The origin of Saturn's rings and moons Ida, Shigeru
Science (American Association for the Advancement of Science),
06/2019, Letnik:
364, Številka:
6445
Journal Article
Recenzirano
Cassini data constrain the age and history of the giant planet's rings
Saturn's rings consist of vast numbers of small icy particles that frequently collide with each other. Within Saturn's Roche ...limit (that is, closer to Saturn's center than twice Saturn's physical radius
R
S
), the icy particles disperse and form rings, whereas outside the Roche limit, they grow through pairwise collisions to form moons. The formation of Saturn's rings and moons should thus be closely related. On pages 1054, 1053, and 1052 of this issue, respectively, Tiscareno
et al.
(
1
), Buratti
et al.
(
2
), and Iess
et al.
(
3
) report results from the Cassini mission's ring-grazing orbit and Grand Finale observations, which reveal the detailed structure of the rings and associated moons. The results strongly suggest that Saturn's rings are much younger than Saturn itself and provide important clues to the origin of the rings and moons.
Planetary migration is a major challenge for planet-formation theories. The speed of type-I migration is proportional to the mass of a protoplanet, while the final decade of growth of a ...pebble-accreting planetary core takes place at a rate that scales with the mass to the two-thirds power. This results in planetary growth tracks (i.e., the evolution of the mass of a protoplanet versus its distance from the star) that become increasingly horizontal (migration dominated) with the rising mass of the protoplanet. It has been shown recently that the migration torque on a protoplanet is reduced proportional to the relative height of the gas gap carved by the growing planet. Here we show from 1D simulations of planet–disc interaction that the mass at which a planet carves a 50% gap is approximately 2.3 times the pebble isolation mass. Our measurements of the pebble isolation mass from 1D simulations match published 3D results relatively well, except at very low viscosities (α < 10−3) where the 3D pebble isolation mass is significantly higher, possibly due to gap edge instabilities that are not captured in 1D. The pebble isolation mass demarks the transition from pebble accretion to gas accretion. Gas accretion to form gas-giant planets therefore takes place over a few astronomical units of migration after reaching first the pebble isolation mass and, shortly after, the 50% gap mass. Our results demonstrate how planetary growth can outperform migration both during core accretion and during gas accretion, even when the Stokes number of the pebbles is small, St ~ 0.01, and the pebble-to-gas flux ratio in the protoplanetary disc is in the nominal range of 0.01–0.02. We find that planetary growth is very rapid in the first million years of the protoplanetary disc and that the probability for forming gas-giant planets increases with the initial size of the protoplanetary disc and with decreasing turbulent diffusion.
Context. Streaming instability is a possible mechanism to form icy planetesimals. It requires special local conditions such as a high solid-to-gas ratio at the midplane and typically more than a ...centimeter in size (Stokes number >0.01). Silicate grains cannot grow to such a size through pairwise collisions. It is important to clarify where and when rocky and icy planetesimals are formed in a viscously evolving disk. Aims. We wish to understand how local runaway pile-up of solids (silicate and water ice) occurs inside or outside the snow line. Methods. We assumed an icy pebble contains micron-sized silicate grains that are uniformly mixed with ice and are released during the ice sublimation. Using a local one-dimensional code, we solved the radial drift and the turbulent diffusion of solids and the water vapor, taking account of their sublimation and condensation around the snow line. We systematically investigated the effects of back-reactions of solids to gas on the radial drift and diffusion of solids, scale height evolution of the released silicate particles, and possible differences in effective viscous parameters between those for turbulent diffusion (αtur) and those for the gas accretion rate onto the central star (αacc). We also studied the dependence on the ratio of the solid mass flux to the gas (Fp/g). Results. We show that the favorable locations for the pile-up of silicate grains and icy pebbles are the regions in the proximity of, both inside and outside, the water snow line, respectively. We find that runaway pile-ups occur when both the back-reactions for radial drift and diffusion are included. In the case with only the back-reaction for the radial drift, runaway pile-up is not found except in extremely high pebble flux, while the condition of streaming instability can be satisfied for relatively large Fp/g as found in the past literature. If the back-reaction for radial diffusion is considered, the runaway pile-up occurs for a reasonable value of pebble flux. The runaway pile-up of silicate grains that would lead to formation of rocky planetesimals occurs for αtur ≪ αacc, while the runaway pile-up of icy pebbles is favored for αtur ~ αacc. Based on these results, we discuss timings and locations of rocky and icy planetesimals in an evolving disk.
