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Determining how and when Mars formed has been a long-standing challenge for planetary scientists. The size and orbit of Mars are difficult to reproduce in classical simulations of planetary accretion, and this has inspired models of inner solar system evolution that are tuned to produce Mars-like planets. However, such models are not always coupled to geochemical constraints. Analyses of Martian meteorites using the extinct hafnium–tungsten (Hf–W) radioisotopic system, which is sensitive to the timing of core formation, have indicated that the Martian core formed within a few million years of the start of the solar system itself. This has been interpreted to suggest that, unlike Earth’s protracted accretion, Mars grew to its modern size very rapidly. These arguments, however, generally rely on simplified growth histories for Mars. Here, we combine likely accretionary histories from a large number of N-body simulations with calculations of metal–silicate partitioning and Hf–W isotopic evolution during core formation to constrain the range of conditions that could have produced Mars. We find that there is no strong correlation between the final masses or orbits of simulated Martian analogs and their 182W anomalies, and that it is readily possible to produce Mars-like Hf–W isotopic compositions for a variety of accretionary conditions. The Hf–W signature of Mars is very sensitive to the oxygen fugacity (fO2) of accreted material because the metal–silicate partitioning behavior of W is strongly dependent on redox conditions. The average fO2 of Martian building blocks must fall in the range of 1.3–1.6 log units below the iron–wüstite buffer to produce a Martian mantle with the observed Hf/W ratio. Other geochemical properties (such as sulfur content) also influence Martian 182W signatures, but the timing of accretion is a more important control. We find that while Mars must have accreted most of its mass within ~5 million years of solar system formation to reproduce the Hf–W isotopic constraints, it may have continued growing afterwards for over 50 million years. There is a high probability of simultaneously matching the orbit, mass, and Hf–W signature of Mars even in cases of prolonged accretion if giant impactor cores were poorly equilibrated and merged directly with the proto-Martian core.

The Sun’s astrophysical birth environment affected the formation and composition of the Solar System. Primitive meteorites display mass-independent oxygen isotope anomalies that were likely caused by ultraviolet (UV) photochemistry of CO gas-phase molecules, either (i) in the outer solar nebula by light from the young Sun or (ii) in the parent molecular cloud by light from nearby stars. However, measurements of oxygen isotopes alone cannot unambiguously constrain the UV spectrum of the source responsible for the photochemistry. Sulfur, with four stable isotopes, can be used as a more direct probe of the astrophysical environment of mass-independent photochemistry. Here, we report the in situ isotopic analysis of paired oxygen and sulfur isotope systematics in cosmic symplectite (COS), magnetite-pentlandite intergrowths, in the primitive ungrouped carbonaceous chondrite Acfer 094. We show that COS grains contain mass-independent sulfur isotope anomalies (weighted means of Δ33S = +3.84 ± 0.72‰ and Δ36S = −6.05 ± 2.25‰, 2SE) consistent with H2S photochemistry by UV from massive O and B stars close to the Solar System’s parent molecular cloud, and inconsistent with UV from the protosun. The presence of coupled mass-independent sulfur and oxygen (Δ17O = 86 ± 6‰, 2SE) isotope anomalies in COS imply that these anomalies originated in the same astrophysical environment. We propose that this environment is the photodissociation region (PDR) of the Solar System’s parent molecular cloud, where nearby massive stars irradiated the edge of the cloud. We conclude that the Sun’s stellar neighbors, likely O and B stars in a massive-star-forming region, affected the composition of the Solar System’s primordial building blocks.

