orbit an oblate planet, the components of a resonance split into several closely spaced resonances.

7. 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.

8. 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.

9. The 2:1ecc/4:2inc Resonance Rate: 10 -6 R J /dayRate: 10 -11 R J /day

10. The 2:1/4:2 Resonance – Summary Eccentricity KicksInclination Kicks

11. 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.

12. The 3:1 Resonance at 10 -9 R J /day

13. The 3:1 Resonance at 10 -12 R J /day

14. The 3:1 Resonance - Summary If the Laplace resonance is primordial, Ganymede must have passed through the 3:1 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.

15. 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.

16. The 5:2 Resonance at 10 -11 R J /day

17. The 5:2 Resonance at 10 -12 R J /day

18. So, what have we learned? 2:1 resonance – can deliver an early heat pulse to both satellites, but preferentially heats Ganymede. 5:2 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. 3:1 resonance – probably did not occur. If we can confirm this result, then the primordial assembly of the satellites into resonance did not take place.

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