Context. The ocean mass of the Earth is only 2.3 × 10−4 of the whole planet mass. Even including water in the interior, the water fraction would be at most 10−3−10−2. Ancient Mars may have had a ...similar or slightly smaller water fraction. What controlled the amount of water in these planets has not been clear, although several models have been proposed. It is important to clarify the control mechanism to discuss water delivery to rocky planets in habitable zones in exoplanetary systems, as well as that to Earth and Mars in our solar system. Aims. We consider water delivery to planets by icy pebbles after the snowline inwardly passes planetary orbits. We derive the water mass fraction (fwater) of the final planet as a function of disk parameters and discuss the parameters that reproduce a small value of fwater comparable to that inferred for the Earth and ancient Mars. Methods. We calculated the growth of icy dust grains to pebbles and the pebble radial drift with a 1D model, by simultaneously solving the snowline migration and dissipation of a gas disk. With the obtained pebble mass flux, we calculated accretion of icy pebbles onto planets after the snowline passage to evaluate fwater of the planets. Results. We find that fwater is regulated by the total mass (Mres) of icy dust materials preserved in the outer disk regions at the timing (t = tsnow) of the snowline passage of the planetary orbit. Because Mres decays rapidly after the pebble formation front reaches the disk outer edge (at t = tpff), fwater is sensitive to the ratio tsnow∕tpff, which is determined by the disk parameters. We find tsnow∕tpff < 10 or > 10 is important. By evaluating Mres analytically, we derive an analytical formula of fwater that reproduces the numerical results. Conclusions. Using the analytical formula, we find that fwater of a rocky planet near 1 au is similar to the Earth, i.e., ~10−4−10−2, in disks with an initial disk size of 30–50 au and an initial disk mass accretion rate of ~(10−8−10−7) M⊙ yr−1 for disk depletion timescale of approximately a few M yr. Because these disks may be median or slightly compact/massive disks, our results suggest that the water fraction of rocky planets in habitable zones may often be similar to that of the Earth if icy pebble accretion is responsible for water delivery.
Abstract
The observationally inferred crystalline abundance in silicates in comets, which should have been formed in the outer region of a protoplanetary disk, is relatively high (∼10%–60%), although ...crystalline silicates would be formed by the annealing of amorphous precursors in the inner disk region. In order to quantitatively address this puzzle, we performed a Monte Carlo simulation of the advection/diffusion of silicate particles in a turbulent disk in a setting based on the pebble accretion model: pebbles consisting of many small amorphous silicates embedded in an icy mantle are formed in the outer disk region, silicate particles are released at the snow line, crystalline silicate particles are produced at the annealing line, silicate particles diffuse beyond the snow line, and they eventually stick to drifting pebbles to return to the snow line. In the simple case without sticking and with steady pebble flux, we show through the simulations and analytical arguments that the crystalline components in silicate materials beyond the snow line are robustly and uniformly ≃5%. On the other hand, in a more realistic case with sticking and with a decaying pebble flux, the crystalline abundance increases to ∼20%–25%, depending on the ratio of the decay to diffusion timescales. This abundance is consistent with the observations. In this investigation, we assume a simple steady-accretion disk. The simulations coupled with the disk evolution are needed for a more detailed comparison with observed data.
Standard accretion disk models suggest that the snow line in the solar nebula migrated interior to the Earth’s orbit in a late stage of nebula evolution. In this late stage, a significant amount of ...ice could have been delivered to 1 AU from outer regions in the form of mm to dm-sized pebbles. This raises the question why the present Earth is so depleted of water (with the ocean mass being as small as 0.023% of the Earth mass). Here we quantify the amount of icy pebbles accreted by terrestrial embryos after the migration of the snow line assuming that no mechanism halts the pebble flow in outer disk regions. We use a simplified version of the coagulation equation to calculate the formation and radial inward drift of icy pebbles in a protoplanetary disk. The pebble accretion cross section of an embryo is calculated using analytic expressions presented by recent studies. We find that the final mass and water content of terrestrial embryos strongly depends on the radial extent of the gas disk, the strength of disk turbulence, and the time at which the snow lines arrives at 1 AU. The disk’s radial extent sets the lifetime of the pebble flow, while turbulence determines the density of pebbles at the midplane where the embryos reside. We find that the final water content of the embryos falls below 0.023 wt% only if the disk is compact (<100 AU), turbulence is strong at 1 AU, and the snow line arrives at 1 AU later than 2–4 Myr after disk formation. If the solar nebula extended to 300 AU, initially rocky embryos would have evolved into icy planets of 1–10 Earth masses unless the snow-line migration was slow. If the proto-Earth contained water of ~1 wt% as might be suggested by the density deficit of the Earth’s outer core, the formation of the proto-Earth was possible with weaker turbulence and with earlier (>0.5–2 Myr) snow-line migration.
