衛星系形成過程のレビュー（と最近の話題）
谷川北海道大学享行
低温科学研究所

Formation of Giant Planets
Giant planet formation
Solid planets (~10 Earth masses)
Instability of the atmosphere
Onset of gas accretion
Gap formation
Dissipation of the disk
Satellites are thought to be formed at very end of theformation of gas-giant planets2

What is a Satellite System?
– Systems that consist of multiple objects
rotating around planets– Generally exist around gas giant planets– Regular satellites and irregular satellites• Regular satellites:– Nearly circular orbits, orbital plane ~= equatorial plane– Account for most of the total mass of satellites• → Formed from circum-planetary disks?

Satellites of outer planets

Jupiter and Galilean satellites

3What is Circum-Planetary Disk?
planetSun
Proto-planetary disk

Formation of a circumplanetary disk

• Circumplanetary disks are natural
by-products of giant planet
formation• Gas is accreted downward fromabove, not through the midplane.
Tanigawa, Ohtsuki, and Machida 2012
Visualization by T. Takeda (CfCA, NAOJ)

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“Classical” Minimum Mass SubNebula Model• 現在の衛星総質量 ~ 10-4 Mp のダストとその約100倍のガスからなる円盤を初期条件– 当時(?)の太陽系形成理論に準じている– 最初から衛星総質量をもつ重い円盤• 問題点– 高過ぎる面密度– → 円盤温度高温 → H2O が気相になり、材料物質として使えない– → 速すぎる集積時間 → 大きな集積熱 → カリストの未分化説明不可– → 速すぎる Type I 移動 → 衛星が出来ても惑星へ落下
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Courtesy of A. Crida

Two plausible models• Gas-starved disk model– Canup and Ward 2002, 2006

Canup and Ward 2002

• Spreading tidal-disk model– Crida and Charnoz 20126

Gas-starved disk model
Canup and Ward 2002, 2006
Viscous spreading

PlanetGas

Steady mass supply

7Gas-starved disk model
Canup and Ward 2002, 2006

PlanetGasSteady mass supply

Growth from outside8Gas-starved disk model
Canup and Ward 2002, 2006

PlanetGas

Steady mass supply

Growth from outside
Larger satellites move inward
Inner objects are swept9Gas-starved disk model
Canup and Ward 2002, 2006

PlanetGas

Steady mass supply

Growth from outside
Larger planets move inward
Inner objects are swept

Continue until the mass supply terminates.
Current satellites are the last generation of this cycle10Gas-starved disk model
Canup and Ward 2002, 2006
Total mass of satellites / planet mass
Canup and Ward 2006

PlanetGasSize (mass) is automatically determinedby the balance between growth and fall11

Summary ofGas-starved disk model• Looks good!
– Total mass of the regular satellites• Weak dependence on model parameters– Partial differentiation of Callisto– Rock fraction of Galilean satellites• Open questions– Size distribution of satellites?
• Difference between Jovian system (4 large moons) andSaturnian system (one large moon and several mid-sizedmoons)?
• Whether inner edge exists or not? (Sasaki et al. 2010)– How about Neptunian system?
• Triton (large retrograde satellite)?

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Spreading tidal disk model
Crida and Charnoz 2012

Size increases with orbital radius13

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Ring viscous evolution model
Salmon et al. 2010
• Translational viscosity– Random motion– “local” component• Collisional viscosity– Collision betweenparticles– “non-local” component• Gravitational viscosity– Gravity wakes
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Forming process in detailA • The tidal disk continuously supplies particles••

beyond Roche radius through viscosity
Particles outside Roche radius aggregate to
be a moonlet
Tidal torque moves the moonlet outward

B • The second moonlet starts to form when the•first moon migrates far enough.
The second moon catch up with the first moonand merge.

C • Outer moons are easier to be caught up by
inner moons and be merged.
• Farther moons moves slower• Larger moons moves fasterPyramidal size distribution!
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Size distribution

Distance from Roche limit

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Courtesy of A. Crida

Summary ofSpreading tidal-disk model• Look better! (maybe)– Pyramidal size distribution (not absolute size)• Size increases with orbital radius– Earth’s moon and Charon can also be explain• Open questions– Origin of the tidal disk
• Disruption of satellites• Tidally disrupted comets– Jovian system (Galilean satellites)– Mutual gravitational interaction between satellites• Increase of orbital eccentricity?

– Density dispersion of Saturnian system• Giant impacts? (Sekine and Genda 2012, Asphaug & Reufer 2013)• LHB? (Nimmo & Korycansky 2012)– Resonance of Saturnian system• Mimas-Tethys, Enceladus-Dione18

Comparison
Gas-starved disk model
Canup and WardSpreading tidal-disk model
Crida and Charnoz

Jupiter△×

Saturn○○Uranus○○

Neptune△○19Comparison
Gas-starved disk
disk modelmodeGas-starved
Canup and Ward

Spreading tidal-disk model
Crida and Charnoz

Jupiter△×

Saturn○○Uranus○○

Neptune△○Earth-○

Pluto-○

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Summary• Two formation models for the satellite systems arereviewed.
– Gas-starved disk model (Canup and Ward)• Total mass of regular satellite systems (not the size distribution)– Weak parameter dependence• Rock fraction and partial differentiation• Size distribution (Galilean satellites)• Neptunian system– Spreading tidal-disk model (Crida and Charnoz)• Pyramidal size distribution (not absolute mass)• Earth and Pluto system• Origin of tidal disk
• Jovian system
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