Formation of Jupiter and Conditions for Accretion of the Galilean Satellites

Abstract

We present an overview of the formation of Jupiter and its associated circumplanetary disk. Jupiter forms via a combination of planetesimal accretion and gravitational accumulation of gas from the surrounding solar nebula. The formation of the circumjovian gaseous disk, or subnebula, straddles the transitional stage between runaway gas accretion and Jupiter's eventual isolation from the circumsolar disk. This isolation, which effectively signals the termination of Jupiter's accretion, takes place as Jupiter opens a deep gas gap in the solar nebula, or the solar nebula gas dissipates. The gap-opening stage is relevant to subnebula formation because the radial extent of the circumjovian disk is determined by the specific angular momentum of gas that enters Jupiter's gravitational sphere of influence. Prior to opening a well-formed, deep gap in the circumsolar disk, Jupiter accretes low specific angular momentum gas from its vicinity, resulting in the formation of a rotationally-supported compact disk whose size is comparable to the radial extent of the Galilean satellites. This process may allocate similar amounts of angular momentum to the planet and the disk, leading to the formation of an ab-initio massive disk compared to the mass of the satellites. As Jupiter approaches its final mass and the gas gap deepens, a more extended, less massive disk forms because the gas inflow, which must come from increasingly farther away from the planet's semimajor axis, has high specific angular momentum. Thus, the size of the circumplanetary gas disk upon inflow is dependent on whether or not a gap is present. We describe the conditions for accretion of the Galilean satellites, including the timescales for their formation and mechanisms for their survival, all within the context of key constraints for satellite formation models. The environment in which the regular satellites form is tied to the timescale for circumplanetary disk dispersal, which depends on the nature and persistence of turbulence. In the case that subnebula turbulence decays as gas inflow wanes, we present a novel mechanism for satellite survival involving gap opening by the largest satellites. On the other hand, assuming that sustained turbulence drives subnebula evolution on a short timescale compared to the satellite formation timescale, we review a model that emphasizes collisional processes to explain satellite observations. We briefly discuss the mechanisms by which solids may be delivered to the circumplanetary disk. At the tail end of Jupiter's accretion, most of the mass in solids resides in planetesimals of size > 1 km; however, planetesimals in Jupiter's feeding zone undergo a period of intense collisional grinding, placing a significant amount of mass in fragments < 1 km. Inelastic or gravitational collisions within Jupiter's gravitational sphere of influence allow for the mass contained in these planetesimal fragments to be delivered to the circumplanetary disk either through direct collisional/gravitational capture, or via ablation through the c...

TITAN'S BULK COMPOSITION CONSTRAINED BYCASSINI-HUYGENS: IMPLICATION FOR INTERNAL OUTGASSING

“…Titan, like the other regular satellites of Saturn, is likely to have formed in a circumplanetary disk during the latest stages of Saturn's accretion (e.g., Lunine et al 2009 and references therein). Solid materials incorporated into the disk and leading to the formation of Titan would be supplied either by direct transport of small icy particles into the Saturnian disk with the gas inflow (Canup & Ward 2002;Sasaki et al 2010), or by gas drag, ablation, and/or collisional capture of metric to kilometric heliocentric planetesimals (Estrada & Mosqueira 2006;Estrada et al 2009;Mosqueira et al 2010aMosqueira et al , 2010b. Whatever the supply mechanism, the major part of the building blocks that formed Titan likely originated from the feeding zone of Saturn.…”

Effects of the Planetary Temperature on the Circumplanetary Disk and on the Gap

“…However, as the planet radiates away its formation heat and the circumstellar disk gas content is dissipating, presumably the gas-giant's vicinity also cools off, eventually reaching low enough temperature for satellite formation to occur. The timescale of this is still unknown (Canup & Ward 2002Estrada et al 2009;Mosqueira & Estrada 2003a,b) and certainly depends on number of factors, such as semi-major axis, opacity of the gas and dust, the temperature and cooling rate of the planet. Motivated by this and the observational efforts of the circumplanetary disks, in this paper we study how the planetary temperature affects the circumplanetary gas, namely its temperature, mass, and kinematic properties.…”

Constraints from Comets on the Formation and Volatile Acquisition of the Planets and Satellites

“…However, the discovery that Amalthea, a small regular moon orbiting closer to Jupiter than Io, had a higher ice-to-rock ratio than the Galilean satellites (Anderson et al 2005) challenged this assumption. This discovery brought about the possibility that all of the Jovian moons could have accreted significant amounts of ice, and that subsequent heating-either tidal heating or bombardment-could have driven off much of the water ice from Io and Europa (Estrada et al 2009). However, this does not rule out the possibility that Amalthea formed later than the giant satellites or in a colder region of the subnebula and migrated to its current location (Anderson et al 2005).…”

“…This parameter is either an indication of their formation distance in Jupiter's subnebula (e.g. Canup and Ward 2002;Mousis and Gautier 2004) or they initially had high ice-to-rock ratios, and have lost part or all of their ice inventory as a result of tidal heating (Estrada et al 2009). Due to extensive thermal processing and radiolitic chemistry on the surface, it is difficult to provide any further constraints on the origin of these moons based on what is currently known about their composition.…”

“…Both Europa and Ganymede are found to be tectonically active due to tidal heating and have experienced extensive resurfacing. Callisto is subjected to the least amount of tidal heating and has a surface that is covered in impact craters suggesting little to no geological activity over its history (Estrada et al 2009). The primordial composition of Io is difficult to constrain due to the extensive heating to which it has been subjected, but Europa, Ganymede and Callisto have all been able to retain substantial amounts of water ice throughout their history.…”