Anomalies among the daughter nuclei of the extinct short-lived radionuclides (SLRs) in the calcium-aluminum-rich inclusions (CAIs) indicate that the Solar System must have been born near a source of the SLRs so that they could be incorporated before they decayed away. $\gamma$-rays from one such living SLR, $^{26}$Al, are detected in only a few nearby star-forming regions. Here we employ multi-wavelength observations to demonstrate that one such region, Ophiuchus, containing many pre-stellar cores that may serve as analogs for the emerging Solar System, is inundated with $^{26}$Al from the neighboring Upper-Scorpius association, and so may provide concrete guidance for how SLR enrichment proceeded in the Solar System complementary to the meteoritics. We demonstrate via Bayesian forward modeling drawing on a wide range of observational and theoretical results that this $^{26}$Al likely 1) arises from supernova explosions, 2) arises from multiple stars, 3) has enriched the gas prior to the formation of the cores, and 4) gives rise to a broad distribution of core enrichment spanning about two orders of magnitude. This means that if the spread in CAI ages is small, as it is in the Solar System, protoplanetary disks must suffer a global heating event., Comment: Published in Nature Astronomy, August 16, 2021

In this work, we propose a novel campaign for constraining relativistically compact massive compact halo object (MACHO) dark matter, such as primordial black holes (PBHs), using the Moon as a detector. PBHs of about 1019 to 1022 g may be sufficiently abundant to have collided with the Moon in the history of the Solar system. We show that the crater profiles of a PBH collision differ from traditional impactors and may be detectable in high-resolution lunar surface scans now available. Any candidates may serve as sites for in situ measurements to identify high-pressure phases of matter which may have formed near the PBH during the encounter. While we primarily consider PBH dark matter, the discussion generalizes to the entire family of MACHO candidates with relativistic compactness. Moreover, we focus on the Moon since it has been studied well, but the same principles can be applied to other rocky bodies in our Solar system without an atmosphere.

The developed Hypersphere World-Universe Model (WUM) is consistent with all Concepts of the World [1]. In WUM, we postulate the principal role of Angular Momentum and Dark Matter in Cosmological theories of the World. The most widely accepted model of Solar System formation, known as the Nebular hypothesis, does not solve the Angular Momentum problem—why is the orbital momentum of Jupiter larger than rotational momentum of the Sun? WUM is the only cosmological model in existence that is consistent with this Fundamental Law. The Nebular hypothesis does not solve Internal Heating and Diversity problems for all Planets and Moons in Solar system—why the actual mean surface temperature of them is higher than their effective temperature calculated based on the Sun’s heat for them and how could each one be so different if all of them came from the same nebula? The proposed concept of Dark Matter Reactors in Cores of all gravitationally-rounded Macroobjects successfully resolves these problems.

We have performed in situ analyses of Li-Be-B and Al-Mg isotope systematics, and abundances of rare earth elements (REEs) in two Ca-Al-rich inclusions (CAIs) from the Ornans-like carbonaceous chondrite Yamato-81020 (CO3.05). The present CO CAIs are depleted in ultra-refractory heavy REEs (group II REE pattern), suggesting condensation of these CAIs or their precursors from the solar nebula. Initial 26Al/27Al ratios, (26Al/27Al)0, of these CO CAIs are found to be (4.8 ± 0.5) × 10–5 and (4.9 ± 0.3) × 10–5 (95% confidence), indicating their contemporaneous formation with a majority of CAIs from Vigarano-like carbonaceous (CV) chondrites. Melilite grains in the present CO CAIs show clear excesses in 10B, ranging from ∼370 to ∼4300‰ relative to the chondritic B isotopic composition, which are correlated well with 9Be/11B ratios. The correlation indicates in situ decay of 10Be in the present CO CAIs and yields initial 10Be/9Be ratios, (10Be/9Be)0, for the individual CAIs of (2.9 ± 0.6) × 10–3 and (2.2 ± 1.0) × 10–3 (95% confidence), which are significantly greater than the average (10Be/9Be)0 = ∼0.7 × 10–3 recorded in CAIs from CV chondrites. The apparent variation in (10Be/9Be)0 between the CO and CV CAIs, despite having indistinguishable (26Al/27Al)0 of ∼5 × 10–5, provides evidence for heterogeneous distribution of 10Be in the CAI forming-regions at the very beginning of the Solar System. The elevated (10Be/9Be)0 and group II REE patterns in the CO CAIs may reflect that compared with the CV CAIs having unfractionated REEs the present CO CAIs have formed closer to the Sun where 10Be was produced more efficiently through solar cosmic ray irradiation caused by solar flares. Alternatively, if the present CO CAIs and CV CAIs formed in the same region, and 26Al was distributed homogeneously at the CAI-forming region, our results indicate that solar cosmic ray fluxes at the forming region have fluctuated by a factor of six within a short duration (∼0.2 million years) inferred from the Al-Mg chronology.

