1. Capture of Irregular Satellites during Planetary Encounters
David Nesvorny
David Vokrouhlicky
(SwRI)
Alessandro Morbidelli
(CNRS)
In this talk I will show you that
Southwest Research Institute is at the forefront of understanding the solar wind that fills the space around our solar system and its interaction at the galactic frontier!
Cassini image of Phoebe

2. Irregular Satellites
95 known objects: 54 at Jupiter, 26 at Saturn, at Uranus, 6 at Neptune (excluding Triton) (Gladman, Sheppard, Jewitt, Holman and others)
1-km to 340-km diameters
Colors ranging from ‘gray’ to ‘light red’ (Grav et al.)
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)

3. Origin of Irregular Satellites
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)
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:
‘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

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

7. Capture during Planetary Encounters
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

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

9. Capture during Planetary Encounters
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
Typically several hundred planetary encounters

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

11. Capture during Planetary Encounters
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
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

12. Capture during Planetary Encounters
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
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
Our model accounts for the encounter sequence where satellites are captured, removed or may switch between parent planets

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

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
~320 stable satellites captured around Neptune in this experiment (out of 3 million test particles)
~ capture probability per one particle in the disk
Late generations
Early generations

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

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

17. Comparison with SFD of known irregular moons
Jupiter
Saturn
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
Indicates that the SFD of the planetesimal disk may have been shallow during planetary encounters
Uranus
Neptune

18. Conclusions
Planetary encounters in the Nice model remove pre-existing irregular satellites and create large populations of the new ones
Shallow SFD of planetesimals at 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 clear correlation between color and heliocentric distance

19. Captures via Exchange Reactions
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
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. Distribution of encounter speeds between planets and planetesimals
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
Requires large, planetary-sized mass of the binary

21. Orbits of objects captured by
exchange reactions
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
Encounters taken from migration runs
Good capture efficiency but produced orbits have large e or small a

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