1. David Nesvorny David Vokrouhlicky (SwRI) Alessandro Morbidelli (CNRS) David Nesvorny David Vokrouhlicky (SwRI) Alessandro Morbidelli (CNRS) Capture of Irregular Satellites during Planetary Encounters Cassini image of Phoebe Cassini image of Phoebe

2. Irregular Satellites 95 known objects: 54 at Jupiter, 26 at Saturn, 9 at Uranus, 6 at Neptune (excluding Triton) 95 known objects: 54 at Jupiter, 26 at Saturn, 9 at Uranus, 6 at Neptune (excluding Triton) 1-km to 340-km diameters 1-km to 340-km diameters Colors ranging from ‘gray’ to ‘very red’ Colors ranging from ‘gray’ to ‘very red’ Irregular satellites have large, eccentric and predominantly retrograde orbits Irregular satellites have large, eccentric and predominantly retrograde orbits Origin distinct from the one of regular moons ( which formed by accretion in a circumplanetary disk) Origin distinct from the one of regular moons ( which formed by accretion in a circumplanetary disk)

3. Origin of Irregular Satellites Capture from the circumsolar planetesimal disk (aerodynamic gas drag, planet’s growth and expansion of its Hill sphere, etc.) Capture from the circumsolar planetesimal disk (aerodynamic gas drag, planet’s growth and expansion of its Hill sphere, etc.) All have one important drawback: formed IR satellites are dynamically removed later when planets migrate in the planetesimal disk (e.g., Beauge et al. 2002) All have one important drawback: formed IR satellites are dynamically removed later when planets migrate in the planetesimal disk (e.g., Beauge et al. 2002) In the Nice model (planets migrate, Jupiter & Saturn cross 2:1, excited orbits of Uranus & Neptune stabilized by dynamical friction): any original populations of irregular satellites are removed during encounters between planets (Tsiganis et al. 2005) In the Nice model (planets migrate, Jupiter & Saturn cross 2:1, excited orbits of Uranus & Neptune stabilized by dynamical friction): any original populations of irregular satellites are removed during encounters between planets (Tsiganis et al. 2005)

4. New model for Capture We propose a new model: We propose a new model: ‘Irregular satellites were captured during planetary encounters when background planetesimals were deflected into bound orbits around planets as a result of 3-body gravitational interactions’ ‘Irregular satellites were captured during planetary encounters when background planetesimals were deflected into bound orbits around planets as a result of 3-body gravitational interactions’

5. Capture during Planetary Encounters We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits

6. Nice model: example simulation (seed1) Neptune Neptune Uranus Uranus Saturn Saturn Jupiter Jupiter 2:1

7. Capture during Planetary Encounters We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits Planetary orbits and state of the planetesimal disk were recorded during every planetary encounter Planetary orbits and state of the planetesimal disk were recorded during every planetary encounter

8. Most orbits beyond ~30 AU are dynamically cold Most orbits beyond ~30 AU are dynamically cold Encounter happens at ~19 AU Encounter happens at ~19 AU Excited orbits in the encounter zone: ~0.2, ~10 o Excited orbits in the encounter zone: ~0.2, ~10 o State of the planetesimal disk recorded at the last encounter in the seed1 run

9. Capture during Planetary Encounters We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits Planetary orbits and state of the planetesimal disk were recorded during every planetary encounter Planetary orbits and state of the planetesimal disk were recorded during every planetary encounter Typically several hundred planetary encounters Typically several hundred planetary encounters

10. In the seed1 run: In the seed1 run: 219 encounters between Uranus and Neptune 3 encounters between Saturn and Neptune 1-3 km/s encounter speeds Planetary encounters

11. Capture during Planetary Encounters We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits Planetary orbits and state of the planetesimal disk were recorded during every planetary encounter Planetary orbits and state of the planetesimal disk were recorded during every planetary encounter Typically several hundred planetary encounters but not enough disk particles to record captures directly Typically several hundred planetary encounters but not enough disk particles to record captures directly Bulirsch-Stoer integrations, 3 million objects (clones of original disk particles) were injected into the encounter zone at each recorded encounter Bulirsch-Stoer integrations, 3 million objects (clones of original disk particles) were injected into the encounter zone at each recorded encounter

12. Capture during Planetary Encounters We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits Planetary orbits and state of the planetesimal disk were recorded during every planetary encounter Planetary orbits and state of the planetesimal disk were recorded during every planetary encounter Typically several hundred planetary encounters but not enough disk particles to record captures directly Typically several hundred planetary encounters but not enough disk particles to record captures directly Bulirsch-Stoer integrations, 3 million objects (clones of original disk particles) were injected into the encounter zone at each recorded encounter Bulirsch-Stoer integrations, 3 million objects (clones of original disk particles) were injected into the encounter zone at each recorded encounter Our model accounts for the encounter sequence where satellites are captured, removed or may switch between parent planets Our model accounts for the encounter sequence where satellites are captured, removed or may switch between parent planets

