Origin of the orbital architecture of the giant planets of the Solar System

Dating the Solar System's giant planet orbital instability using enstatite meteorites

The giant planets of the Solar System formed on initially compact orbits, which transitioned to the current wider configuration by means of an orbital instability. The timing of that instability is poorly constrained. In this work, we use dynamical simulations to demonstrate that the instability implanted planetesimal fragments from the terrestrial planet region into the asteroid main belt. We use meteorite data to show that the implantation occurred >60 million years (Myr) after the Solar System began to form. Combining this constraint with a previous upper limit derived from Jupiter's trojan asteroids, we conclude that the orbital instability occurred 60 to 100 Myr after the beginning of Solar System formation. The giant impact that formed the Moon occurred within this range, so it might be related to the giant planet instability.

Chapter 3: The Origins and Evolution of Planetary Systems

The materials that form the diverse chemicals and structures on Earth-from mountains to oceans and biological organisms-all originated in a universe dominated by hydrogen and helium. Over billions of years, the composition and structure of the galaxies and stars evolved, and the elements of life, CHONPS, were formed through nucleosynthesis in stellar cores. Climactic events such as supernovae and stellar collisions produced heavier elements and spread them throughout the cosmos, often to be incorporated into new, more metal-rich stars. Stars typically form in molecular clouds containing small amounts of dust through the collapse of a high-density core. The surrounding nebular material is then pulled into a protoplanetary disk, from which planets, moons, asteroids, and comets eventually accrete. During the accretion of planetary systems, turbulent mixing can expose matter to a variety of different thermal and radiative environments. Chemical and physical changes in planetary system materials occur before and throughout the process of accretion, though many factors such as distance from the star, impact history, and level of heating experienced combine to ultimately determine the final geophysical characteristics. In Earth's planetary system, called the Solar System, after the orbits of the planets had settled into their current configuration, large impacts became rare, and the composition of and relative positions of objects became largely fixed. Further evolution of the respective chemical and physical environments of the planets-geosphere, hydrosphere, and atmosphere-then became dependent on their local geochemistry, their atmospheric interactions with solar radiation, and smaller asteroid impacts. On Earth, the presence of land, air, and water, along with an abundance of important geophysical and geochemical phenomena, led to a habitable planet where conditions were right for life to thrive.

Organic Matter in the Asteroid Ryugu: What We Know So Far

The Hayabusa2 mission was tasked with returning samples from the C-complex asteroid Ryugu (1999 JU3), in order to shed light on the formation, evolution and composition of such asteroids. One of the main science objectives was to understand whether such bodies could have supplied the organic matter required for the origin of life on Earth. Here, a review of the studies concerning the organic matter within the Ryugu samples is presented. This review will inform the reader about the Hayabusa2 mission, the nature of the organic matter analyzed and the various interpretations concerning the analytical findings including those concerning the origin and evolution of organic matter from Ryugu. Finally, the review puts the findings and individual interpretations in the context of the current theories surrounding the formation and evolution of Ryugu. Overall, the summary provided here will help to inform those operating in a wide range of interdisciplinary fields, including planetary science, astrobiology, the origin of life and astronomy, about the most recent developments concerning the organic matter in the Ryugu return samples and their relevance to understanding our solar system and beyond. The review also outlines the issues that still remain to be solved and highlights potential areas for future work.

Synthesis of prebiotic organics from CO2 by catalysis with meteoritic and volcanic particles

The emergence of prebiotic organics was a mandatory step toward the origin of life. The significance of the exogenous delivery versus the in-situ synthesis from atmospheric gases is still under debate. We experimentally demonstrate that iron-rich meteoritic and volcanic particles activate and catalyse the fixation of CO₂, yielding the key precursors of life-building blocks. This catalysis is robust and produces selectively aldehydes, alcohols, and hydrocarbons, independent of the redox state of the environment. It is facilitated by common minerals and tolerates a broad range of the early planetary conditions (150-300 °C, ≲ 10-50 bar, wet or dry climate). We find that up to 6 × 10⁸ kg/year of prebiotic organics could have been synthesized by this planetary-scale process from the atmospheric CO₂ on Hadean Earth.

