Origins of Regularand

Irregular Satellites

ASTR5830March 19, 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.

Giant Planet Formation

• Core Accretion Model• Extended envelope that fills the planet’s Hill

sphere. (rH,J = 744 RJ)• Gap Opening– Mp= 100ME

– Continuing Accretion• Disk Formation– Accretion– Spin-out

Observational Constraints on Regular Satellite Formation

• Coplanar, Circular orbits– e ~ 0.01 and i < few degrees– formed in a disk, miniature solar system

• MS= 10-4 MP

– similar processes.• 50/50 Ice-Rock Fraction

– low temperatures• Decreasing Ice-Rock fraction with distance

– Disk gradients or subsequent evolution?• Incomplete differentiation of Callisto and Titan

– Long formation timescales: >105 yr

• Formed at the tail end of Giant planet formation.

Minimum Mass Sub-Nebula (MMSN)

• Lunine and Stevenson (1982)• Augment solid mass of satellites to solar

composition and spread out mass based on satellite locations.

• Results in a very massive disk with numerous problems.

ΣS ≈ 3×103 g cm−2

Problems with MMSN Approach

• Rapid Accretion of Satellites

• Orbital Decay– Gas drag on small particles: 103 yrs– Type I migration on larger bodies: 102 yrs– Type II migration on largest bodies: 103 yrs

τ A = 50 yr RS

2500 km⎛⎝⎜

⎞⎠⎟

ρS

2 g cm−3

⎛⎝⎜

⎞⎠⎟

10

Fg

⎛

⎝⎜

⎞

⎠⎟

3×103 g cm−2

ΣS

⎛

⎝⎜⎞

⎠⎟r

15 R J

⎛

⎝⎜⎞

⎠⎟

MS

M J

>10−5

Problems with MMSN Approach

• Temperature too hot unless disk is inviscid.

– Implies a disk lifetime of ~ 106 yrs

• Dynamical Constraints– Forced eccentricity of satellites– Obliquity of Jupiter

α ≤10−6

Gas-Starved Disk Model

• Canup and Ward (CW; 2002)• Solids build up slowly over time, analogous to

the accumulation of solids in a water pipe over time.

M J

&M J

=τ J =106 yr

ΣS = few ×102 g cm−2

Tanigawa et al. (2012)

CW semi-analytic disk models

α =5 ×10−3

τ J = 5 ×106 yr

Canup and Ward (2002)

Problems Solved by CW model

• Longer formation timescales

• Lower temperatures allow for condensation of ices.• Subsequent tidal evolution causes inner satellites to

thermally evolve and differentiate.• Solids are delivered by entrainment in accretion flow.

– Small enough to capture, small enough to deliver• Differential migration places satellites in Laplace

resonances.

τ A ≥ 105 yr for τ J ≥ 105 yr

Satellite Formation and Survival

• Multiple generations of satellites are formed and lost through migration into the host planet.

• Quasi-steady state is achieved with ~10-4 MP in satellites retained in the disk.

• Inflow cutoff from the solar nebula may explain Jupiter-Saturn dichotomy.

Common Mass Scaling for Satellite Systems of Gaseous Planets

Canup and Ward (2006)

The total mass in satellites, MT, scaled to the planet’s mass, MP, is shown versus time. The green, blue and red lines corresponding respectively to simulations with (a/f) = 10-6, 5x10-5 and 5x10-2.

Jupiter-Saturn Dichotomy

Sasaki et al. (2010)

Two-Phase Disk Model

• Mosqueira and Estrada (ME; 2003a,b)• Two-component disk based on the mass of

satellites, with a massive inner disk and a less massive outer disk.

• Requires very low viscosities.• Relies on planetessimal capture for delivery of

solids.• Satellites survive against migration by opening

gaps in the circumplanetary disk.

What is Needed?

• Better understanding of the viscous processes at work in circumplanetary disks.

• Higher resolution, non-isothermal, viscous simulations of infall from the solar nebula onto circumplanetary disks.