Deciphering the origin of the regular satellites of gaseous giants – Iapetus: The Rosetta ice-moon

Abstract

Ever since their discovery the regular satellites of Jupiter and Saturn have held out the promise of providing an independent set of observations with which to test theories of planet formation. Yet elucidating their origin has proven elusive. Here we show that Iapetus can serve to discriminate between satellite formation models. Its accretion history can be understood in terms of a two-component gaseous subnebula, with a relatively dense inner region, and an extended tail out to the location of the irregular satellites, as in the SEMM model of Mosqueira and Estrada (2003a,b). Following giant planet formation, planetesimals in the feeding zone of Jupiter and Saturn become dynamically excited, and undergo a collisional cascade. Ablation and capture of planetesimal fragments crossing the gaseous circumplanetary disks delivers enough collisional rubble to account for the mass budgets of the regular satellites of Jupiter and Saturn. This process can result in rock/ice fractionation as long as the make up of the population of disk crossers is non-homogeneous, thus offering a natural explanation for the marked compositional differences between outer solar nebula objects and those that accreted in the subnebulae of the giant planets. For a given size, icy objects are easier to capture and to ablate, likely resulting in an overall enrichment of ice in the subnebula. Furthermore, capture and ablation of rocky fragments become inefficient far from the planet for two reasons: the gas surface density of the subnebula is taken to drop outside the centrifugal radius, and the velocity of interlopers decreases with distance from the planet. Thus, rocky objects crossing the outer disks of Jupiter and Saturn never reach a temperature high enough to ablate either due to melting or vaporization, and capture is also greatly diminished there. In contrast, icy objects crossing the outer disks of each planet ablate due to the melting and vaporization of water-ice. Consequently, our model leads to an enhancement of the ice content of Iapetus, and to a lesser degree those of Titan, Callisto and Ganymede, and accounts for the (non-stochastic) compositions of these large, low-porosity outer regular satellites of Jupiter and Saturn. For this to work, the primordial population of planetesimals in the Jupiter-Saturn region must be partially differentiated, so that the ensuing collisional cascade produces an icy population of 1 m size fragments to be ablated during subnebula crossing. We argue this is likely because the first generation of solar nebula ∼ 10 km planetesimals in the Jupiter-Saturn region incorporated significant quantities of 26 Al. This is the first study successfully to provide a direct connection between nebula planetesimals and subnebulae mixtures with quantifiable and observable consequences for the bulk properties of the regular satellites of Jupiter and Saturn, and the only explanation presently available for Iapetus' low density and ice-rich composition.

Saturn’s Formation and Early Evolution at the Origin of Jupiter’s Massive Moons

“…Another potential mechanism to deliver solid material to the CPD is the capture/ablation of larger planetesimals located in the vicinity of Jupiter due to either collisions in a gas poor environment (Estrada & Mosqueira 2006) or gas drag within a gas rich CPD (Mosqueira et al 2010). The latter process has been numerically investigated by several authors (Fujita et al 2013;Suetsugu et al 2016;Suetsugu & Ohtsuki 2017;D'Angelo & Podolak 2015).…”

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.…”

Despinning and shape evolution of Saturn’s moon Iapetus triggered by a giant impact

“…Mosqueira et al, 2010;Levison et al, 2011;Sekine and Genda, 2012;Zhang and Nimmo, 2012;Asphaug and Reufer, 2013). In a way analogous to the standard model of terrestrial planet formation (e.g.…”

“…The irregularly shaped satellite Hyperion may be considered as a remnant of this accretion process (e.g. Mosqueira et al, 2010). In the case of Iapetus, a giant impact must have been strong enough to significantly reduce its spin, but not too violent to disrupt the satellite.…”