David Nesvorny (Southwest Research Institute) David Nesvorny (Southwest Research Institute) Capture of Irregular Satellites during Planetary Encounters Cassini image of Phoebe Cassini image of Phoebe

Irregular Satellites 104 known objects: 54 at Jupiter, 35 at Saturn, 9 at Uranus, 6 at Neptune (excluding Triton) 104 known objects: 54 at Jupiter, 35 at Saturn, 9 at Uranus, 6 at Neptune (excluding Triton)

Discovery Rate

Numbers & Sizes of Irregular Satellites Planet Number of known Irregular Satellites Smallest Detectable Radius (km) Jupiter551 Saturn352 Uranus95 Neptune615 To infer the numbers of irregular satellites at each planet the numbers of known satellites must be corrected for observational incompleteness To infer the numbers of irregular satellites at each planet the numbers of known satellites must be corrected for observational incompleteness

Corrected Size Distributions The numbers of irregular satellites present at individual planets may be SIMILAR. The numbers of irregular satellites present at individual planets may be SIMILAR. Jewitt & Sheppard

Irregular Satellites 104 known objects: 54 at Jupiter, 35 at Saturn, 9 at Uranus, 6 at Neptune (excluding Triton) 104 known objects: 54 at Jupiter, 35 at Saturn, 9 at Uranus, 6 at Neptune (excluding Triton) 1-km to 340-km diameters 1-km to 340-km diameters

Irregular Satellites 104 known objects: 54 at Jupiter, 35 at Saturn, 9 at Uranus, 6 at Neptune (excluding Triton) 104 known objects: 54 at Jupiter, 35 at Saturn, 9 at Uranus, 6 at Neptune (excluding Triton) 1-km to 340-km diameters 1-km to 340-km diameters Diversity of colors (neutral to reddish) Diversity of colors (neutral to reddish)

Colors of Irregular Satellites Observations show color diversity Observations show color diversity Colors range from neutral to reddish Colors range from neutral to reddish A hint of color gradient with heliocentric distance A hint of color gradient with heliocentric distance Neutral Neutral Reddish Reddish

Colors of Irregular Satellites Observations show color diversity Observations show color diversity Colors range from neutral to reddish Colors range from neutral to reddish A hint of color gradient with heliocentric distance A hint of color gradient with heliocentric distance

Irregular Satellites 104 known objects: 54 at Jupiter, 35 at Saturn, 9 at Uranus, 6 at Neptune (excluding Triton) 104 known objects: 54 at Jupiter, 35 at Saturn, 9 at Uranus, 6 at Neptune (excluding Triton) 1-km to 340-km diameters 1-km to 340-km diameters Diversity of colors (neutral to reddish) Diversity of colors (neutral to reddish) Irregular satellites have large, eccentric and predominantly retrograde orbits Irregular satellites have large, eccentric and predominantly retrograde orbits

Orbits of Irregular Satellites Retrograde Prograde

Irregular Satellites 104 known objects: 54 at Jupiter, 35 at Saturn, 9 at Uranus, 6 at Neptune (excluding Triton) 104 known objects: 54 at Jupiter, 35 at Saturn, 9 at Uranus, 6 at Neptune (excluding Triton) 1-km to 340-km diameters 1-km to 340-km diameters Diversity of colors (neutral to reddish) Diversity of colors (neutral to reddish) 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)

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)

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’

Capture during Planetary Encounters Uranus Neptune Hill Sphere Numerous disk planetesimals

Capture during Planetary Encounters

Captured Irregular Satellites

Our Model We performed 50 new simulations of the Nice model, ~14 successful runs produced correct planetary orbits We performed 50 new simulations of the Nice model, ~14 successful runs produced correct planetary orbits

Example simulations of Planet Migration Neptune Neptune Uranus Uranus Saturn Saturn Jupiter Jupiter

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

Encounter happens at ~23 AU Encounter happens at ~23 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 job #47

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

In the #47 job: In the #47 job: 408 encounters between Uranus and Neptune 35 encounters between Saturn and Neptune 1-3 km/s encounter speeds Planetary encounters

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

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

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

Number of Captured Satellites 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 ~1368 stable satellites captured around Neptune in this experiment (out of 3 million test particles) ~1368 stable satellites captured around Neptune in this experiment (out of 3 million test particles) ~ capture probability per one particle in the disk ~ capture probability per one particle in the disk Uranus Neptune Uranus Neptune

Capture during Planetary Encounters

Good agreement between model and real orbits. Good agreement between model and real orbits. Orbit distributions of captured objects Satellites of Uranus Satellites of Neptune

Orbit distributions of captured objects Satellites of Jupiter Satellites of Saturn Good agreement between model and real orbits. Good agreement between model and real orbits.

35 Earth masses, Bernstein et al.’s SFD of present Kuiper belt, & our capture efficiency 35 Earth masses, Bernstein et al.’s SFD of present Kuiper belt, & our capture efficiency Planetary encounters produce more small irregular satellites than needed, their SFD slope is steep Planetary encounters produce more small irregular satellites than needed, their SFD slope is steep Indicates that the SFD of irregular satellites may have changed by collisional disruptions Indicates that the SFD of irregular satellites may have changed by collisional disruptions Comparison with SFD of known irregular moons

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 The difference between model and real SFDs indicates that the SFDs of the irregular satellites changed by collisional disruptions The difference between model and real SFDs indicates that the SFDs of the irregular satellites changed by collisional disruptions Results consistent with spectroscopic observations of IR moons that show diverse colors Results consistent with spectroscopic observations of IR moons that show diverse colors

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 Exchange reactions suggested by Agnor & Hamilton (2006) as an attractive model to capture Neptune’s Triton 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

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

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

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