Transcript Origins of Regular and Irregular Satellites

Origins of Regular
and
Irregular Satellites
ASTR5830
March 21, 2013
12:30-1:45 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.
sin 
2
e 
2
P
m ax
for

 0 .4 csc i P
2
P
1
6
1  5 cos 2 i P 
39.2  i  140.8
o
o
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
http://nssdc.gsfc.nasa.gov/planetary/factsheet/
•
•
•
•
Pluto
M = 1.25x1022 kg
R = 1,195 km
rho = 1,795 kg/m3
Albedo = 0.5-0.7
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.