Some Pertinent Issues for Interstellar Panspermia Raised after the Discovery of 1I/'Oumuamua

The interstellar objects 1I/'Oumuamua and 2I/Borisov confirm the long-held expectation that bodies from one stellar system will be carried to another, allowing, in principle, interstellar panspermia. Life might be transferred between stellar systems, depending on the nature of the bodies and how they escaped their systems. 2I/Borisov appears to be a comet, with no more likelihood of carrying life than Solar System comets. In contrast, the nature of 1I/'Oumuamua has been difficult to determine. We review various hypotheses for its origin, including ejection of N₂ ice from the surface of an exo-Pluto, formation in a molecular cloud by freezing of H₂, and a derelict solar sail of alien construction. Of these, the N₂ ice fragment hypothesis is uniquely falsifiable, plausible, and completely consistent with all observations. The possibility of interstellar panspermia would be made more probable if 'Oumuamua originated on a dwarf planet rather than a comet, although substantial challenges to transfer of life would remain. Of proposed mechanisms for interstellar panspermia, transfer of life via rocky meteoroids is perhaps less improbable.

Distinguishing the Origin of Asteroid (16) Psyche

The asteroid (16) Psyche may be the metal-rich remnant of a differentiated planetesimal, or it may be a highly reduced, metal-rich asteroidal material that never differentiated. The NASA Psyche mission aims to determine Psyche's provenance. Here we describe the possible solar system regions of origin for Psyche, prior to its likely implantation into the asteroid belt, the physical and chemical processes that can enrich metal in an asteroid, and possible meteoritic analogs. The spacecraft payload is designed to be able to discriminate among possible formation theories. The project will determine Psyche's origin and formation by measuring any strong remanent magnetic fields, which would imply it was the core of a differentiated body; the scale of metal to silicate mixing will be determined by both the neutron spectrometers and the filtered images; the degree of disruption between metal and rock may be determined by the correlation of gravity with composition; some mineralogy (e.g., modeled silicate/metal ratio, and inferred existence of low-calcium pyroxene or olivine, for example) will be detected using filtered images; and the nickel content of Psyche's metal phase will be measured using the GRNS.

Early Solar System instability triggered by dispersal of the gaseous disk

The Solar System's orbital structure is thought to have been sculpted by an episode of dynamical instability among the giant planets^1-4. However, the instability trigger and timing have not been clearly established^5-9. Hydrodynamical modelling has shown that while the Sun's gaseous protoplanetary disk was present the giant planets migrated into a compact orbital configuration in a chain of resonances^2,10. Here we use dynamical simulations to show that the giant planets' instability was probably triggered by the dispersal of the gaseous disk. As the disk evaporated from the inside out, its inner edge swept successively across and dynamically perturbed each planet's orbit in turn. The associated orbital shift caused a dynamical compression of the exterior part of the system, ultimately triggering instability. The final orbits of our simulated systems match those of the Solar System for a viable range of astrophysical parameters. The giant planet instability therefore took place as the gaseous disk dissipated, constrained by astronomical observations to be a few to ten million years after the birth of the Solar System¹¹. Terrestrial planet formation would not complete until after such an early giant planet instability^12,13; the growing terrestrial planets may even have been sculpted by its perturbations, explaining the small mass of Mars relative to Earth¹⁴.

On the origin and evolution of the asteroid Ryugu: A comprehensive geochemical perspective

Presented here are the observations and interpretations from a comprehensive analysis of 16 representative particles returned from the C-type asteroid Ryugu by the Hayabusa2 mission. On average Ryugu particles consist of 50% phyllosilicate matrix, 41% porosity and 9% minor phases, including organic matter. The abundances of 70 elements from the particles are in close agreement with those of CI chondrites. Bulk Ryugu particles show higher δ¹⁸O, Δ¹⁷O, and ε⁵⁴Cr values than CI chondrites. As such, Ryugu sampled the most primitive and least-thermally processed protosolar nebula reservoirs. Such a finding is consistent with multi-scale H-C-N isotopic compositions that are compatible with an origin for Ryugu organic matter within both the protosolar nebula and the interstellar medium. The analytical data obtained here, suggests that complex soluble organic matter formed during aqueous alteration on the Ryugu progenitor planetesimal (several 10's of km), <2.6 Myr after CAI formation. Subsequently, the Ryugu progenitor planetesimal was fragmented and evolved into the current asteroid Ryugu through sublimation.

Late accretionary history of Earth and Moon preserved in lunar impactites

Late accretion describes the final addition of Earth’s mass following Moon formation and includes a period of Late Heavy Bombardment (LHB), which occurred either as a short-lived cataclysm triggered by a late giant planet orbital instability or a declining bombardment during late accretion. Using genetically characteristic ruthenium and molybdenum isotope compositions of lunar impact–derived rocks, we show that the impactors during the LHB and the entire period of late accretion were the same type of bodies and that they originated in the terrestrial planet region. Because a cataclysmic LHB would have, in part, resulted in compositionally distinct projectiles, we conclude that the LHB reflects the tail end of accretion. This implies that the giant planet orbital instability occurred during the main phase of planet formation. Last, because of their inner solar system origin, late-accreted bodies cannot be the primary source of Earth’s water.

