Origins of Regular and
Irregular Satellites
ASTR5830
March 21, 2013
12:30-1:45 pm

Regular:
• Coplanar, low eccentricity and small inclination orbits.
• Typically, larger.
• Thought to have formed in situ.
• Inhabit a small fraction of host planet’s Hill sphere.
Irregular:
• Exist in a large range of e and i .
• Typically, smaller.
• Thought to be captured from heliocentric orbit.
• Orbits extend to ~ 0.5 r
H
.

Observational Constraints on Irregular
Satellite Origins
• Large orbital radii: a ~ 0.5r
H
• Large eccentricities: ( 0.1 ~ e ~ 0.7
)
• Large inclinations: i up to 180˚
– Yet, none are found 60˚ < i
< 120˚
•
Many have retrograde orbits!
• 100’s known to exist (numerically dominant).
•
Generally smaller than Regular satellites.
•
Exist in families with similar dynamics.

Why Study the Capture of
Irregular Satellites?
• Requires dissipative mechanism!
– Tidal dissipation
– Pull-down capture
– Gas-drag capture
– Three-body capture (two flavors)
• No such processes exist in solar system today.
• Must have occurred at an early epoch.

Jewitt and Haghighipour (2007)

Kozai Resonances!
Jewitt and Haghighipour (2007)

Jewitt and Haghighipour (2007)

Jewitt and Haghighipour (2007)

• Initial population of irregular satellites was perhaps an order of magnitude larger.
• Collisions can occur that remove, or alter, small bodies.
– Collisions w/ planet.
– Collisions with other satellites ( 10 6 yr ).
– Collisions with interlopers.
– Creation of dynamical families.
• Dynamical instabilities can remove satellites from the system – Kozai Resonances.
– Retrograde satellites are more stable than their prograde counterparts.

• Method of pumping up eccentricities, and reducing the periapses, of orbits by reducing inclinations.
sin
2

P

0.4 csc
2 i
P
 
P max

1
6

1

 
P
 for 39.2
o  i

140.8
o

• Pull-down
– Sudden increase in Hill radius causes capture.
– Works well for retrograde satellites.
– Requires runaway growth.
• Gas-drag
– Friction with gas in an extended envelope.
– Capture efficiency is a a function of size.
– Also requires sufficient gas.

• Does not require circumplanetary gas.
• Two main flavors.
– Interactions with existing satellite(s).
– Wide binary (Kuiper belt) object.
• Density of small objects was much higher at early epochs.

• Yet to be identified!
• Either local or non-local.
• Physical properties of irregular satellites dissimilar to Kuiper belt objects.
• Yet, theories of giant planet formation and migration predict a large amount of objects will be scattered into the inner solar system from a disk of planetesimals exterior to
Neptune.

• Largest satellite of Neptune.
– d = 2706 km
– rho = 2.061 g cm -3
– a = 354,800 km = 14.4 R
N
= 0.003 r
– Retrograde orbit: i = 156.8˚
H
•
Probable Kuiper belt object
• Tidal evolution probably changed Triton’s orbit and evolved its surface.

• Tidal capture is unlikely because Triton is too far for tides to act on a short enough timescale.
• Gas-drag capture unlikely because of lack of gas around Neptune.
• N-body capture most likely scenario!
– Disruption of regular satellite system.
– Originally had a binary companion.

Triton
• M = 2.14x1022 kg
Pluto
• M = 1.25x1022 kg
• R = 1,353.4 km
• rho = 2,050 kg/m3 • rho = 1,795 kg/m3
• Albedo = 0.76
• R = 1,195 km
• Albedo = 0.5-0.7
http://nssdc.gsfc.nasa.gov/planetary/factsheet/

Agnor and Hamilton (2006)

Schmit and Mitchell
(Unpublished)

Schmit and Mitchell
(Unpublished)

Schmit and Mitchell
(Unpublished)

• Irregular satellites are most likely captured objects.
• Their source region and capture mechanism have yet to be identified.
• The sizes, colors and total number of irregulars has likely been altered since they were captured.