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Earth’s water, intrinsic oxidation state and metal core density are fundamental chemical features of our planet. Studies of exoplanets provide a useful context for elucidating the source of these chemical traits. Planet formation and evolution models demonstrate that rocky exoplanets commonly formed with hydrogen-rich envelopes that were lost over time 1 . These findings suggest that Earth may also have formed from bodies with hydrogen-rich primary atmospheres. Here we use a self-consistent thermodynamic model to show that Earth’s water, core density and overall oxidation state can all be sourced to equilibrium between hydrogen-rich primary atmospheres and underlying magma oceans in its progenitor planetary embryos. Water is produced from dry starting materials resembling enstatite chondrites as oxygen from magma oceans reacts with hydrogen. Hydrogen derived from the atmosphere enters the magma ocean and eventually the metal core at equilibrium, causing metal density deficits matching that of Earth. Oxidation of the silicate rocks from solar-like to Earth-like oxygen fugacities also ensues as silicon, along with hydrogen and oxygen, alloys with iron in the cores. Reaction with hydrogen atmospheres and metal–silicate equilibrium thus provides a simple explanation for fundamental features of Earth’s geochemistry that is consistent with rocky planet formation across the Galaxy. Thermodynamic modelling shows that Earth’s water, core density and overall oxidation state can be explained by the formation of Earth from planetary embryos with hydrogen-rich primary atmospheres and underlying magma oceans.

Earth has had oceans for nearly four billion years 1 and Mars had lakes and rivers 3.5–3.8 billion years ago 2 . However, it is still unknown whether water has ever condensed on the surface of Venus 3 , 4 because the planet—now completely dry 5 —has undergone global resurfacing events that obscure most of its history 6 , 7 . The conditions required for water to have initially condensed on the surface of Solar System terrestrial planets are highly uncertain, as they have so far only been studied with one-dimensional numerical climate models 3 that cannot account for the effects of atmospheric circulation and clouds, which are key climate stabilizers. Here we show using three-dimensional global climate model simulations of early Venus and Earth that water clouds—which preferentially form on the nightside, owing to the strong subsolar water vapour absorption—have a strong net warming effect that inhibits surface water condensation even at modest insolations (down to 325 watts per square metre, that is, 0.95 times the Earth solar constant). This shows that water never condensed and that, consequently, oceans never formed on the surface of Venus. Furthermore, this shows that the formation of Earth’s oceans required much lower insolation than today, which was made possible by the faint young Sun. This also implies the existence of another stability state for present-day Earth: the ‘steam Earth’, with all the water from the oceans evaporated into the atmosphere. Global climate model simulations of early Venus and Earth show that differences in the cloud regimes prevented ocean formation on Venus but not on Earth.

NASA’S Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at the near-Earth asteroid (101955) Bennu, a primitive body that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth1. Bennu is a low-albedo B-type asteroid2 that has been linked to organic-rich hydrated carbonaceous chondrites3. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Therefore, the primary mission objective is to return a sample of Bennu to Earth that is pristine—that is, not affected by these processes4. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu’s global properties, support the selection of a sampling site and document that site at a sub-centimetre scale5,6,7,8,9,10,11. Here we consider early OSIRIS-REx observations of Bennu to understand how the asteroid’s properties compare to pre-encounter expectations and to assess the prospects for sample return. The bulk composition of Bennu appears to be hydrated and volatile-rich, as expected. However, in contrast to pre-encounter modelling of Bennu’s thermal inertia12 and radar polarization ratios13—which indicated a generally smooth surface covered by centimetre-scale particles—resolved imaging reveals an unexpected surficial diversity. The albedo, texture, particle size and roughness are beyond the spacecraft design specifications. On the basis of our pre-encounter knowledge, we developed a sampling strategy to target 50-metre-diameter patches of loose regolith with grain sizes smaller than two centimetres4. We observe only a small number of apparently hazard-free regions, of the order of 5 to 20 metres in extent, the sampling of which poses a substantial challenge to mission success.

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 System 11 . 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 Earth 14 . Dynamical simulations of the early Solar System show that the giant planets’ instability was triggered by the dispersal of the Sun’s gaseous disk, constrained by astronomical observations to be a few to ten million years after the birth of the Solar System.