GAUSS - genesis of asteroids and evolution of the solar system: A sample return mission to Ceres

The goal of Project GAUSS (Genesis of Asteroids and evolUtion of the Solar System) is to return samples from the dwarf planet Ceres. Ceres is the most accessible candidate of ocean worlds and the largest reservoir of water in the inner Solar System. It shows active volcanism and hydrothermal activities in recent history. Recent evidence for the existence of a subsurface ocean on Ceres and the complex geochemistry suggest past habitability and even the potential for ongoing habitability. GAUSS will return samples from Ceres with the aim of answering the following top-level scientific questions: What is the origin of Ceres and what does this imply for the origin of water and other volatiles in the inner Solar System?What are the physical properties and internal structure of Ceres? What do they tell us about the evolutionary and aqueous alteration history of dwarf planets?What are the astrobiological implications of Ceres? Is it still habitable today?What are the mineralogical connections between Ceres and our current collections of carbonaceous meteorites?

Olivine-rich achondrites from Vesta and the missing mantle problem

Mantles of rocky planets are dominantly composed of olivine and its high-pressure polymorphs, according to seismic data of Earth's interior, the mineralogy of natural samples, and modelling results. The missing mantle problem represents the paucity of olivine-rich material among meteorite samples and remote observation of asteroids, given how common differentiated planetesimals were in the early Solar System. Here we report the discovery of new olivine-rich meteorites that have asteroidal origins and are related to V-type asteroids or vestoids. Northwest Africa 12217, 12319, and 12562 are dunites and lherzolite cumulates that have siderophile element abundances consistent with origins on highly differentiated asteroidal bodies that experienced core formation, and with trace element and oxygen and chromium isotopic compositions associated with the howardite-eucrite-diogenite meteorites. These meteorites represent a step towards the end of the shortage of olivine-rich material, allowing for full examination of differentiation processes acting on planetesimals in the earliest epoch of the Solar System.

Chromium Isotopic Evidence for Mixing of NC and CC Reservoirs in Polymict Ureilites: Implications for Dynamical Models of the Early Solar System

Nucleosynthetic isotope anomalies show that the first few million years of solar system history were characterized by two distinct cosmochemical reservoirs, CC (carbonaceous chondrites and related differentiated meteorites) and NC (the terrestrial planets and all other groups of chondrites and differentiated meteorites), widely interpreted to correspond to the outer and inner solar system, respectively. At some point, however, bulk CC and NC materials became mixed, and several dynamical models offer explanations for how and when this occurred. We use xenoliths of CC materials in polymict ureilite (NC) breccias to test the applicability of such models. Polymict ureilites represent regolith on ureilitic asteroids but contain carbonaceous chondrite-like xenoliths. We present the first 54Cr isotope data for such clasts, which, combined with oxygen and hydrogen isotopes, show that they are unique CC materials that became mixed with NC materials in these breccias. It has been suggested that such xenoliths were implanted into ureilites by outer solar system bodies migrating into the inner solar system during the gaseous disk phase ~3-5 Myr after CAI, as in the "Grand Tack" model. However, combined textural, petrologic, and spectroscopic observations suggest that they were added to ureilitic regolith at ~50-60 Myr after CAI, along with ordinary, enstatite, and Rumuruti-type chondrites, as a result of breakup of multiple parent bodies in the asteroid belt at this time. This is consistent with models for an early instability of the giant planets. The C-type asteroids from which the xenoliths were derived were already present in inner solar system orbits.

The interiors of Uranus and Neptune: current understanding and open questions

Uranus and Neptune form a distinct class of planets in our Solar System. Given this fact, and ubiquity of similar-mass planets in other planetary systems, it is essential to understand their interior structure and composition. However, there are more open questions regarding these planets than answers. In this review, we concentrate on the things we do not know about the interiors of Uranus and Neptune with a focus on why the planets may be different, rather than the same. We next summarize the knowledge about the planets' internal structure and evolution. Finally, we identify the topics that should be investigated further on the theoretical front as well as required observations from space missions. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.

On the Origin and Evolution of the Material in 67P/Churyumov-Gerasimenko

Primitive objects like comets hold important information on the material that formed our solar system. Several comets have been visited by spacecraft and many more have been observed through Earth- and space-based telescopes. Still our understanding remains limited. Molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. The samples returned by the Stardust mission to comet Wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. The recent Rosetta mission acquired a wealth of new data on the composition of comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) and complemented earlier observations of other comets. The isotopic, elemental, and molecular abundances of the volatile, semi-volatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. The emerging picture after Rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. Deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the Stardust results. Post-formation evolution of the material might play an important role, which further complicates the picture. This paper discusses these major findings of the Rosetta mission with respect to the origin of the material and puts them in the context of what we know from other comets and solar system objects.

