Understanding the formation of Saturn's regular moons in the context of giant planet moons formation scenarios
Michel Blanc, Aurélien Crida, Yuhito Shibaike, Sebastien Charnoz, Maryame El Moutamid, Paul Estrada, Olivier Mousis, Julien Salmon, Antoine Schneeberger, Pierre Vernazza
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44 pages, 13 figures, accepted for publication in Space Science Reviews, as a part of the collection "New Vision of the Saturnian System in the Context of a Highly Dissipative Saturn" presenting the results from the ISSI Workhop "New Vision of the Saturnian System in the Context of a Highly Dissipative Saturn"
This article explores the different formation scenarios of the Kronian moons system in the context of a highly dissipative Saturn, with the objective of identifying the most likely of these scenarios. First, we review the diversity of objects - moons and rings - orbiting solar system giant planets, and the diversity of their architectures, which formation scenarios must reproduce. We then identify in this broader context the specific features of the Saturn system, such as the particularly large spectrum of its moon masses, the uniqueness of Titan and the presence of both dense and tenuous rings, before discussing the applicability of the different giant planet moon formation scenarios to the Saturn case. We discuss each of the most relevant scenarios and their respective merits. Finally, we tentatively propose a "favorite" scenario and we identify the key observations to be made by future space missions and/or Earth-based telescopic observations to validate this scenario or possibly alternative ones.
We propose a mechanism for the observed non-keplerian motion (Tiscareno et al. 2010, ApJL) of "propeller" moons embedded in Saturn's rings. Our mechanism, in which radial variations in surface density -- external to, and unaffected by, the embedded moon -- result in an equilibrium semimajor axis for the moon due to "Type I" angular momentum exchange (Crida et al. 2010, AJ), provides a good fit to the observations. Future observations should distinguish between our model and others recently proposed.
Context. Saturn has an excess of irregular moons. This is thought to be the result of past collisional events. Debris produced during such episodes in the neighborhood of a host planet can evolve into co-orbitals trapped in quasi-satellite and/or horseshoe resonant states. A recently announced centaur, 2013 VZ70, follows an orbit that could be compatible with those of prograde Saturn's co-orbitals. Aims. We perform an exploration of the short-term dynamical evolution of 2013 VZ70 to confirm or reject a co-orbital relationship with Saturn. A possible connection with Saturn's irregular moon popu…
The Cassini spacecraft revealed the spectacular, highly irregular shapes of the small inner moons of Saturn, ranging from the unique "ravioli-like" forms of Pan and Atlas to the highly elongated structure of Prometheus. Closest to Saturn, these bodies provide important clues regarding the formation process of small moons in close orbits around their host planet, but their range of irregular shapes has not been explained yet. Here we show that the spectrum of shapes among Saturn's small moons is a natural outcome of merging collisions among similar-sized moonlets possessing physical properties …
During the thirteen years in orbit around Saturn before its final plunge, the Cassini spacecraft provided more than ten thousand astrometric measurements. Such large amounts of accurate data enable the search for extremely faint signals in the orbital motion of the saturnian moons. Among these, the detection of the dynamical feedback of the rotation of the inner moons of Saturn on their respective orbits becomes possible. Using all the currently available astrometric data associated with Atlas, Prometheus, Pandora, Janus and Epimetheus, we first provide a detailed analysis of the Cassini Imagi…
The origin of Saturn's inner mid-sized moons (Mimas, Enceladus, Tethys, Dione and Rhea) and Saturn's rings is debated. Charnoz et al. (2010) introduced the idea that the smallest inner moons could form from the spreading of the rings' edge while Salmon et al. (2010) showed that the rings could have been initially massive, and so was the ring's progenitor itself. One may wonder if the mid-sized moons may have formed also from the debris of a massive ring progenitor, as also suggested in Canup (2010). However, the process driving mid-sized moons accretion from the icy debris disks has not been i…
The inner moons of Jupiter and Saturn migrate outwards due to tidal energy dissipation within the planets, the details of which remain poorly understood. We demonstrate that resonance locking between moons and internal oscillation modes of the planet can produce rapid tidal migration. Resonance locking arises due to the internal structural evolution of the planet and typically produces an outward migration rate comparable to the age of the solar system. Resonance locking predicts a similar migration timescale but a different effective tidal quality factor $Q$ governing the migration of each mo…