Timing Solar System formation The oldest solids that formed in the Solar System are calcium-aluminium–rich inclusions (CAIs), small metallic droplets that were later incorporated into meteorites. The ages of CAIs are conventionally taken as the age of the Solar System, but which exact moment in star formation they correspond to has been unclear. Brennecka et al. measured molybdenum isotope ratios in CAIs and found a wide range of origins in both the inner and outer Solar System. They propose that CAIs formed from heterogeneous material accreting from the presolar nebula and that the ages of CAIs coincide with the Sun's transition from a protostar to a pre–main sequence star. Science , this issue p. 837

In its 16 years of scientific measurements, the Spitzer Space Telescope performed ground-breaking and key infrared measurements of Solar System objects near and far. Targets ranged from the smallest planetesimals to the giant planets; Spitzer helped us to reshape our understanding of these objects while also laying the groundwork for future infrared space-based observations like those to be undertaken by the James Webb Space Telescope in the 2020s. In this Review Article, we describe how Spitzer advanced our knowledge of Solar System formation and evolution through observations of small outer Solar System planetesimals—that is, comets, centaurs and Kuiper belt objects (KBOs). Relics from the early formation era of our Solar System, these objects hold important information about the processes that created them.We group Spitzer’s key contributions into three broad classes: characterization of new Solar System objects (comets D/ISON 2012 S1, C/2016 R2 and 1I/‘Oumuamua); large population surveys of known objects (comets, centaurs and KBOs); and compositional studies through spectral measurements of body surfaces and emitted materials. In the Spitzer Space Telescope’s 16 years of operation, it observed many Solar System objects and environments. In this first Review Article of a pair, Spitzer’s insights into comets, centaurs and Kuiper belt objects—all remnants of the Solar System’s formation—are summarized.

The discovery of living organisms under extreme environmental conditions of pressure, temperature, and chemical composition on Earth has opened up the possibility of existence and persistence of life in extreme environment pockets across the solar system. These environments range from the many intriguing moons, to the deep atmospheres of Venus and even the giant gas planets, to the small icy worlds of comets and Kuiper Belt Objects (KBOs). Exploring these environments can ascertain the range of conditions that can support life and can also identify planetary processes that are responsible for generating and sustaining habitable worlds. These environments are also time capsules into early formation of the solar system and will provide vital clues of how our early solar system gave way to the current planets and moons. Over the last few decades, numerous missions started with flyby spacecraft, followed by orbiting satellites and missions with orbiter/lander capabilities. Since then, there have been numerous missions that have utilized rovers of ever-increasing size and complexity, equipped with state-of-the-art laboratories on wheels. Although current generations of rovers achieve mobility through wheels, there are fundamental limitations that prevent these rovers from accessing rugged environments, cliffs, canyons, and caves. These rugged environments are often the first places geologist look to observe stratification from geohistorical processes. There is an important need for new robot mobility solutions, like hopping, rolling, crawling, and walking that can access these rugged environments like cliffs, canyons, and caves. These new generations of rovers have some extraordinary capabilities including being able to grip onto rocks like NASA/JPL LEMUR 2, operate in swarms such as MIT’s microbots, or have high-specific energy fuel cell power supply that is approximately 40-fold higher than conventional lithium ion batteries to Stanford/NASA JPL’s Hedgehog which is able to hop and somersault in low-gravity environments such asteroids. All of these mobility options and supporting technologies have been proposed and developed to explore these hard-to-reach unconventional environments. This article provides a review of the robotic systems developed over the past few decades, in addition to new state-of-the-art concepts that are leading contenders for future missions to explore extreme environments on Earth and off-world.

