Abstract
Isotopic anomalies in several elements, as recently observed in meteorites, are generally interpreted to indicate nonequilibrium environments in the protoplanetary disk (PPD). Here we study ...reported Xe isotopic compositions on planets Earth and Mars, in a comet, and in meteorites for precursor discrepancies. Abundance variations of inferred presolar nano-diamonds, the carrier phase of the Xe-HL component, appear to be the primary source of nonuniformity of Xe precursors in the PPD, together with mechanisms of mass-dependent fractionation. While planet Mars kept a record of initial solar Xe isotopic abundances, such a record is missing for planet Earth. Xe isotopic abundances in paleo-atmospheres of both planets represent secondary reservoirs that show mass-dependent fractionation effects, but the inferred compositions of their PPD precursors differ: Mars atmospheric precursor Xe had solar isotopic composition, while Earth’s Xe precursor is consistent with a PPD reservoir of low nano-diamond abundance. Strong mass-dependent fractionation effects are observed in Xe components of IAB irons and in Yamato carbonaceous (CY) chondrites, and show that fractionation mechanisms are not restricted to planetary atmospheres. These records show that Xe isotopes in solar system reservoirs are useful tracers of evolutionary processes and of nonequilibrated presolar components in the PPD.
The interior structure of Saturn, the depth of its winds, and the mass and age of its rings constrain its formation and evolution. In the final phase of the Cassini mission, the spacecraft dived ...between the planet and its innermost ring, at altitudes of 2600 to 3900 kilometers above the cloud tops. During six of these crossings, a radio link with Earth was monitored to determine the gravitational field of the planet and the mass of its rings. We find that Saturn's gravity deviates from theoretical expectations and requires differential rotation of the atmosphere extending to a depth of at least 9000 kilometers. The total mass of the rings is (1.54 ± 0.49) × 10
kilograms (0.41 ± 0.13 times that of the moon Mimas), indicating that the rings may have formed 10
to 10
years ago.
Containing only a few percentages of the mass of the moon, the current asteroid belt is around three to four orders of magnitude smaller than its primordial mass inferred from disk models. Yet ...dynamical studies have shown that the asteroid belt could not have been depleted by more than about an order of magnitude over the past ∼4 Gyr. The remainder of the mass loss must have taken place during an earlier phase of the solar system's evolution. An orbital instability in the outer solar system occurring during the process of terrestrial planet formation can reproduce the broad characteristics of the inner solar system. Here, we test the viability of this model within the constraints of the main belt's low present-day mass and orbital structure. Although previous studies modeled asteroids as massless test particles because of limited computing power, our work uses graphics processing unit acceleration to model a fully self-gravitating asteroid belt. We find that depletion in the main belt is related to the giant planets' exact evolution within the orbital instability. Simulations that produce the closest matches to the giant planets' current orbits deplete the main belt by two to three orders of magnitude. These simulated asteroid belts are also good matches to the actual asteroid belt in terms of their radial mixing and broad orbital structure.
Several properties of the Solar System, including the wide radial spacing of the giant planets, can be explained if planets radially migrated by exchanging orbital energy and momentum with outer disk ...planetesimals. Neptune's planetesimal-driven migration, in particular, has a strong advocate in the dynamical structure of the Kuiper belt. A dynamical instability is thought to have occurred during the early stages with Jupiter having close encounters with a Neptune-class planet. As a result of the encounters, Jupiter acquired its current orbital eccentricity and jumped inward by a fraction of an astronomical unit, as required for the survival of the terrestrial planets and from asteroid belt constraints. Planetary encounters also contributed to capture of Jupiter Trojans and irregular satellites of the giant planets. Here we discuss the dynamical evolution of the early Solar System with an eye to determining how models of planetary migration/instability can be constrained from its present architecture. Specifically, we review arguments suggesting that the Solar System may have originally contained a third ice giant on a resonant orbit between Saturn and Uranus. This hypothesized planet was presumably ejected into interstellar space during the instability. The Kuiper belt kernel and other dynamical structures in the trans-Neptunian region may provide evidence for the ejected planet. We favor the early version of the instability where Neptune migrated into the outer planetesimal disk within a few tens of millions of years after the dispersal of the protosolar nebula. If so, the planetary migration/instability was not the cause of the Late Heavy Bombardment. Mercury's orbit may have been excited during the instability.
Abstract
The large-scale structure of the solar system has been shaped by a transient dynamical instability that may have been triggered by the interaction of the giants planets with a massive ...primordial disk of icy debris. In this work, we investigate the conditions under which this primordial disk could have coalesced into planets using analytic and numerical calculations. In particular, we perform numerical simulations of the solar system’s early dynamical evolution that account for the viscous stirring and collisional damping within the disk. We demonstrate that if collisional damping would have been sufficient to maintain a temperate velocity dispersion, Earth-mass trans-Neptunian planets could have emerged within a timescale of 10 Myr. Therefore, our results favor a scenario wherein the dynamical instability of the outer solar system began immediately upon the dissipation of the gaseous nebula to avoid the overproduction of Earth-mass planets in the outer solar system.
The origin of inner Solar System water Alexander, Conel M. O'D.