Gas-giant planets are widely thought to form from solid ‘cores’ of roughly ten Earth masses; simulations now show that such cores can be produced from ‘pebbles’ (centimetre-to-metre-sized objects) provided that the pebbles form sufficiently slowly, leading to the formation of one to four gas giants in agreement with the observed structure of the Solar System. Accretion model for giant planet formation 'Pebble accretion' models for the formation of the solid cores of gas giant planets assume that centimetre- to metre-sized objects or pebbles are concentrated by aerodynamic drag and then gravitationally collapse to form 100–1,000 km objects. These 'planetesimals' would then efficiently accrete leftover pebbles to produce cores in only a few thousand years. However, simulations suggest that rather than creating a few large planetary cores, the models produce a population of hundreds of Earth-mass objects. Harold Levison et al . resolve this difficulty by showing that if the pebbles form sufficiently slowly, gravitational interactions between the planetesimals can lead to the formation of one to four gas giants, in agreement with the observed structure of the Solar System. 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 1 , 2 . 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 4 , 5 . 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 6 , 7 , 8 , 9 . These ‘planetesimals’ can then efficiently accrete left-over pebbles 10 and directly form the cores of giant planets 11 , 12 . This model is known as ‘pebble accretion’; theoretically, it can produce cores of ten Earth masses in only a few thousand years 11 , 13 . Unfortunately, full simulations of this process 13 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.

The formation of planetesimals is expected to occur via particle-gas instabilities that concentrate dust into self-gravitating clumps 1 – 3 . Triggering these instabilities requires the prior pile-up of dust in the protoplanetary disk 4 , 5 . This has been successfully modelled exclusively at the disk’s snowline 6 – 9 , whereas rocky planetesimals in the inner disk were only obtained by assuming either unrealistically large particle sizes 10 , 11 or an enhanced global disk metallicity 12 . However, planetesimal formation solely at the snowline is difficult to reconcile with the early and contemporaneous formation of iron meteorite parent bodies with distinct oxidation states 13 , 14 and isotopic compositions 15 , indicating formation at different radial locations in the disk. Here, by modelling the evolution of a disk with ongoing accretion of material from the collapsing molecular cloud 16 – 18 , we show that planetesimal formation may have been triggered within the first 0.5 million years by dust pile-up at both the snowline (at ~5  au ) and the silicate sublimation line (at ~1  au ), provided turbulent diffusion was low. Particle concentration at ~1  au is due to the early outward radial motion of gas 19 and is assisted by the sublimation and recondensation of silicates 20 , 21 . Our results indicate that, although the planetesimals at the two locations formed about contemporaneously, those at the snowline accreted a large fraction of their mass (~60%) from materials delivered to the disk in the first few tens of thousands of years, whereas this fraction is only 30% for the planetesimals formed at the silicate line. Thus, provided that the isotopic composition of the delivered material changed with time 22 , these two planetesimal populations should have distinct isotopic compositions, consistent with observations 15 . An evolutionary model of the solar protoplanetary disk that includes the decrease of its viscosity with time and the accretion of gas from the interstellar medium shows that planetesimals formed simultaneously in two locations: at the water snowline (~5  au ) and at the silicate sublimation line (~1  au ), explaining the observed isotopic dichotomy of iron meteorites.

dust particles at comet 67P/Churyumov–Gerasimenko confirm that the particles are aggregates of smaller, elongated grains even at the smallest sizes examined. Rosetta's brush with cometary dust Mark Bentley et al . report in situ measurements of dust particles at comet 67P/Churyumov–Gerasimenko, obtained using the MIDAS instrument on the Rosetta spacecraft, the first atomic force microscope to be launched into space. The particles are revealed to be aggregates of smaller, elongated grains, with structures at distinct sizes indicating hierarchical aggregation. The dust particles show a variety of morphologies, from compact single grains to large porous aggregate particles, similar to chondritic porous interplanetary dust particles, supporting the suggestion that the latter are of cometary origin. Comets are thought to preserve almost pristine dust particles, thus providing a unique sample of the properties of the early solar nebula. The microscopic properties of this dust played a key part in particle aggregation during the formation of the Solar System 1 , 2 . Cometary dust was previously considered to comprise irregular, fluffy agglomerates on the basis of interpretations of remote observations in the visible and infrared 3 , 4 , 5 , 6 and the study of chondritic porous interplanetary dust particles 7 that were thought, but not proved, to originate in comets. Although the dust returned by an earlier mission 8 has provided detailed mineralogy of particles from comet 81P/Wild, the fine-grained aggregate component was strongly modified during collection 9 . Here we report in situ measurements of dust particles at comet 67P/Churyumov–Gerasimenko. The particles are aggregates of smaller, elongated grains, with structures at distinct sizes indicating hierarchical aggregation. Topographic images of selected dust particles with sizes of one micrometre to a few tens of micrometres show a variety of morphologies, including compact single grains and large porous aggregate particles, similar to chondritic porous interplanetary dust particles. The measured grain elongations are similar to the value inferred for interstellar dust and support the idea that such grains could represent a fraction of the building blocks of comets. In the subsequent growth phase, hierarchical agglomeration could be a dominant process 10 and would produce aggregates that stick more easily at higher masses and velocities than homogeneous dust particles 11 . The presence of hierarchical dust aggregates in the near-surface of the nucleus of comet 67P also provides a mechanism for lowering the tensile strength of the dust layer and aiding dust release 12 .