The disruption of an icy moon in a collision with an interloping comet a few hundred million years ago is a simple way to create Saturn's rings. A ring parent moon with a mass comparable to Mimas could be trapped in mean motion resonance with Enceladus and Dione in an orbit near the current outer edge of the rings just beyond the Roche zone. I present collisional N-body simulations of cometary impacts that lead to the partial disruption of a differentiated moon with a rocky core and icy mantle. The core can survive largely intact while the debris from the mantle settles into a ring of predomin…
We explore the past evolution of Saturn's moons using direct numerical integrations. We find that the past Tethys-Dione 3:2 orbital resonance predicted in standard models likely did not occur, implying that the system is less evolved than previously thought. On the other hand, the orbital inclinations of Tethys, Dione and Rhea suggest that the system did cross the Dione-Rhea 5:3 resonance, which is closely followed by a Tethys-Dione secular resonance. A clear implication is that either the moons are significantly younger than the planet, or that their tidal evolution must be extremely slow (Q …
We report the discovery of several large "propeller" moons in the outer part of Saturn's A ring, objects large enough to be followed over the 5-year duration of the Cassini mission. These are the first objects ever discovered that can be tracked as individual moons, but do not orbit in empty space. We infer sizes up to 1--2 km for the unseen moonlets at the center of the propeller-shaped structures, though many structural and photometric properties of propeller structures remain unclear. Finally, we demonstrate that some propellers undergo sustained non-keplerian orbit motion. (Note: This arXi…
Saturn's rings are rock-poor, containing 90%-95% ice by mass. As a group, Saturn's moons interior to and including Tethys are also about 90% ice. Tethys itself contains <6% rock by mass, in contrast to its similar-mass outer neighbor Dione, which contains >40% rock. Here we simulate the evolution of a massive primordial ice-rich ring and the production of satellites as ring material spreads beyond the Roche limit. We describe the Roche-interior ring with an analytic model, and use an N-body code to describe material beyond the Roche limit. We track the accretion and interactions of spawned sat…
The motions of small moons through Saturn's rings provide excellent tests of radial migration models. In theory, torque exchange between these moons and ring particles leads to radial drift. We predict that moons with Hill radii r_H ~ 2-24 km should migrate through the A ring in 1000 yr. In this size range, moons orbiting in an empty gap or in a full ring eventually migrate at the same rate. Smaller moons or moonlets -- such as the propellers (e.g., Tiscareno et al. 2006) -- are trapped by diffusion of disk material into corotating orbits, creating inertial drag. Larger moons -- such as Pan or…
Lainey et al. (2012), by re-analyzing long-baseline astrometry of Saturn's moons, have found that the moons' tidal evolution is much faster than previously thought, implying an order of magnitude stronger tidal dissipation within Saturn. This result is controversial and implies recent formation of at least some of the mid-sized icy moons of Saturn. Here we show that this more intensive tidal dissipation is in full agreement with the evolved state of the Titan-Hyperion resonance. This resonance was previously thought to be non-tidal in origin, as the amount of tidal evolution required for its a…
We examine the surface brightnesses of Saturn's smaller satellites using a photometric model that explicitly accounts for their elongated shapes and thus facilitates comparisons among different moons. Analyses of Cassini imaging data with this model reveals that the moons Aegaeon, Methone and Pallene are darker than one would expect given trends previously observed among the nearby mid-sized satellites. On the other hand, the trojan moons Calypso and Helene have substantially brighter surfaces than their co-orbital companions Tethys and Dione. These observations are inconsistent with the moons…
The formation and orbital evolution of Saturn's inner mid-sized moons are still debated. The most puzzling aspects are 1) how the Tethys-Dione pair and the Mimas-Enceladus pair passed through their strong 3:2 mean-motion resonances during the tidal orbital evolution, and 2) the current strong heat flow from Enceladus, which is a few orders of magnitude higher than the tidal energy dissipation caused by the present orbital eccentricity of Enceladus. Here we perform N-body simulations of the moons' orbital evolution from various initial conditions -- assuming that the moons were formed from Satu…