Two ancient asteroids with distinctive diamond-like shapes are revealing clues not only about their own formation, but that of the solar system as well. For the past three years, two spacecraft have been studying a pair of the most primitive solar system objects ever explored. The asteroids, Ryugu and Bennu, born of violent collisions in the asteroid belt, are some of the smallest objects ever visited by spacecraft. Ryugu is just over a kilometer across, and Bennu is about half as wide. The asteroid Bennu ejects particles from its surface in January 2019—a phenomenon captured in an image composed from multiple photos. Image credit: NASA/Goddard/University of Arizona/Lockheed Martin. But these small bodies are promising a huge bounty of data to astronomers over the next few years. Although the findings could revolutionize our understanding of the history, formation, and composition of asteroids, they may also tell us a little about the solar system during its infancy. That’s because these carbon-rich (1) asteroids, with their coal-dark surfaces, are also among the darkest bodies known in the solar system. Although that can make it difficult to observe them from Earth, studying them could tell us about the building blocks of life that such asteroids may have delivered to Earth early in the history of the solar system. Before researchers could hope to study these asteroids, they first had to reach them. This, by itself, was an enormous challenge. Only a handful of spacecraft have visited asteroids, and fewer still have managed to orbit one of these tiny, irregularly shaped objects. The Hayabusa spacecraft, launched by the Japanese Aerospace Exploration Agency (JAXA) in 2003, became the first to collect a sample of an asteroid. But the mission was beset by engine problems. Hayabusa’s prospects for returning with a sample remained uncertain for much of …

The isotopic composition of meteorites and terrestrial planets holds important clues about the earliest history of the Solar System and the processes of planet formation. Recent work has shown that meteorites exhibit a fundamental isotopic dichotomy between non-carbonaceous (NC) and carbonaceous (CC) groups, which most likely represent material from the inner and outer Solar System, respectively. Here we review the isotopic evidence for this NC–CC dichotomy, discuss its origin and highlight the far-reaching implications for the dynamics of the solar protoplanetary disk. The NC–CC dichotomy combined with the chronology of meteorite parent-body accretion mandate an early and prolonged spatial separation of inner (NC) and outer (CC) disk reservoirs, lasting between ~1 and ~4 Myr after Solar System formation. This is most easily reconciled with the early and rapid growth of Jupiter’s core, inhibiting substantial exchange of material from inside and outside its orbit. The growth and migration of Jupiter also led to the later implantation of CC bodies into the inner Solar System and, therefore, can explain the co-occurrence of NC and CC bodies in the asteroid belt, and the delivery of volatile and water-rich CC bodies to the terrestrial planets. Cosmochemical measurements reveal the existence of two distinct reservoirs of non-carbonaceous and carbonaceous materials, originating from the inner and outer protoplanetary disk, respectively, which separated after the first million years after the birth of the Solar System, possibly due to the rapid growth of Jupiter’s core.

Type 3 chondritic meteorites often contain chondrules and refractory inclusions that are coated with accretionary, fine-grained rims (FGRs). FGRs are of low porosity, were subject to centrally directed pressure, may contain high temperature products like microchondrules and there is a linear relationship between the rim thickness and the radius of the enclosed object. FGRs are thought to have formed by the gentle adhesion of dust onto the central object with the subsequent compression of this fluffy rim within the parent body. However, this model does not explain the low porosity, micro-chondrules and centralized pressure. This model also has difficulties explaining the linear relationship between rim thickness and object size including the existence of a non-zero constant in that linear relationship. We propose that FGRs formed by the relatively high-speed interaction between dust and the object, where high initial impact speed produced abrasion and, possibly, microchondrules. FGR formation occurred over a range of lower speeds aided by vacuum adhesion of fragments from the impacting dust particles. This model naturally produces the rim thickness linear relationship with non-zero constant, low porosity and centrally directed pressure. We call this process kinetic dust aggregation (KDA), which is another name for the aerosol deposition processes used in industry. KDA may be a tentative, part explanation of how dust aggregation occurs in protostellar disks on the pathway from dust to planets., 34 pages, 7 figures