Isotopic evidence for volatile replenishment of the Moon during the Late Accretion

The traditional view of a dry, volatile-poor Moon has been challenged by the identification of water and other volatiles in lunar samples, but the volatile budget delivery time(s), source(s) and temporal evolution remain poorly constrained. Here we show that hydrogen and chlorine isotopic ratios in lunar apatite changed significantly during the Late Accretion (LA, 4.1-3.8 billion years ago). During this period, deuterium/hydrogen ratios in the Moon changed from initial carbonaceous-chondrite-like values to values consistent with an influx of ordinary-chondrite-like material and pre-LA elevated δ37Cl values drop towards lower chondrite-like values. Inferred pre-LA lunar interior water contents are significantly lower than pristine values suggesting degassing, followed by an increase during the LA. These trends are consistent with dynamic models of solar-system evolution, suggesting that the Moon's (and Earth's) initial volatiles were replenished ∼0.5 Ga after their formation, with their final budgets reflecting a mixture of sources and delivery times.

Discovery of fossil asteroidal ice in primitive meteorite Acfer 094

Carbonaceous chondrites are meteorites believed to preserve our planet's source materials, but the precise nature of these materials still remains uncertain. To uncover pristine planetary materials, we performed synchrotron radiation-based x-ray computed nanotomography of a primitive carbonaceous chondrite, Acfer 094, and found ultraporous lithology (UPL) widely distributed in a fine-grained matrix. UPLs are porous aggregates of amorphous and crystalline silicates, Fe─Ni sulfides, and organics. The porous texture must have been formed by removal of ice previously filling pore spaces, suggesting that UPLs represent fossils of primordial ice. The ice-bearing UPLs formed through sintering of fluffy icy dust aggregates around the H₂O snow line in the solar nebula and were incorporated into the Acfer 094 parent body, providing new insight into asteroid formation by dust agglomeration.

Percolation clusters of organics in interstellar ice grains as the incubators of life

Biomolecules can be synthesized in interstellar ice grains subject to UV radiation and cosmic rays. I show that on time scales of ≳10⁶ years, these processes lead to the formation of large percolation clusters of organic molecules. Some of these clusters would have ended up on proto-planets where large, loosely bound aggregates of clusters (superclusters) would have formed. The interior regions of such superclusters provided for chemical micro-environments that are filtered versions of the outside environment. I argue that models for abiogenesis are more likely to work when considered inside such micro-environments. As the supercluster breaks up, biochemical systems in such micro-environments gradually become subject to a less filtered environment, allowing them to get adapted to the more complex outside environment. A particular system originating from a particular location on some supercluster would have been the first to get adapted to the raw outside environment and survive there, thereby becoming the first microbe. A collision of a microbe-containing proto-planet with the Moon could have led to fragments veering off back into space, microbes in small fragments would have been able to survive a subsequent impact with the Earth.

Not a simple relationship between Neptune's migration speed and Kuiper belt inclination excitation

We present numerical simulations of giant planet migration in our solar system and examine how the speed of planetary migration affects inclinations in the resulting population of small bodies (test particles) scattered outward and subsequently captured into Neptune's 3:2 mean motion resonance (the Plutinos) as well as the hot classical Kuiper belt population. We do not find a consistent relationship between the degree of test particle inclination excitation and e-folding planet migration timescales in the range 5 - 50 Myr. Our results present a counter-example to Nesvorný (2015)'s finding that the Plutino and hot classical inclinations showed a marked increase with increasing e-folding timescales for Neptune's migration. We argue that these differing results are likely due to differing secular architectures of the giant planets during and after migration. Small changes in the planets' initial conditions and differences in the numerical implementation of planet migration can result in different amplitudes of the planets' inclination secular modes, and this can lead to different final inclination distributions for test particles in the simulations. We conclude that the observed large inclination dispersion of Kuiper belt objects does not require Neptune's migration to be slow; planetary migration with e-folding timescales of 5, 10, 30, and 50 Myr can all yield inclination dispersions similar to the observed Plutino and hot classical populations, with no correlation between the degree of inclination excitation and migration speed.