Saturn's mid-sized moons (satellites) have a puzzling orbital configuration with trapping in mean-motion resonances with every other pairs (Mimas-Tethys 4:2 and Enceladus-Dione 2:1). To reproduce their current orbital configuration on the basis of Crida & Charnoz's model of satellite formation from a hypothetical ancient massive rings, adjacent pairs must pass 1st-order mean-motion resonances without being trapped. The trapping could be avoided by fast orbital migration and/or excitation of the satellite's eccentricity caused by gravitational interactions between the satellites and the rings (…
Measurements of the polarized reflected sunlight from atmosphereless solar system bodies, over a range of phase angles, provide information about the surface structure and composition. With this work, we provide analysis of the polarimetric observations of the bright side of Iapetus at five different phase angles, and over the full useful wavelength range (400-800nm), so as to assess the light scattering behaviour of a typical surface water ice. Using FORS2 of the ESO VLT, we have performed linear spectro-polarimetric observations of Iapetus' bright side from 2009 to 2011 at five different pha…
The details of the Solar system's formation are still heavily debated. Questions remain about the formation locations of the giant planets, and the degree to which volatile material was mixed throughout the proto-planetary system. One diagnostic which offers great promise in helping to unravel the history of planet formation is the study of the level of deuteration in various Solar system bodies. For example, the D/H ratio of methane in the atmosphere of Titan can be used as a diagnostic of the initial conditions of the solar nebula within the region of giant planet formation, and can help us …
Saturn has a dynamically rich satellite system, which includes at least three orbital resonances between three pairs of moons: Mimas-Tethys 4:2, Enceladus-Dione 2:1, and Titan-Hyperion 4:3 mean-motion resonances. Studies of the orbital history of Saturn's moons usually assume that their past dynamics was also dominated solely by two-body resonances. Using direct numerical integrations, we find that three-body resonances among Saturnian satellites were quite common in the past, and could result in a relatively long-term, but finite capture time (10 Myr or longer). We find that these three-body …
The four massive Galilean satellites are believed to have formed within a circumplanetary disk during the last stages of Jupiter's formation. While the existence of a circum-jovian disk is supported by hydrodynamic simulations, no consensus exists regarding the origin and delivery mechanisms of the building blocks of the forming satellites. The opening of a gap in the circumsolar disk would have efficiently isolated Jupiter from the main sources of solid material. However, a reservoir of planetesimals should have existed at the outer edge of Jupiter's gap, where solids were trapped and accumul…
Spacecraft observations of Saturn's rings show evidence of an active aggregation-disaggregation process triggered by periodic influences from the nearby moons. This leads to clumping and break-up of the ring particles at time-scales of the order of a few hours. A mathematical model has been developed to explain these dynamics in the Saturn's F-ring and B-ring [3], the implications of which are in close agreement with the empirical results. In this paper, we conduct a rigorous analysis of the proposed forced dynamical system for a class of continuous, periodic and zero-mean forcing functions th…
The stability of satellites in the solar system is affected by the so-called evection resonance. The moons of Saturn, in particular, exhibit a complex dynamical architecture in which co-orbital configurations occur, especially close to the planet where this resonance is present. We address the dynamics of the evection resonance, with particular focus on the Saturn system, and compare the known behavior of the resonance for a single moon to that of a pair of moons in co-orbital trojan configuration. We developed an analytic expansion of the averaged Hamiltonian of a trojan pair of bodies, inclu…
The Cassini spacecraft discovered a propeller-shaped structure in Saturn's A ring. This propeller structure is thought to be formed by gravitational scattering of ring particles by an unseen embedded moonlet. Self-gravity wakes are prevalent in dense rings due to gravitational instability. Strong gravitational wakes affect the propeller structure. Here, we derive the condition for formation of a propeller structure by a moonlet embedded in a dense ring with gravitational wakes. We find that a propeller structure is formed when the wavelength of the gravitational wakes is smaller than the Hill …
