David NesvornyDavid Nesvorny

(Southwest Research (Southwest Research Institute)Institute)

David NesvornyDavid Nesvorny

(Southwest Research (Southwest Research Institute)Institute)

Capture of Irregular Satellites during Planetary EncountersCapture of Irregular Satellites during Planetary Encounters

Cassini image of PhoebeCassini image of Phoebe

IrregularIrregular Satellites Satellites

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

Discovery RateDiscovery Rate

Numbers & Sizes of Irregular SatellitesNumbers & Sizes of Irregular Satellites

PlanetPlanet Number Number of known of known Irregular Irregular Satellites Satellites

Smallest Smallest DetectableDetectableRadius (km)Radius (km)

JupiterJupiter 5555 11

SaturnSaturn 3535 22

UranusUranus 99 55

NeptuneNeptune 66 1515

To infer the numbers of irregular satellites at each To infer the numbers of irregular satellites at each planet the numbers of known satellites must be planet the numbers of known satellites must be corrected for observational incompleteness corrected for observational incompleteness

Corrected Size DistributionsCorrected Size Distributions

The numbers The numbers of irregular of irregular satellites satellites present at present at individualindividualplanets may planets may be be SIMILARSIMILAR..

Jewitt & SheppardJewitt & Sheppard

Irregular SatellitesIrregular Satellites

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

1-km to 340-km diameters1-km to 340-km diameters

Irregular SatellitesIrregular Satellites

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

1-km to 340-km diameters1-km to 340-km diameters

Diversity of colors (neutral to reddish) Diversity of colors (neutral to reddish)

Colors of Irregular SatellitesColors of Irregular Satellites

Observations Observations show color show color diversitydiversity

Colors rangeColors rangefrom neutral from neutral to reddishto reddish

A hint of A hint of color gradient color gradient with with heliocentricheliocentricdistance distance

NeutralNeutral

ReddishReddish

Colors of Irregular SatellitesColors of Irregular Satellites

Observations Observations show color show color diversitydiversity

Colors rangeColors rangefrom neutral from neutral to reddishto reddish

A hint of A hint of color gradient color gradient with with heliocentricheliocentricdistance distance

Irregular SatellitesIrregular Satellites

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

1-km to 340-km diameters1-km to 340-km diameters

Diversity of colors (neutral to reddish) Diversity of colors (neutral to reddish)

Irregular satellites have large, eccentric and Irregular satellites have large, eccentric and predominantly retrograde orbitspredominantly retrograde orbits

Orbits of Irregular SatellitesOrbits of Irregular Satellites

RetrogradeRetrograde ProgradePrograde

Irregular SatellitesIrregular Satellites

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

1-km to 340-km diameters1-km to 340-km diameters

Diversity of colors (neutral to reddish) Diversity of colors (neutral to reddish)

Irregular satellites have large, eccentric and Irregular satellites have large, eccentric and predominantly retrograde orbitspredominantly retrograde orbits

Origin distinct from the one of regular moonsOrigin distinct from the one of regular moons (which formed by accretion in a circumplanetary disk)(which formed by accretion in a circumplanetary disk)

Origin of Irregular SatellitesOrigin of Irregular Satellites

Capture from the circumsolar planetesimal diskCapture from the circumsolar planetesimal disk (aerodynamic gas drag, planet’s growth and expansion (aerodynamic gas drag, planet’s growth and expansion of its Hill sphere, etc.)of its Hill sphere, etc.)

All have one important drawback: formed IR satellites are All have one important drawback: formed IR satellites are dynamically removed later when planets migrate in the dynamically removed later when planets migrate in the planetesimal disk planetesimal disk (e.g., Beauge et al. 2002)(e.g., Beauge et al. 2002)

In the Nice modelIn the Nice model (planets migrate, Jupiter & Saturn cross 2:1, (planets migrate, Jupiter & Saturn cross 2:1, excited orbits of Uranus & Neptune stabilized by dynamical friction):excited orbits of Uranus & Neptune stabilized by dynamical friction):

any original populations of irregular satellites are removed any original populations of irregular satellites are removed during encounters between planetsduring encounters between planets (Tsiganis et al. 2005)(Tsiganis et al. 2005)

New model for CaptureNew model for Capture

We propose a new model:We propose a new model:

‘ ‘Irregular satellites were captured during Irregular satellites were captured during planetary encounters when background planetary encounters when background planetesimals were deflected into bound planetesimals were deflected into bound orbits around planets as a result of 3-bodyorbits around planets as a result of 3-bodygravitational interactions’ gravitational interactions’

Capture during Planetary EncountersCapture during Planetary Encounters

UranusUranus

NeptuneNeptune

Hill SphereHill Sphere

Hill SphereHill Sphere

Numerous disk planetesimalsNumerous disk planetesimals

Capture during Planetary EncountersCapture during Planetary Encounters

Capture during Planetary EncountersCapture during Planetary Encounters

Captured Irregular SatellitesCaptured Irregular Satellites

Our ModelOur Model

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

Example simulations ofExample simulations ofPlanet MigrationPlanet Migration

NeptuneNeptune

UranusUranus

SaturnSaturnJupiterJupiter

Capture during Planetary EncountersCapture during Planetary Encounters

We performed 50 new simulations of the Nice model, We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits ~20 successful runs produced correct planetary orbits

Planetary orbits and state of the planetesimal disk were Planetary orbits and state of the planetesimal disk were recorded during every planetary encounterrecorded during every planetary encounter

Encounter happens at Encounter happens at ~23 AU~23 AU

Excited orbits in the Excited orbits in the encounter zone: encounter zone: <e>~0.2, <i>~10 <e>~0.2, <i>~10oo

State of the planetesimal disk recorded State of the planetesimal disk recorded at the last encounter in job #47 at the last encounter in job #47

Capture during Planetary EncountersCapture during Planetary Encounters

We performed 50 new simulations of the Nice model, We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits ~20 successful runs produced correct planetary orbits

Planetary orbits and state of the planetesimal disk were Planetary orbits and state of the planetesimal disk were recorded during every planetary encounterrecorded during every planetary encounter

Typically several hundred planetary encounters Typically several hundred planetary encounters

In the #47 job: In the #47 job:

408 encounters between408 encounters betweenUranus and Neptune Uranus and Neptune

35 encounters between 35 encounters between Saturn and NeptuneSaturn and Neptune

1-3 km/s encounter speeds1-3 km/s encounter speeds

Planetary encountersPlanetary encounters

Capture during Planetary EncountersCapture during Planetary Encounters

We performed 50 new simulations of the Nice model, We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits ~20 successful runs produced correct planetary orbits

Planetary orbits and state of the planetesimal disk were Planetary orbits and state of the planetesimal disk were recorded during every planetary encounterrecorded during every planetary encounter

Typically several hundred planetary encounters Typically several hundred planetary encounters but not but not enough disk particles to record captures directlyenough disk particles to record captures directly

Capture during Planetary EncountersCapture during Planetary Encounters

We performed 50 new simulations of the Nice model, We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits ~20 successful runs produced correct planetary orbits

Planetary orbits and state of the planetesimal disk were Planetary orbits and state of the planetesimal disk were recorded during every planetary encounterrecorded during every planetary encounter

Typically several hundred planetary encounters but not Typically several hundred planetary encounters but not enough disk particles to record captures directlyenough disk particles to record captures directly

Bulirsch-Stoer integrations, 3 million objects (clones of Bulirsch-Stoer integrations, 3 million objects (clones of original disk particles) were injected into the encounter original disk particles) were injected into the encounter zone at each recorded encounterzone at each recorded encounter

Capture during Planetary EncountersCapture during Planetary Encounters

We performed 50 new simulations of the Nice model, We performed 50 new simulations of the Nice model, ~20 successful runs produced correct planetary orbits ~20 successful runs produced correct planetary orbits

Planetary orbits and state of the planetesimal disk were Planetary orbits and state of the planetesimal disk were recorded during every planetary encounterrecorded during every planetary encounter

Typically several hundred planetary encounters but not Typically several hundred planetary encounters but not enough disk particles to record captures directlyenough disk particles to record captures directly

Bulirsch-Stoer integrations, 3 million objects (clones of Bulirsch-Stoer integrations, 3 million objects (clones of original disk particles) were injected into the encounter original disk particles) were injected into the encounter zone at each recorded encounterzone at each recorded encounter

Our model accounts for the encounter sequence where Our model accounts for the encounter sequence where satellites are captured, removed or may switch between satellites are captured, removed or may switch between parent planetsparent planets

Number of Captured SatellitesNumber of Captured Satellites

Generations of satellitesGenerations of satellitescaptured during early captured during early planetary encounters planetary encounters do not contribute muchdo not contribute muchto the final populationto the final population

~1368 stable satellites~1368 stable satellitescaptured around Neptunecaptured around Neptunein this experimentin this experiment(out of 3 million test (out of 3 million test particles) particles)

~10~10-7-7-10-10-8-8 capture capture probability per one probability per one particle in the diskparticle in the disk

UranusUranus NeptuneNeptune

UranusUranus NeptuneNeptune

Capture during Planetary EncountersCapture during Planetary Encounters

Good agreement between Good agreement between model and real orbits. model and real orbits.