History of the Terminal Cataclysm Paradigm: Epistemology of a Planetary Bombardment That Never (?) Happened

This study examines the history of the paradigm concerning a lunar (or solar-systemwide)terminal cataclysm (also called “Late Heavy Bombardment” or LHB), a putative, brief spikein impacts at ~3.9 Ga ago, preceded by low impact rates. We examine origin of the ideas, why theywere accepted, and why the ideas are currently being seriously revised, if not abandoned. Thepaper is divided into the following sections:1. Overview of paradigm.2. Pre-Apollo views (1949-1969).3. Initial suggestions of cataclysm (ca. 1974).4. Ironies.5. Alternative suggestions, megaregolith evolution (1970s).6. Impact melt rocks “establish” cataclysm (1990).7. Imbrium redux (ca. 1998).8. Impact melt clasts (early 2000s).9. Dating of front-side lunar basins?10. Dynamical models “explain” the cataclysm (c. 2000s).11. Asteroids as a test case.12. Impact melts predating 4.0 Ga ago (ca. 2008-present.).13. Biological issues.14. Growing doubts (ca. 1994-2014).15. Evolving Dynamical Models (ca. 2001-present).16. Connections to lunar origin.17. Dismantling the paradigm (2015-2018).18. “Megaregolith Evolution Model” for explaining the data.19. Conclusions and new directions for future work.The author hopes that this open-access discussion may prove useful for classroom discussionsof how science moves forward through self-correction of hypotheses.

Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object

The Kuiper Belt is a distant region of the outer Solar System. On 1 January 2019, the New Horizons spacecraft flew close to (486958) 2014 MU₆₉, a cold classical Kuiper Belt object approximately 30 kilometers in diameter. Such objects have never been substantially heated by the Sun and are therefore well preserved since their formation. We describe initial results from these encounter observations. MU₆₉ is a bilobed contact binary with a flattened shape, discrete geological units, and noticeable albedo heterogeneity. However, there is little surface color or compositional heterogeneity. No evidence for satellites, rings or other dust structures, a gas coma, or solar wind interactions was detected. MU₆₉'s origin appears consistent with pebble cloud collapse followed by a low-velocity merger of its two lobes.

Instabilities in the Early Solar System due to a Self-gravitating Disk

Modern studies of the early solar system routinely invoke the possibility of an orbital instability among the giant planets triggered by gravitational interactions between the planets and a massive exterior disk of planetesimals. Previous works have suggested that this instability can be substantially delayed (~100s Myr) after the formation of the giant planets. Bodies in the disk are typically treated in a semi-active manner, wherein their gravitational force on the planets is included, but interactions between the planetesimals are ignored. We perform N-body numerical simulations using GENGA, which makes use of GPUs to allow for the inclusion of all gravitational interactions between bodies. Although our simulated Kuiper belt particles are more massive than the probable masses of real primordial Kuiper belt objects, our simulations indicate that the self-stirring of the primordial Kuiper belt is very important to the dynamics of the giant planet instability. We find that interactions between planetesimals dynamically heat the disk and typically prevent the outer solar system instability from being delayed by more than a few tens of million years after giant planet formation. Longer delays occur in a small fraction of systems that have at least 3.5 AU gaps between the planets and planetesimal disk. Our final planetary configurations match the solar system at a rate consistent with other previous works in most regards. Pre-instability heating of the disk typically yields final Jovian eccentricities comparable to the modern solar system value, which has been a difficult constraint to match in past works.

Evolution of Saturn's Mid-Sized Moons

The orbits of Saturn's inner mid-sized moons (Mimas, Enceladus, Tethys, Dione, and Rhea) have been notably difficult to reconcile with their geology. Here, we present numerical simulations coupling thermal, geophysical, and simplified orbital evolution for 4.5 billion years that reproduce observed characteristics of their orbits and interiors, provided that the outer four moons are old. Tidal dissipation within Saturn expands the moons' orbits over time. Dissipation within the moons decreases their eccentricities, which are episodically increased by moon-moon interactions, causing past or present oceans in the interior of Enceladus, Dione, and Tethys. In contrast, Mimas' proximity to Saturn's rings generates interactions that cause such rapid orbital expansion that Mimas must have formed only 0.1-1 Gyr ago if it postdates the rings. The resulting lack of radionuclides keeps it geologically inactive. These simulations can explain the Mimas-Enceladus dichotomy, reconcile the moons' orbital properties and geological diversity, and self-consistently produce a recent ocean on Enceladus.