The ungrouped iron meteorite Nedagolla is the first meteorite with bulk Mo, Ru, and Ni isotopic compositions that are intermediate between those of the non-carbonaceous (NC) and carbonaceous (CC) meteorite reservoirs. The Hf-W chronology of Nedagolla indicates that this mixed NC-CC isotopic composition was established relatively late, more than 7 million years after Solar System formation. The mixed NC-CC isotopic composition is consistent with the chemical composition of Nedagolla, which combines signatures of metal segregation under more oxidizing conditions (relative depletions in Mo and W), characteristic for CC bodies, and more reducing conditions (high Si and Cr contents), characteristic for some NC bodies, in a single sample. These data combined suggest that Nedagolla formed as the result of collisional mixing of NC and CC core material, which partially re-equilibrated with silicate mantle material that predominantly derives from the NC body. These mixing processes might have occurred during a hit-and-run collision between two differentiated bodies, which also provides a possible pathway for Nedagolla's extreme volatile element depletion. As such, Nedagolla provides the first isotopic evidence for early collisional mixing of NC and CC bodies that is expected as a result of Jupiter's growth., Accepted for publication in Meteoritics & Planetary Science

Description, Paleomagnetic measurements of chondrules indicate the presence of a substructure in the solar nebula., Astronomical observations and isotopic measurements of meteorites suggest that substructures are common in protoplanetary disks and may even have existed in the solar nebula. Here, we conduct paleomagnetic measurements of chondrules in CO carbonaceous chondrites to investigate the existence and nature of these disk substructures. We show that the paleomagnetism of chondrules in CO carbonaceous chondrites indicates the presence of a 101 ± 48 μT field in the solar nebula in the outer solar system (~3 to 7 AU from the Sun). The high intensity of this field relative to that inferred from inner solar system (~

We describe the AMBITION project, a mission to return the first-ever cryogenically-stored sample of a cometary nucleus, that has been proposed for the ESA Science Programme Voyage 2050. Comets are the leftover building blocks of giant planet cores and other planetary bodies, and fingerprints of Solar System’s formation processes. We summarise some of the most important questions still open in cometary science and Solar System formation after the successful Rosetta mission. We show that many of these scientific questions require sample analysis using techniques that are only possible in laboratories on Earth. We summarize measurements, instrumentation and mission scenarios that can address these questions. We emphasize the need for returning a sample collected at depth or, still more challenging, at cryogenic temperatures while preserving the stratigraphy of the comet nucleus surface layers. We provide requirements for the next generation of landers, for cryogenic sample acquisition and storage during the return to Earth. Rendezvous missions to the main belt comets and Centaurs, expanding our knowledge by exploring new classes of comets, are also discussed. The AMBITION project is discussed in the international context of comet and asteroid space exploration.

The formation of the solar system's giant planets predated the ultimate epoch of massive impacts that concluded the process of terrestrial planet formation. Following their formation, the giant planets' orbits evolved through an episode of dynamical instability. Several qualities of the solar system have recently been interpreted as evidence of this event transpiring within the first ~100 Myr after the Sun's birth; around the same time as the final assembly of the inner planets. In a series of recent papers we argued that such an early instability could resolve several problems revealed in classic numerical studies of terrestrial planet formation; namely the small masses of Mars and the asteroid belt. In this paper, we revisit the early instability scenario with a large suite of simulations specifically designed to understand the degree to which Earth and Mars' formation are sensitive to the specific evolution of Jupiter and Saturn's orbits. By deriving our initial terrestrial disks directly from recent high-resolution simulations of planetesimal accretion, our results largely confirm our previous findings regarding the instability's efficiency of truncating the terrestrial disk outside of the Earth-forming region in simulations that best replicate the outer solar system. Moreover, our work validates the primordial 2:1 Jupiter-Saturn resonance within the early instability framework as a viable evolutionary path for the solar system. While our simulations elucidate the fragility of the terrestrial system during the epoch of giant planet migration, many realizations yield outstanding solar system analogs when scrutinized against a number of observational constraints. Finally, we highlight the inability of models to form adequate Mercury-analogs and the low eccentricities of Earth and Venus as the most significant outstanding problems for future numerical studies to resolve., 24 pages, 10 figures, 6 tables, accepted for publication in Icarus