One of the unique aspects of Earth is that it has a fractionally large Moon, which is thought to have formed from a Moon-forming disk generated by a giant impact. The Moon stabilizes the Earth's spin ...axis at least by several degrees and contributes to Earth's stable climate. Given that impacts are common during planet formation, exomoons, which are moons around planets in extrasolar systems, should be common as well, but no exomoon has been confirmed. Here we propose that an initially vapor-rich moon-forming disk is not capable of forming a moon that is large with respect to the size of the planet because growing moonlets, which are building blocks of a moon, experience strong gas drag and quickly fall toward the planet. Our impact simulations show that terrestrial and icy planets that are larger than ~1.3-1.6R
produce entirely vapor disks, which fail to form a fractionally large moon. This indicates that (1) our model supports the Moon-formation models that produce vapor-poor disks and (2) rocky and icy exoplanets whose radii are smaller than ~1.6R
are ideal candidates for hosting fractionally large exomoons.
The mass and semimajor axis distribution of gas giants in exoplanetary systems obtained by radial velocity surveys shows that super-Jupiter-mass planets are piled up at 1 au, while ...Jupiter/sub-Jupiter-mass planets are broadly distributed from ∼0.03 au to beyond 1 au. This feature has not been explained by theoretical predictions. In order to reconcile this inconsistency, we investigate evolution of gas giants with a new type II migration formula by Kanagawa et al., by comparing the migration, growth timescales of gas giants, and disk lifetime, and by population synthesis simulation. While the classical migration model assumes that a gas giant opens up a clear gap in the protoplanetary disk and the planet migration is tied to the disk gas accretion, recent high-resolution simulations show that the migration of gap-opening planets is decoupled from the disk gas accretion and Kanagawa et al. proposed that type II migration speed is nothing other than type I migration speed with the reduced disk gas surface density in the gap. We show that with this new formula, type II migration is significantly reduced for super-Jupiter-mass planets, if the disk accretion is driven by the disk wind as suggested by recent magnetohydrodynamic simulations. Population synthesis simulations show that super-Jupiter-mass planets remain at 1 au without any additional ingredient such as disk photoevaporation. Therefore, the mystery of the pile-up of gas giants at 1 au will be theoretically solved if the new formula is confirmed and wind-driven disk accretion dominates.
The Galilean Satellites Formed Slowly from Pebbles Shibaike, Yuhito; Ormel, Chris W.; Ida, Shigeru ...
Astrophysical journal/The Astrophysical journal,
11/2019, Letnik:
885, Številka:
1
Journal Article
Recenzirano
Odprti dostop
It is generally accepted that the four major (Galilean) satellites formed out of the gas disk that accompanied Jupiter's formation. However, understanding the specifics of the formation process is ...challenging, as both small particles (pebbles) and the satellites are subject to fast migration processes. Here we hypothesize a new scenario for the origin of the Galilean system, based on the capture of several planetesimal seeds and subsequent slow accretion of pebbles. To halt migration, we invoke an inner disk truncation radius, and other parameters are tuned for the model to match physical, dynamical, compositional, and structural constraints. In our scenario it is natural that Ganymede's mass is determined by pebble isolation. Our slow pebble accretion scenario then reproduces the following characteristics: (1) the mass of all the Galilean satellites; (2) the orbits of Io, Europa, and Ganymede captured in mutual 2:1 mean motion resonances; (3) the ice mass fractions of all the Galilean satellites; and (4) the unique ice-rock partially differentiated Callisto and the complete differentiation of the other satellites. Our scenario is unique to simultaneously reproduce these disparate properties.
Abstract
Dust particles in protoplanetary disks experience various chemical reactions under different physicochemical conditions through their accretion and diffusion, which results in the radial ...chemical gradient of dust. We performed three-dimensional Monte Carlo simulations to evaluate the dust trajectories and the progress of fictitious irreversible reactions, of which kinetics is expressed by the Johnson–Mehl–Avrami equation. The distribution of the highest temperature that each particle experiences before the degree of reaction exceeds a certain level shows the lognormal distribution, and its mode temperature was used as the effective reaction temperature. Semi-analytical prediction formulas of the effective reaction temperature and its dispersion were derived by comparing a reaction timescale with a diffusive transport timescale of dust as a function of the reaction parameters and the disk parameters. The formulas reproduce the numerical results of the effective reaction temperatures and their dispersions within 5.5% and 24%, respectively, in a wide temperature range (200–1400 K). We applied the formulas for the crystallization of amorphous silicate dust and its oxygen isotope exchange with the H
2
O vapor based on the experimentally determined kinetics. For submicron-sized amorphous forsterite dust, the predicted effective reaction temperature for the oxygen isotope exchange was lower than that of crystallization without overlap even considering their dispersions. This suggests that the amorphous silicate dust in the protosolar disk could exchange their oxygen isotopes efficiently with the
16
O-poor H
2
O vapor, resulting in the distinct oxygen isotope compositions from the Sun.
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