Constraining the Time Interval for the Origin of Life on Earth

Estimates of the time at which life arose on Earth make use of two types of evidence. First, astrophysical and geophysical studies provide a timescale for the formation of Earth and the Moon, for large impact events on early Earth, and for the cooling of the early magma ocean. From this evidence, we can deduce a habitability boundary, which is the earliest point at which Earth became habitable. Second, biosignatures in geological samples, including microfossils, stromatolites, and chemical isotope ratios, provide evidence for when life was actually present. From these observations we can deduce a biosignature boundary, which is the earliest point at which there is clear evidence that life existed. Studies with molecular phylogenetics and records of the changing level of oxygen in the atmosphere give additional information that helps to determine the biosignature boundary. Here, we review the data from a wide range of disciplines to summarize current information on the timings of these two boundaries. The habitability boundary could be as early as 4.5 Ga, the earliest possible estimate of the time at which Earth had a stable crust and hydrosphere, or as late as 3.9 Ga, the end of the period of heavy meteorite bombardment. The lack of consensus on whether there was a late heavy meteorite bombardment that was significant enough to prevent life is the largest uncertainty in estimating the time of the habitability boundary. The biosignature boundary is more closely constrained. Evidence from carbon isotope ratios and stromatolite fossils both point to a time close to 3.7 Ga. Life must have emerged in the interval between these two boundaries. The time taken for life to appear could, therefore, be within 200 Myr or as long as 800 Myr. Key Words: Origin of life-Astrobiology-Habitability-Biosignatures-Geochemistry-Early Earth. Astrobiology 18, 343-364.

Tilting Styx and Nix but not Uranus with a Spin-Precession-Mean-motion resonance

A Hamiltonian model is constructed for the spin axis of a planet perturbed by a nearby planet with both planets in orbit about a star. We expand the planet-planet gravitational potential perturbation to first order in orbital inclinations and eccentricities, finding terms describing spin resonances involving the spin precession rate and the two planetary mean motions. Convergent planetary migration allows the spinning planet to be captured into spin resonance. With initial obliquity near zero, the spin resonance can lift the planet's obliquity to near 90 or 180 degrees depending upon whether the spin resonance is first or zero-th order in inclination. Past capture of Uranus into such a spin resonance could give an alternative non-collisional scenario accounting for Uranus's high obliquity. However we find that the time spent in spin resonance must be so long that this scenario cannot be responsible for Uranus's high obliquity. Our model can be used to study spin resonance in satellite systems. Our Hamiltonian model explains how Styx and Nix can be tilted to high obliquity via outward migration of Charon, a phenomenon previously seen in numerical simulations.

Radial mixing and Ru-Mo isotope systematics under different accretion scenarios

The Ru-Mo isotopic compositions of inner Solar System bodies may reflect the provenance of accreted material and how it evolved with time, both of which are controlled by the accretion scenario these bodies experienced. Here we use a total of 116 N-body simulations of terrestrial planet accretion, run in the Eccentric Jupiter and Saturn (EJS), Circular Jupiter and Saturn (CJS), and Grand Tack scenarios, to model the Ru-Mo anomalies of Earth, Mars, and Theia analogues. This model starts by applying an initial step function in Ru-Mo isotopic composition, with compositions reflecting those in meteorites, and traces compositional evolution as planets accrete. The mass-weighted provenance of the resulting planets reveals more radial mixing in Grand Tack simulations than in EJS/CJS simulations, and more efficient mixing among late-accreted material than during the main phase of accretion in EJS/CJS simulations. We find that an extensive homogenous inner disk region is required to reproduce Earth's observed Ru-Mo composition. EJS/CJS simulations require a homogeneous reservoir in the inner disk extending to ≥3-4 AU (≥74-98% of initial mass) to reproduce Earth's composition, while Grand Tack simulations require a homogeneous reservoir extending to ≥3-10 AU (≥97-99% of initial mass), and likely to ≥6-10 AU. In the Grand Tack model, Jupiter's initial location (the most likely location for a discontinuity in isotopic composition) is ~3.5 AU; however, this step location has only a 33% likelihood of producing an Earth with the correct Ru-Mo isotopic signature for the most plausible model conditions. Our results give the testable predictions that Mars has zero Ru anomaly and small or zero Mo anomaly, and the Moon has zero Mo anomaly. These predictions are insensitive to wide variations in parameter choices.

Cataclysm No More: New Views on the Timing and Delivery of Lunar Impactors

If properly interpreted, the impact record of the Moon, Earth's nearest neighbour, can be used to gain insights into how the Earth has been influenced by impacting events since its formation ~4.5 billion years (Ga) ago. However, the nature and timing of the lunar impactors - and indeed the lunar impact record itself - are not well understood. Of particular interest are the ages of lunar impact basins and what they tell us about the proposed "lunar cataclysm" and/or the late heavy bombardment (LHB), and how this impact episode may have affected early life on Earth or other planets. Investigations of the lunar impactor population over time have been undertaken and include analyses of orbital data and images; lunar, terrestrial, and other planetary sample data; and dynamical modelling. Here, the existing information regarding the nature of the lunar impact record is reviewed and new interpretations are presented. Importantly, it is demonstrated that most evidence supports a prolonged lunar (and thus, terrestrial) bombardment from ~4.2 to 3.4 Ga and not a cataclysmic spike at ~3.9 Ga. Implications for the conditions required for the origin of life are addressed.