The origin of Titan's atmospheric methane is a key issue for understanding the origin of the Saturnian satellite system. It has been proposed that serpentinization reactions in Titan's interior could lead to the formation of the observed methane. Meanwhile, alternative scenarios suggest that methane was incorporated in Titan's planetesimals before its formation. Here, we point out that serpentinization reactions in Titan's interior are not able to reproduce the deuterium over hydrogen (D/H) ratio observed at present in methane in its atmosphere, and would require a maximum D/H ratio in Titan's…
We explore scenarios for the accretion of ornamental ridges on Saturn's moons Pan, Atlas, and Daphnis from material in Saturn's rings. Accretion of complex shaped ridges from ring material should be possible when the torque from accreted material does not exceed the tidal torque from Saturn that ordinarily maintains tidal lock. This gives a limit on the maximum accretion rate and the minimum duration for equatorial ridge growth. We explore the longitude distribution of ridges accreted from ring material, initially in circular orbits, onto a moon that is on a circular, inclined or eccentric orb…
Using astrometric observations spanning more than a century and including a large set of Cassini data, we determine Saturn's tidal parameters through their current effects on the orbits of the eight main and four coorbital moons. We have used the latter to make the first determination of Saturn's Love number, $k_2=0.390 \pm 0.024$, a value larger than the commonly used theoretical value of 0.341 (Gavrilov & Zharkov, 1977), but compatible with more recent models (Helled & Guillot, 2013) for which $k_2$ ranges from 0.355 to 0.382. Depending on the assumed spin for Saturn's interior, the new cons…
Tidal effects in planetary systems are the main driver in the orbital migration of natural satellites. They result from physical processes occurring deep inside celestial bodies, whose effects are rarely observable from surface imaging. For giant planet systems, the tidal migration rate is determined by poorly understood dissipative processes in the planet, and standard theories suggest an orbital expansion rate inversely proportional to the power 11/2 in distance, implying little migration for outer moons such as Saturn's largest moon, Titan. Here, we use two independent measurements obtained…
It is widely accepted that Titan and the mid-sized regular satellites around Saturn were formed in the circum-Saturn disk. Thus, if these mid-sized satellites were simply accreted by collisions of similar ice-rock satellitesimals in the disk, the observed wide diversity in density (i.e., the rock fraction) of the Saturnian mid-sized satellites is enigmatic. A recent circumplanetary disk model suggests satellite growth in an actively supplied circumplanetary disk, in which Titan-sized satellites migrate inward by interaction with the gas and are eventually lost to the gas planet. Here we report…
The hypothesis of a recent origin of Saturn's rings and its mid-sized moons is actively debated. It was suggested that a proto-Rhea and a proto-Dione might have collided recently, giving birth to the modern system of mid-sized moons. It is also suggested that the rapid viscous spreading of the debris may have implanted mass inside Saturn's Roche limit, giving birth to the modern Saturn's ring system. However, this scenario has been only investigated in very simplified way for the moment. This paper investigates it in detail to assess its plausibility by using $N$-body simulations and analytica…
Interior to the gaseous envelopes of Saturn, Uranus, and Neptune, there are high-density cores with masses larger than 10 Earth masses. According to the conventional sequential accretion hypothesis, such massive cores are needed for the onset of efficient accretion of their gaseous envelopes. However, Jupiter's gaseous envelope is more massive and core may be less massive than those of Saturn. In order to account for this structural diversity and the super-solar metallicity in the envelope of Jupiter and Saturn, we investigate the possibility that they may have either merged with other gas gia…
We demonstrate that habitable Earth-mass planets and moons can exist in the Kepler-16 system, known to host a Saturn-mass planet around a stellar binary, by investigating their orbital stability in the standard and extended habitable zone (HZ). We find that Earth-mass planets in satellite-like (S-type)orbits are possible within the standard HZ in direct vicinity of Kepler-16b, thus constituting habitable exomoons. However, Earth-mass planets cannot exist in planetary-like (P-type) orbits around the two stellar components within the standard HZ. Yet, P-type Earth-mass planets can exist superior…
Models of the dynamical evolution of the early solar system following the dispersal of the gaseous protoplanetary disk have been widely successful in reconstructing the current orbital configuration of the giant planets. Statistically, some of the most successful dynamical evolution simulations have initially included a hypothetical fifth giant planet, of ice giant mass, which gets ejected by a gas giant during the early solar system's proposed instability phase. We investigate the likelihood of an ice giant ejection event by either Jupiter or Saturn through constraints imposed by the current …