We present a chronology of the formation and early evolution of the Oort cloud by simulations. These simulations start with the Solar System being born with planets and asteroids in a stellar cluster orbiting the Galactic center. Upon ejection from its birth environment, we continue to follow the evolution of the Solar System while it navigates the Galaxy as an isolated planetary system. We conclude that the range in semi-major axis between 100au and several 10$^3$\,au still bears the signatures of the Sun being born in a 1000MSun/pc$^3$ star cluster, and that most of the outer Oort cloud formed after the Solar System was ejected. The ejection of the Solar System, we argue, happened between 20Myr and 50Myr after its birth. Trailing and leading trails of asteroids and comets along the Sun's orbit in the Galactic potential are the by-product of the formation of the Oort cloud. These arms are composed of material that became unbound from the Solar System when the Oort cloud formed. Today, the bulk of the material in the Oort cloud ($\sim 70$\%) originates from the region in the circumstellar disk that was located between $\sim 15$\,au and $\sim 35$\,au, near the current location of the ice giants and the Centaur family of asteroids. According to our simulations, this population is eradicated if the ice-giant planets are born in orbital resonance. Planet migration or chaotic orbital reorganization occurring while the Solar System is still a cluster member is, according to our model, inconsistent with the presence of the Oort cloud. About half the inner Oort cloud, between 100 and $10^4$\,au, and a quarter of the material in the outer Oort cloud, $\apgt 10^4$\,au, could be non-native to the Solar System but was captured from free-floating debris in the cluster or from the circumstellar disk of other stars in the birth cluster., Corrected proofs for publication in A&A

The evolutionary system of mineralogy relies on varied physical and chemical attributes, including trace elements, isotopes, solid and fluid inclusions, and other information-rich characteristics, to understand processes of mineral formation and to place natural condensed phases in the deep-time context of planetary evolution. Part I of this system reviewed the earliest refractory phases that condense at T > 1000 K within the turbulent expanding and cooling atmospheres of highly evolved stars. Part II considers the subsequent formation of primary crystalline and amorphous phases by condensation in three distinct mineral-forming environments, each of which increased mineralogical diversity and distribution prior to the accretion of planetesimals >4.5 billion years ago. (1). INTERSTELLAR MOLECULAR SOLIDS: Varied crystalline and amorphous molecular solids containing primarily H, C, O, and N are observed to condense in cold, dense molecular clouds in the interstellar medium (10 < T < 20 K; P < 10(−13) atm). With the possible exception of some nanoscale organic condensates preserved in carbonaceous meteorites, the existence of these phases is documented primarily by telescopic observations of absorption and emission spectra of interstellar molecules in radio, microwave, or infrared wavelengths. (2). NEBULAR AND CIRCUMSTELLAR ICE: Evidence from infrared observations and laboratory experiments suggest that cubic H(2)O (“cubic ice”) condenses as thin crystalline mantles on oxide and silicate dust grains in cool, distant nebular and circumstellar regions where T ~100 K. (3). PRIMARY CONDENSED PHASES OF THE INNER SOLAR NEBULA: The earliest phase of nebular mineralogy saw the formation of primary refractory minerals that solidified through high-temperature condensation (1100 < T < 1800 K; 10(−6) < P < 10(−2) atm) in the solar nebula more than 4.565 billion years ago. These earliest mineral phases originating in our solar system formed prior to the accretion of planetesimals and are preserved in calcium-aluminum-rich inclusions, ultra-refractory inclusions, and amoeboid olivine aggregates.