Cometary science after Rosetta

The European Space Agency's Rosetta mission ended operations on 30 September 2016 having spent over 2 years in close proximity to its target comet, 67P/Churyumov-Gerasimenko. Shortly before this, in summer 2016, a discussion meeting was held to examine how the results of the mission could be framed in terms of cometary and solar system science in general. This paper provides a brief history of the Rosetta mission, and gives an overview of the meeting and the contents of this associated special issue.This article is part of the themed issue 'Cometary science after Rosetta'.

Setting the scene: what did we know before Rosetta?

This paper provides an overview of our state of knowledge about comets prior to the Rosetta mission encounter. Starting with the historical perspective, this paper discusses the development of comet science up to the modern era of space exploration. The extent to which comets are tracers of solar system formation processes or preserve pristine interstellar material has been investigated for over four decades. There is increasing evidence that in contrast with the distinct dynamical comet reservoirs we see today, comet formation regions strongly overlapped in the protoplanetary disc and there was significant migration of material in the disc during the epoch of comet formation. Comet nuclei are now known to be very low-density highly porous bodies, with very low thermal inertia, and have a range of sizes which exhibit a deficiency of very small bodies. The low thermal inertia suggests that comets may preserve pristine materials close to the surface, and that this might be accessible to sample return missions.This article is part of the themed issue 'Cometary science after Rosetta'.

ACCRETION OF SATURN'S INNER MID-SIZED MOONS FROM A MASSIVE PRIMORDIAL ICE RING

Saturn's rings are rock-poor, containing 90 to 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 satellites, including tidal interaction with the planet, assuming a tidal dissipation factor for Saturn of Q ~ 10⁴. We find that ring torques and capture of moons into mutual resonances produces a system of ice-rich inner moons that extends outward to approximately Tethys's orbit in 10⁹ years, even with relatively slow orbital expansion due to tides. The resulting mass and semi-major axis distribution of spawned moons resembles that of Mimas, Enceladus and Tethys. We estimate the mass of rock delivered to the moons by external cometary impactors during a late-heavy bombardment. We find that the inner moons receive a mass in rock comparable to their current total rock content, while Dione and Rhea receive an order-of-magnitude less rock than their current rock content. This suggests that external contamination may have been the primary source of rock in the inner moons, and that Dione and Rhea formed from much more rock-rich source material. Reproducing the distribution of rock among the current inner moons is challenging, and appears to require large impactors and stochasticity and/or the presence of some rock in the initial ring.

The transiting dust clumps in the evolved disc of the Sun-like UXor RZ Psc

RZ Psc is a young Sun-like star, long associated with the UXor class of variable stars, which is partially or wholly dimmed by dust clumps several times each year. The system has a bright and variable infrared excess, which has been interpreted as evidence that the dimming events are the passage of asteroidal fragments in front of the host star. Here, we present a decade of optical photometry of RZ Psc and take a critical look at the asteroid belt interpretation. We show that the distribution of light curve gradients is non-uniform for deep events, which we interpret as possible evidence for an asteroidal fragment-like clump structure. However, the clumps are very likely seen above a high optical depth midplane, so the disc's bulk clumpiness is not revealed. While circumstantial evidence suggests an asteroid belt is more plausible than a gas-rich transition disc, the evolutionary status remains uncertain. We suggest that the rarity of Sun-like stars showing disc-related variability may arise because (i) any accretion streams are transparent and/or (ii) turbulence above the inner rim is normally shadowed by a flared outer disc.

Inner solar system material discovered in the Oort cloud

We have observed C/2014 S3 (PANSTARRS), a recently discovered object on a cometary orbit coming from the Oort cloud that is physically similar to an inner main belt rocky S-type asteroid. Recent dynamical models successfully reproduce the key characteristics of our current solar system; some of these models require significant migration of the giant planets, whereas others do not. These models provide different predictions on the presence of rocky material expelled from the inner solar system in the Oort cloud. C/2014 S3 could be the key to verifying these predictions of the migration-based dynamical models. Furthermore, this object displays a very faint, weak level of comet-like activity, five to six orders of magnitude less than that of typical ice-rich comets on similar Orbits coming from the Oort cloud. For the nearly tailless appearance, we are calling C/2014 S3 a Manx object. Various arguments convince us that this activity is produced by sublimation of volatile ice, that is, normal cometary activity. The activity implies that C/2014 S3 has retained a tiny fraction of the water that is expected to be present at its formation distance in the inner solar system. We may be looking at fresh inner solar system Earth-forming material that was ejected from the inner solar system and preserved for billions of years in the Oort cloud.