The migration and encounter histories of the giant planets in our Solar System can be constrained by the obliquities of Jupiter and Saturn. We have performed secular simulations with imposed migration and N-body simulations with planetesimals to study the expected obliquity distribution of migrating planets with initial conditions resembling those of the smooth migration model, the resonant Nice model and two models with five giant planets initially in resonance (one compact and one loose configuration). For smooth migration, the secular spin-orbit resonance mechanism can tilt Saturn's spin ax…
Saturn's E ring consists of micron-sized particles launched from Enceladus by that moon's geological activity. A variety of small-scale structures in the E-ring's brightness have been attributed to tendrils of material recently launched from Enceladus. However, one of these features occurs at a location where Enceladus' gravitational perturbations should concentrate background E-ring particles into structures known as satellite wakes. While satellite wakes have been observed previously in ring material drifting past other moons, these E-ring structures would be the first examples of wakes invo…
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 c…
The Earth-Moon system is suggested to have formed through a single giant collision, in which the Moon accreted from the impact-generated debris disk. However, such giant impacts are rare, and during its evolution the Earth experienced many more smaller impacts, producing smaller satellites that potentially coevolved. In the multiple-impact hypothesis of lunar formation, the current Moon was produced from the mergers of several smaller satellites (moonlets), each formed from debris disks produced by successive large impacts. In the Myrs between impacts, a pre-existing moonlet tidally evolves ou…
Irregular moons are a class of satellite found orbiting all of the Solar System's giant planets: as their orbits don't match those of their planets, they are theorised to have formed elsewhere in the Solar System and were subsequently captured into their observed orbits. Missions such as Cassini have contributed significant empirical data on irregular moons in the present day but this paper aims to develop our currently limited theoretical understanding of their origins and capture as it presents one of the first projects to connect moon capture with another feature common to all giant planets…
We show that the fate of moons of a close-in giant planet is mainly determined by the migration history of the planet in the protoplanetary disk. As the planet migrates in the disk from beyond the snow line towards a multi-day period orbit, the formed and forming moons become unstable as the planet's sphere of influence shrinks. Disk-driven migration is faster than the moons' tidal orbital evolution. Moons are eventually ejected from around close-in exoplanets or forced into collision with them before tides from the star affect their orbits. If moons are detected around close-in exoplanets, th…
The Moon is believed to have formed in the aftermath of a giant impact between a planetary mass body and the proto-Earth. In a typical giant impact scenario, a disk of vapor, liquid, and solid debris forms around the proto-Earth and--after possibly decades of evolution--condenses to form the Moon. Using state-of-the-art numerical simulations, we investigate the dynamical effects of magnetic fields on the Moon-forming giant impact. We show that turbulence generated by the collision itself, shear in the boundary layer between the post-impact debris field and the proto-Earth, and turbulence in th…
We carry out a series of high resolution ($1024\times 1024$) hydrodynamical simulations to investigate the orbital evolution of Jupiter and Saturn embedded in a gaseous protostellar disk. Our work extends the results in the classical papers of Masset & Snellgrove (2001) and Morbidelli & Crida (2007) by exploring various surface density profiles ($σ$), where $σ\propto r^{-α}$. The stability of the mean motion resonances(MMRs) caused by the convergent migration of the two planets is studied as well. Our results show that:(1) The gap formation process of Saturn is greatly delayed by the tidal per…
The giant impact hypothesis is the dominant theory of how the Earth-Moon system was formed. Models have been created that can produce a disk of debris with the proper mass and composition to create our Moon. Models have also been created which start with a disk of debris that eventually coalesces into a Moon. To date, no model has been created that produces a stable Earth-Moon system in a single simulation. Here we combine two recently published ideas in this field, along with a new gravity-centered model, and generate such a simulation. In addition, we show how the method can produce a hetero…
