Origins of Regular and Irregular Satellites

1. Origins of RegularandIrregular Satellites ASTR5830 March 19, 2013 12:30-1:45 pm

2. Regular vs. Irregular Satellites Regular: 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. • 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.

3. 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

4. Observational Constraints on Regular Satellite Formation • Coplanar, Circular orbits • e ~ 0.01 andi < 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.

5. 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.

6. 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

7. 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

8. 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.

9. Tanigawa et al. (2012)

10. CW semi-analytic disk models Canup and Ward (2002)

11. 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.

12. Satellite Formation and Survival • Multiple generations of satellites are formed and lost through migration into the host planet. • Quasi-steady state is achieved with ~10-4MP in satellites retained in the disk. • Inflow cutoff from the solar nebula may explain Jupiter-Saturn dichotomy.

13. Common Mass Scaling for Satellite Systems of Gaseous Planets 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. Canup and Ward (2006)

14. Jupiter-Saturn Dichotomy Sasaki et al. (2010)

15. 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.

16. 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.