Orbit distributions of captured objectsOrbit distributions of captured objects

Satellites of UranusSatellites of Uranus

Satellites of NeptuneSatellites of Neptune

Orbit distributions of captured objectsOrbit distributions of captured objects

Satellites of JupiterSatellites of Jupiter

Satellites of SaturnSatellites of Saturn

Good agreement between Good agreement between model and real orbits. model and real orbits.

35 Earth masses, 35 Earth masses, Bernstein et al.’s Bernstein et al.’s SFD of SFD of present Kuiper belt, present Kuiper belt, & our capture efficiency& our capture efficiency

Planetary encounters Planetary encounters produce more small produce more small irregular satellites than irregular satellites than needed, their SFD slope needed, their SFD slope is steepis steep

Indicates that the SFD Indicates that the SFD of irregular satellitesof irregular satellitesmay have changed by may have changed by collisional disruptionscollisional disruptions

Comparison with SFD of known Comparison with SFD of known irregular moonsirregular moons

ConclusionsConclusions

Planetary encounters in the Nice model remove Planetary encounters in the Nice model remove pre-existing irregular satellites and create large pre-existing irregular satellites and create large populations of the new ones populations of the new ones

The difference between model and real SFDsThe difference between model and real SFDsindicates that the SFDs of the irregular satellites indicates that the SFDs of the irregular satellites changed by collisional disruptionschanged by collisional disruptions

Results consistent with spectroscopic Results consistent with spectroscopic observations of IR moons that show diverse observations of IR moons that show diverse colors colors

Captures via Exchange ReactionsCaptures via Exchange Reactions

Observed large fraction of binaries in Kuiper BeltObserved large fraction of binaries in Kuiper Belt

Exchange reactions suggested by Exchange reactions suggested by Agnor & Agnor &

Hamilton (2006)Hamilton (2006) as an attractive model to capture as an attractive model to capture Neptune’s TritonNeptune’s Triton

We have studied exchange reactions forWe have studied exchange reactions forirregular satellites via numerical simulations of irregular satellites via numerical simulations of the late phase of planet migration andthe late phase of planet migration andvia millions of scattering experimentsvia millions of scattering experiments

Speeds typically a few km/sSpeeds typically a few km/s

To capture by exchange, To capture by exchange, orbit speed of the binary needs orbit speed of the binary needs to be comparable or larger thanto be comparable or larger thanthe encounter speedthe encounter speed

Requires large, planetary-sizedRequires large, planetary-sizedmass of the binary mass of the binary

Distribution of encounter speeds between Distribution of encounter speeds between planets and planetesimals planets and planetesimals

2 Mars-mass primary2 Mars-mass primaryand several million and several million encounter experimentsencounter experiments

We varied binary’s We varied binary’s semimajor axis, inclinationsemimajor axis, inclinationand orientation of its orbitand orientation of its orbitrelative to the target planerelative to the target plane

Encounters taken from Encounters taken from migration runsmigration runs

Good capture efficiency Good capture efficiency but produced orbits have but produced orbits have large large ee or small or small aa

Orbits of objects captured by Orbits of objects captured by exchange reactions exchange reactions

Exchange reactions during binary-planet Exchange reactions during binary-planet encounters require a planetary-sized primaryencounters require a planetary-sized primary

Captured objects have very large eccentricities Captured objects have very large eccentricities and/or small semimajor axis values and/or small semimajor axis values

Requires additional mechanism that can Requires additional mechanism that can expand captured orbits expand captured orbits (at Neptune, captured and tidally-evolving Triton may (at Neptune, captured and tidally-evolving Triton may

scatter stuff around,scatter stuff around, Cuk & Gladman 2005Cuk & Gladman 2005))

ConclusionsConclusions