Origins of Regularand

Irregular Satellites

ASTR5830March 21, 201312:30-1:45 pm

Regular vs. Irregular Satellites

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 rH.

Observational Constraints on Irregular Satellite Origins

• Large orbital radii: a ~ 0.5rH

• 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)

Jewitt and Haghighipour (2007)

Kozai Resonances!

Jewitt and Haghighipour (2007)

Jewitt and Haghighipour (2007)

Dynamics and Collisions• 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 (106 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.

Kozai Resonance• Method of pumping up eccentricities, and

reducing the periapses, of orbits by reducing inclinations.

sin2 ω P =0.4csc2 iP

eP2( )

m ax=

16

1−5cos 2iP( )⎡⎣ ⎤⎦

for 39.2o ≤i ≤140.8o

Capture Mechanisms

• 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.

Three- and N-body Capture

• 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.

Source Regions

• 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.

Triton: a case study

• Largest satellite of Neptune.– d = 2706 km– rho = 2.061 g cm-3

– a = 354,800 km = 14.4 RN = 0.003 rH

– Retrograde orbit: i = 156.8˚• Probable Kuiper belt object• Tidal evolution probably changed Triton’s

orbit and evolved its surface.

Capture of Triton

• 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 vs. Pluto

Triton• M = 2.14x1022 kg • R = 1,353.4 km • rho = 2,050 kg/m3 • Albedo = 0.76

Pluto• M = 1.25x1022 kg • R = 1,195 km • rho = 1,795 kg/m3 • 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)

Overview

• 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.