Main-belt comets are small Solar System bodies located in the asteroid belt that repeatedly exhibit comet-like activity (that is, dust comae or tails) during their perihelion passages, strongly indicating ice sublimation 1 , 2 . Although the existence of main-belt comets implies the presence of extant water ice in the asteroid belt, no gas has been detected around these objects despite intense scrutiny with the world’s largest telescopes 3 . Here we present James Webb Space Telescope observations that clearly show that main-belt comet 238P/Read has a coma of water vapour, but lacks a significant CO 2 gas coma. Our findings demonstrate that the activity of comet Read is driven by water–ice sublimation, and implies that main-belt comets are fundamentally different from the general cometary population. Whether or not comet Read experienced different formation circumstances or evolutionary history, it is unlikely to be a recent asteroid belt interloper from the outer Solar System. On the basis of these results, main-belt comets appear to represent a sample of volatile material that is currently unrepresented in observations of classical comets and the meteoritic record, making them important for understanding the early Solar System’s volatile inventory and its subsequent evolution. Using James Webb Space Telescope observations, spectroscopic identification of a coma of water vapour but no significant CO 2 gas coma is found for the main-belt comet 238P/Read, indicating water–ice sublimation.

A critical step toward the emergence of planets in a protoplanetary disk is the accretion of planetesimals, bodies 1–1,000 km in size, from smaller disk constituents. This process is poorly understood, partly because we lack good observational constraints on the complex physical processes that contribute to planetesimal formation1. In the outer Solar System, the best place to look for clues is the Kuiper belt, where icy planetesimals survive to this day. Here we report evidence that Kuiper belt planetesimals formed by the streaming instability, a process in which aerodynamically concentrated clumps of pebbles gravitationally collapse into 100-kilometre-class bodies2. Gravitational collapse has previously been suggested to explain the ubiquity of equal-sized binaries in the Kuiper belt3–5. We analyse new hydrodynamical simulations of the streaming instability to determine the model expectations for the spatial orientation of binary orbits. The predicted broad inclination distribution with approximately 80% of prograde binary orbits matches the observations of trans-Neptunian binaries6. The formation models that imply predominantly retrograde binary orbits (for example, ref. 7) can be ruled out. Given its applicability over a wide range of protoplanetary disk conditions8, it is expected that the streaming instability also seeded planetesimal formation elsewhere in the Solar System, and beyond.The predominantly prograde orientation and broad inclination distribution of trans-Neptunian binary objects is reproduced by a three-dimensional hydrodynamical simulation of planetesimal formation driven by the streaming instability, showing evidence of the activation of the streaming instability in the solar protoplanetary disk.

A model of the Moon’s tidal evolution, starting from the fast-spinning, high-obliquity Earth that would be expected after a giant impact, reveals that solar perturbations on the Moon’s orbit naturally produce the current lunar inclination and Earth’s low obliquity. An explanation of Earth's low obliquity Matija Ćuk et al . show that tidal dissipation due to lunar obliquity may have been an important effect during the Moon's tidal evolution, in which case the past lunar inclination would have been larger than can be explained by present theoretical models. They instead propose a tidal evolution model that starts with the Moon in an equatorial orbit around an initially fast-spinning, high-obliquity Earth—a plausible outcome of giant impacts. In this model the solar perturbations on the Moon's orbit naturally induce a large lunar inclination and remove angular momentum from the Earth–Moon system. In the giant-impact hypothesis for lunar origin, the Moon accreted from an equatorial circum-terrestrial disk; however, the current lunar orbital inclination of five degrees requires a subsequent dynamical process that is still unclear 1 , 2 , 3 . In addition, the giant-impact theory has been challenged by the Moon’s unexpectedly Earth-like isotopic composition 4 , 5 . Here we show that tidal dissipation due to lunar obliquity was an important effect during the Moon’s tidal evolution, and the lunar inclination in the past must have been very large, defying theoretical explanations. We present a tidal evolution model starting with the Moon in an equatorial orbit around an initially fast-spinning, high-obliquity Earth, which is a probable outcome of giant impacts. Using numerical modelling, we show that the solar perturbations on the Moon’s orbit naturally induce a large lunar inclination and remove angular momentum from the Earth–Moon system. Our tidal evolution model supports recent high-angular-momentum, giant-impact scenarios to explain the Moon’s isotopic composition 6 , 7 , 8 and provides a new pathway to reach Earth’s climatically favourable low obliquity.