Transcript bertioga06 - SwRI - Southwest Research Institute

Capture of Irregular Satellites
during Planetary Encounters
David Nesvorny
(Southwest Research
Institute)
 Cassini image of Phoebe
Irregular Satellites
 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)
Jupiter
55
1
Saturn
35
2
Uranus
9
5
Neptune
6
15
 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.
Jewitt & Sheppard
Irregular Satellites
 104 known objects: 54 at Jupiter, 35 at Saturn,
9 at Uranus, 6 at Neptune (excluding Triton)
 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)
 1-km to 340-km diameters
 Diversity of colors (neutral to reddish)
Colors of Irregular Satellites
 Observations
Reddish
show color
diversity
 Colors range
from neutral
to reddish
Neutral
 A hint of color
gradient with
heliocentric
distance
Colors of Irregular Satellites
 Observations
show color
diversity
 Colors range
from neutral
to reddish
 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)
 1-km to 340-km diameters
 Diversity of colors (neutral to reddish)
 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)
 1-km to 340-km diameters
 Diversity of colors (neutral to reddish)
 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 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)
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’
Capture during Planetary Encounters
Hill Sphere
Neptune
Uranus
Numerous disk planetesimals
Hill Sphere
Capture during Planetary Encounters
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
Example simulations of
Planet Migration
Neptune
Uranus
Saturn
Jupiter
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
State of the planetesimal disk recorded
at the last encounter in job #47
Encounter happens at
~23 AU
Excited orbits in the
encounter zone:
<e>~0.2, <i>~10o
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
Planetary encounters
In the #47 job:
408 encounters between
Uranus and Neptune
35 encounters between
Saturn and Neptune
1-3 km/s encounter speeds
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
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
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
Number of Captured Satellites
Uranus
Neptune
Generations of satellites
captured during early
planetary encounters
do not contribute much
to the final population
~1368 stable satellites
Uranus
Neptune
captured around Neptune
in this experiment
(out of 3 million test
particles)
~10-7-10-8 capture
probability per one
particle in the disk
Capture during Planetary Encounters
Orbit distributions of captured objects
Satellites of Uranus
 Good agreement between
model and real orbits.
Satellites of Neptune
Orbit distributions of captured objects
Satellites of Jupiter
 Good agreement between
model and real orbits.
Satellites of Saturn
Comparison with SFD of known
irregular moons
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
 Indicates that the SFD
of irregular satellites
may have changed by
collisional disruptions
Conclusions
 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
 Results consistent with spectroscopic
observations of IR moons that show diverse
colors
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
 We have studied exchange reactions for
irregular satellites via numerical simulations of
the late phase of planet migration and
via millions of scattering experiments
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
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
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
(at Neptune, captured and tidally-evolving Triton may
scatter stuff around, Cuk & Gladman 2005)