Origins of Regular and Irregular Satellites ASTR5830 March 19, :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 planets 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.

Giant Planet Formation Core Accretion Model Extended envelope that fills the planets Hill sphere. ( r H,J = 744 R J ) Gap Opening – M p = 100M E – 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 M S = M P – 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: >10 5 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.

Problems with MMSN Approach Rapid Accretion of Satellites Orbital Decay – Gas drag on small particles: 10 3 yrs – Type I migration on larger bodies: 10 2 yrs – Type II migration on largest bodies: 10 3 yrs

Problems with MMSN Approach Temperature too hot unless disk is inviscid. – Implies a disk lifetime of ~ 10 6 yrs Dynamical Constraints – Forced eccentricity of satellites – Obliquity of Jupiter

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.

Tanigawa et al. (2012)

CW semi-analytic disk models 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.

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 M P 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, M T, scaled to the planets mass, M P, 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.