13. Will show in the following the results for the last 22 encounters between Uranus and Neptune in the seed1 run Capture during Planetary Encounters

14. # of satellites captured at Neptune in the last 22 encounters in seed1 Generations of satellites captured during early planetary encounters do not contribute much to the final population Generations of satellites captured during early planetary encounters do not contribute much to the final population ~320 stable satellites captured around Neptune in this experiment (out of 3 million test particles) ~320 stable satellites captured around Neptune in this experiment (out of 3 million test particles) ~10 -7 -10 -8 capture probability per one particle in the disk ~10 -7 -10 -8 capture probability per one particle in the disk Early generations Late generations

15. Wide range of inclinations and eccentricities Wide range of inclinations and eccentricities Semimajor axis values up to ~0.25 AU Semimajor axis values up to ~0.25 AU Orbit distributions of captured objects in the last 22 encounters in seed1 Satellites of Uranus Satellites of Neptune

16. A good agreement for Uranus Two IR satellites of Neptune, S/2002 N4 and S/2003 N1, have a~0.32 AU Comparison with orbits of known irregular moons Satellites of Uranus Satellites of Neptune

17. 35 Earth masses, Bernstein et al. SFD of present Kuiper belt, & our capture efficiency 35 Earth masses, Bernstein et al. SFD of present Kuiper belt, & our capture efficiency Planetary encounters produce more small irregular satellites than needed, their SFD is steeper Planetary encounters produce more small irregular satellites than needed, their SFD is steeper Indicates that the SFD of the planetesimal disk may have been shallow during planetary encounters Indicates that the SFD of the planetesimal disk may have been shallow during planetary encounters Comparison with SFD of known irregular moons Jupiter Neptune Saturn Uranus

18. Conclusions Planetary encounters in the Nice model remove pre-existing irregular satellites and create large populations of the new ones Planetary encounters in the Nice model remove pre-existing irregular satellites and create large populations of the new ones Shallow SFD of planetesimals at 10-30 AU at the time when encounters happened; constraint on timing (early vs. LHB Nice models) Shallow SFD of planetesimals at 10-30 AU at the time when encounters happened; constraint on timing (early vs. LHB Nice models) Results consistent with spectroscopic obs. of IR moons that show no correlation between spectral type and heliocentric distance Results consistent with spectroscopic obs. of IR moons that show no correlation between spectral type and heliocentric distance

19. Captures via Exchange Reactions Observed large fraction of binaries in Kuiper Belt Observed large fraction of binaries in Kuiper Belt Exchange reactions suggested by Agnor & Hamilton (2006) as an attractive model to capture Neptune’s Triton; proposed by H. Levison for irregular satellites Exchange reactions suggested by Agnor & Hamilton (2006) as an attractive model to capture Neptune’s Triton; proposed by H. Levison for irregular satellites We have studied exchange reactions for irregular satellites via numerical simulations of the late phase of planet migration and via millions of scattering experiments We have studied exchange reactions for irregular satellites via numerical simulations of the late phase of planet migration and via millions of scattering experiments

20. Speeds typically a few km/s Speeds typically a few km/s To capture by exchange, orbit speed of the binary needs to be comparable or larger than the encounter speed To capture by exchange, orbit speed of the binary needs to be comparable or larger than the encounter speed Requires large, planetary-sized mass of the binary Requires large, planetary-sized mass of the binary Distribution of encounter speeds between planets and planetesimals

21. 2 Mars-mass primary and several million encounter experiments 2 Mars-mass primary and several million encounter experiments We varied binary’s semimajor axis, inclination and orientation of its orbit relative to the target plane We varied binary’s semimajor axis, inclination and orientation of its orbit relative to the target plane Encounters taken from migration runs Encounters taken from migration runs Good capture efficiency but produced orbits have large e or small a Good capture efficiency but produced orbits have large e or small a Orbits of objects captured by exchange reactions

22. Exchange reactions during binary-planet encounters require a planetary-sized primary Exchange reactions during binary-planet encounters require a planetary-sized primary Captured objects have very large eccentricities and/or small semimajor axis values Captured objects have very large eccentricities and/or small semimajor axis values Requires additional mechanism that can expand captured orbits (at Neptune, captured and tidally-evolving Triton may scatter stuff around, Cuk & Gladman 2005 ) Requires additional mechanism that can expand captured orbits (at Neptune, captured and tidally-evolving Triton may scatter stuff around, Cuk & Gladman 2005 )Conclusions