Revealing the true nature of the gas giant planets in our Solar System is challenging. The masses of Jupiter and Saturn are about 318 and 95 Earth masses, respectively. While they mostly consist of hydrogen and helium, the total mass and distribution of the heavier elements, which reveal information on their origin, are still unknown. Recent accurate measurements of the gravitational fields of Jupiter and Saturn together with knowledge of the behavior of planetary materials at high pressures allow us to better constrain their interiors. Updated structure models of Jupiter and Saturn suggest th…
The determination of Saturn's atmospheric noble gas abundances are critical to understanding the formation and evolution of Saturn, and giant planets in general. These measurements can only be performed with an entry probe. A Saturn probe will address whether enhancement in heavy noble gases, as was found in Jupiter, are a general feature of giant planets, and their ratios will be a powerful constraint on how they form. The helium abundance will show the extent to which helium has phase separated from hydrogen in the planet's deep interior. Jupiter's striking neon depletion may also be tied to…
We examine the effects that planetary encounters have on the moon systems of ejected gas giant planets. We conduct a suite of numerical simulations of planetary systems containing three Jupiter-mass planets (with the innermost planet at 3 AU) up to the point where a planet is ejected from the system. The ejected planet has an initial system of 100 test-particle moons. We determine the survival probability of moons at different distances from their host planet, measure the final distribution of orbital elements, examine the stability of resonant configurations, and characterize the properties o…
Three-dimensional numerical simulations show that large-scale latent heating resulting from condensation of water vapor can produce multiple zonal jets similar to those on the gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune). For plausible water abundances (3-5 times solar on Jupiter/Saturn and 30 times solar on Uranus/Neptune), our simulations produce ~20 zonal jets for Jupiter and Saturn and 3 zonal jets on Uranus and Neptune, similar to the number of jets observed on these planets. Moreover, these Jupiter/Saturn cases produce equatorial superrotation whereas the Uranus/Ne…
We carry out a series of high-resolution (1024 X 1024) hydrodynamic simulations to investigate the orbital evolution of a Saturn-Jupiter pair embedded in a gaseous disk. This work extends the results of our previous work by exploring a different orbital configuration---Jupiter lies outside Saturn (q<1, where q= M_i/M_o is the mass ratio of the inner planet and the outer one). We focus on the effects of different initial separations (d) between the two planets and the various surface density profiles of the disk, where σ\propto r^{-α}. We also compare the results of different orbital configurat…
Reproducing Uranus and Neptune remains a challenge for simulations of solar system formation. The ice giants' peculiar obliquities suggest that they both suffered giant collisions during their formation. Thus, there must have been an epoch of accretion dominated by collisions among large planetary embryos in the primordial outer solar system. We test this idea using N-body numerical simulations including the effects of a gaseous protoplanetary disk. One strong constraint is that the masses of the ice giants are very similar -- the Neptune/Uranus mass ratio is $\sim1.18$. We show that similar-s…
In the Grand Tack (GT) scenario for the young solar system, Jupiter formed beyond 3.5 AU from the Sun and migrated as close as 1.5 AU until it encountered an orbital resonance with Saturn. Both planets then supposedly migrated outward for several $10^5$ yr, with Jupiter ending up at ~5 AU. The initial conditions of the GT and the timing between Jupiter's migration and the formation of the Galilean satellites remain unexplored. We study the formation of Ganymede and Callisto, both of which consist of ~50% H$_2$O and rock, in the GT scenario. We examine why they lack dense atmospheres, while Tit…
The Earth's Moon is thought to have formed from a circumterrestrial disk generated by a giant impact between the proto-Earth and an impactor approximately 4.5 billion years ago. Since the impact was energetic, the disk would have been hot and partially vaporized. This formation process is thought to be responsible for the geochemical observation that the Moon is depleted in volatiles. This model predicts that the Moon should be significantly depleted in water as well, but this appears to contradict some of the recently measured lunar water abundances and D/H ratios that suggest that the Moon i…