Perspectives on Comets, Comet-like Asteroids, and Their Predisposition to Provide an Environment That Is Friendly to Life

In recent years, studies have shown that there are many similarities between comets and asteroids. In some cases, it cannot even be determined to which of these groups an object belongs. This is especially true for objects found beyond the main asteroid belt. Because of the lack of comet fragments, more progress has been made concerning the chemical composition of asteroids. In particular, the SMASSII classification establishes a link between the reflecting spectra and chemical composition of asteroids and meteorites. To find clues for the chemical structure of comets, the parameters of all known asteroids of the SMASSII classification were compared to those of comet groups like the Encke-type comets, the Jupiter-family comets, and the Halley-type comets, as well as comet-like objects like the damocloids and the centaurs. Fifty-six SMASSII objects similar to comets were found and are categorized as comet-like asteroids in this work. Aside from the chemistry, it is assumed that the available energy on these celestial bodies plays an important role concerning habitability. For the determination of the available energy, the effective temperature was calculated. Additionally, the size of these objects was considered in order to evaluate the possibility of a liquid water core, which provides an environment that is more likely to support processes necessary to create the building blocks of life. Further study of such objects could be notable for the period of the Late Heavy Bombardment and could therefore provide important implications for our understanding of the inner workings of the prebiotic evolution within the Solar System since the beginning.

AstRoMap European Astrobiology Roadmap

The European AstRoMap project (supported by the European Commission Seventh Framework Programme) surveyed the state of the art of astrobiology in Europe and beyond and produced the first European roadmap for astrobiology research. In the context of this roadmap, astrobiology is understood as the study of the origin, evolution, and distribution of life in the context of cosmic evolution; this includes habitability in the Solar System and beyond. The AstRoMap Roadmap identifies five research topics, specifies several key scientific objectives for each topic, and suggests ways to achieve all the objectives. The five AstRoMap Research Topics are • Research Topic 1: Origin and Evolution of Planetary Systems • Research Topic 2: Origins of Organic Compounds in Space • Research Topic 3: Rock-Water-Carbon Interactions, Organic Synthesis on Earth, and Steps to Life • Research Topic 4: Life and Habitability • Research Topic 5: Biosignatures as Facilitating Life Detection It is strongly recommended that steps be taken towards the definition and implementation of a European Astrobiology Platform (or Institute) to streamline and optimize the scientific return by using a coordinated infrastructure and funding system.

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

Comets play a dual role in understanding the formation and evolution of the solar system. First, the composition of comets provides information about the origin of the giant planets and their moons because comets formed early and their composition is not expected to have evolved significantly since formation. They, therefore serve as a record of conditions during the early stages of solar system formation. Once comets had formed, their orbits were perturbed allowing them to travel into the inner solar system and impact the planets. In this way they contributed to the volatile inventory of planetary atmospheres. We review here how knowledge of comet composition up to the time of the Rosetta mission has contributed to understanding the formation processes of the giant planets, their moons and small icy bodies in the solar system. We also discuss how comets contributed to the volatile inventories of the giant and terrestrial planets.

Growing the gas-giant planets by the gradual accumulation of pebbles

It is widely held that the first step in forming gas-giant planets, such as Jupiter and Saturn, was the production of solid 'cores' each with a mass roughly ten times that of the Earth. Getting the cores to form before the solar nebula dissipates (in about one to ten million years; ref. 3) has been a major challenge for planet formation models. Recently models have emerged in which 'pebbles' (centimetre-to-metre-sized objects) are first concentrated by aerodynamic drag and then gravitationally collapse to form objects 100 to 1,000 kilometres in size. These 'planetesimals' can then efficiently accrete left-over pebbles and directly form the cores of giant planets. This model is known as 'pebble accretion'; theoretically, it can produce cores of ten Earth masses in only a few thousand years. Unfortunately, full simulations of this process show that, rather than creating a few such cores, it produces a population of hundreds of Earth-mass objects that are inconsistent with the structure of the Solar System. Here we report that this difficulty can be overcome if pebbles form slowly enough to allow the planetesimals to gravitationally interact with one another. In this situation, the largest planetesimals have time to scatter their smaller siblings out of the disk of pebbles, thereby stifling their growth. Our models show that, for a large and physically reasonable region of parameter space, this typically leads to the formation of one to four gas giants between 5 and 15 astronomical units from the Sun, in agreement with the observed structure of the Solar System.