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
05/2017, Letnik:
375, Številka:
2094
Journal Article
Recenzirano
Odprti dostop
Of the potential volatile sources for the terrestrial planets, the CI and CM carbonaceous chondrites are closest to the planets' bulk H and N isotopic compositions. For the Earth, the addition of ...approximately 2-4 wt% of CI/CM material to a volatile-depleted proto-Earth can explain the abundances of many of the most volatile elements, although some solar-like material is also required. Two dynamical models of terrestrial planet formation predict that the carbonaceous chondrites formed either in the asteroid belt ('classical' model) or in the outer Solar System (5-15 AU in the Grand Tack model). To test these models, at present the H isotopes of water are the most promising indicators of formation location because they should have become increasingly D-rich with distance from the Sun. The estimated initial H isotopic compositions of water accreted by the CI, CM, CR and Tagish Lake carbonaceous chondrites were much more D-poor than measured outer Solar System objects. A similar pattern is seen for N isotopes. The D-poor compositions reflect incomplete re-equilibration with H2 in the inner Solar System, which is also consistent with the O isotopes of chondritic water. On balance, it seems that the carbonaceous chondrites and their water did not form very far out in the disc, almost certainly not beyond the orbit of Saturn when its moons formed (approx. 3-7 AU in the Grand Tack model) and possibly close to where they are found today.
This article is part of the themed issue ‘The origin, history and role of water in the evolution of the inner Solar System’.
•We combine N-body accretion and core–mantle differentiation models.•We model the chemical evolution of the mantles and cores of the terrestrial planets.•Tight constraints are placed on the ...compositions of Solar System primitive bodies.•Oxidation occurred in the early Solar System through the inward migration of ice.•Water that accretes to the planets originates in bodies that formed beyond 6–7AU.
In order to test accretion simulations as well as planetary differentiation scenarios, we have integrated a multistage core–mantle differentiation model with N-body accretion simulations. Impacts between embryos and planetesimals are considered to result in magma ocean formation and episodes of core formation. The core formation model combines rigorous chemical mass balance with metal–silicate element partitioning data and requires that the bulk compositions of all starting embryos and planetesimals are defined as a function of their heliocentric distances of origin. To do this, we assume that non-volatile elements are present in Solar System (CI) relative abundances in all bodies and that oxygen and H2O contents are the main compositional variables. The primary constraint on the combined model is the composition of the Earth’s primitive mantle. In addition, we aim to reproduce the composition of the martian mantle and the mass fractions of the metallic cores of Earth and Mars. The model is refined by least squares minimization with up to five fitting parameters that consist of the metal–silicate equilibration pressure and 1–4 parameters that define the starting compositions of primitive bodies. This integrated model has been applied to six Grand Tack N-body accretion simulations. Investigations of a broad parameter space indicate that: (1) accretion of Earth was heterogeneous, (2) metal–silicate equilibration pressures increase as accretion progresses and are, on average, 60–70% of core–mantle boundary pressures at the time of each impact, and (3) a large fraction (70–100%) of the metal of impactor cores equilibrates with a small fraction of the silicate mantles of proto-planets during each core formation event. Results are highly sensitive to the compositional model for the primitive starting bodies and several accretion/core-formation models can thus be excluded. Acceptable fits to the Earth’s mantle composition are obtained only when bodies that originated close to the Sun, at <0.9–1.2AU, are highly reduced and those from beyond this distance are increasingly oxidized. Reasonable concentrations of H2O in Earth’s mantle are obtained when bodies originating from beyond 6–7AU contain 20wt% water ice (icy bodies that originated between the snow line and this distance did not contribute to Earth’s accretion because they were swept up by Jupiter and Saturn). In the six models examined, water is added to the Earth mainly after 60–80% of its final mass has accreted. The compositional evolution of the mantles of Venus and Mars are also constrained by the model. The FeO content of the martian mantle depends critically on the heliocentric distance at which the Mars-forming embryo originated. Finally, the Earth’s core is predicted to contain 8–9wt% silicon, 2–4wt% oxygen and 10–60ppm hydrogen, whereas the martian core is predicted to contain low concentrations (<1wt%) of Si and O.
•We explain why Mars-mass embryos formed in the inner Solar System and massive cores in the outer System.•Accretion of planetesimals cannot explain this mass difference.•The scenario of formation of ...embryos and cores can explain this mass difference.
The basic structure of the Solar System is set by the presence of low-mass terrestrial planets in its inner part and giant planets in its outer part. This is the result of the formation of a system of multiple embryos with approximately the mass of Mars in the inner disk and of a few multi-Earth-mass cores in the outer disk, within the lifetime of the gaseous component of the protoplanetary disk. What was the origin of this dichotomy in the mass distribution of embryos/cores? We show in this paper that the classic processes of runaway and oligarchic growth from a disk of planetesimals cannot explain this dichotomy, even if the original surface density of solids increased at the snowline. Instead, the accretion of drifting pebbles by embryos and cores can explain the dichotomy, provided that some assumptions hold true. We propose that the mass-flow of pebbles is two-times lower and the characteristic size of the pebbles is approximately ten times smaller within the snowline than beyond the snowline (respectively at heliocentric distance r<rice and r>rice, where rice is the snowline heliocentric distance), due to ice sublimation and the splitting of icy pebbles into a collection of chondrule-size silicate grains. In this case, objects of original sub-lunar mass would grow at drastically different rates in the two regions of the disk. Within the snowline these bodies would reach approximately the mass of Mars while beyond the snowline they would grow to ∼20 Earth masses. The results may change quantitatively with changes to the assumed parameters, but the establishment of a clear dichotomy in the mass distribution of protoplanets appears robust provided that there is enough turbulence in the disk to prevent the sedimentation of the silicate grains into a very thin layer.
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