The date of the Moon-forming impact places an important constraint on Earth’s origin. Lunar age estimates range from about 30 Myr to 200 Myr after Solar System formation. Central to this age debate is the greater abundance of 182W inferred for the silicate Moon than for the bulk silicate Earth. This compositional difference has been explained as a vestige of less late accretion to the Moon than to the Earth after core formation. Here we present high-precision trace element composition data from inductively coupled plasma mass spectrometry for a wide range of lunar samples. Our measurements show that the Hf/W ratio of the silicate Moon is higher than that of the bulk silicate Earth. By combining these data with experimentally derived partition coefficients, we found that the 182W excess in lunar samples can be explained by the decay of the now extinct 182Hf to 182W. 182Hf was only extant for the first 60 Myr after the Solar System formation. We conclude that the Moon formed early, approximately 50 Myr after the Solar System, and that the excess 182W of the silicate Moon is unrelated to late accretion. The Moon formed around 50 Myr after the Solar System, suggests a lunar silicate Hf/W ratio higher than that of Earth, from high-precision compositional analysis of lunar rock samples.

A growing body of evidence indicates that binary neutron-star mergers are the primary origin of heavy elements produced exclusively through rapid neutron capture1-4 (the 'r-process'). As neutron-star mergers occur infrequently, their deposition of radioactive isotopes into the pre-solar nebula could have been dominated by a few nearby events. Although short-lived r-process isotopes-with half-lives shorter than 100 million years-are no longer present in the Solar System, their abundances in the early Solar System are known because their daughter products were preserved in high-temperature condensates found in meteorites5. Here we report that abundances of short-lived r-process isotopes in the early Solar System point to their origin in neutron-star mergers, and indicate substantial deposition by a single nearby merger event. By comparing numerical simulations with the early Solar System abundance ratios of actinides produced exclusively through the r-process, we constrain the rate of occurrence of their Galactic production sites to within about 1-100 per million years. This is consistent with observational estimates of neutron-star merger rates6-8, but rules out supernovae and stellar sources. We further find that there was probably a single nearby merger that produced much of the curium and a substantial fraction of the plutonium present in the early Solar System. Such an event may have occurred about 300 parsecs away from the pre-solar nebula, approximately 80 million years before the formation of the Solar System.

著者人数: 13名, Accepted: 2019-01-22, 資料番号: SA1180366000

The dependence of the region of physically admissible values of uranium-thorium isotope ratios on the short-term change in nucleosynthesis rate, which increases just before the formation of the Solar system, is studied. An additional admissible region associated with the presence of the isotope 244Pu and the effect of this region on the calculation of the age of the Universe, TU, are considered within galactic-nucleosynthsis theory. The size of the admissible region and its dependence on the short-term increase in the rate of heavy-element production (enhancement of nucleosynthsis) before the formation of the Solar System is discussed along with the consistency of the predictions for the above ratios with the region of admissible values. It is shown that an enhancement of nucleosynthesis is necessary for attaining agreement between the calculated ratios of cosmochronometer nuclei and the region of admissible values, but this enhancement should not lead to an increase in the abundances of heavy elements that is greater than 1 to 3% of the total amount of synthesized heavy elements.

Impacts on undifferentiated asteroidal bodies provide invaluable information for understanding the solar system evolution. Nevertheless, dating early and small-scale impact events is technically challenging. Uranium-lead (U-Pb) systematics of U-bearing phases within shock-induced melt veins (SMVs) of chondrites may be significantly disturbed by localized heating, thus providing opportunities to date these impact events. As one of the major U hosts in chondrites, apatite in the Suizhou (L6) chondrite has been compositionally and structurally modified to varying degrees by shock metamorphism. Apatite grains in the host remained largely intact during the impact and have recorded the initial thermal cooling time (similar to 4550 Ma) on their parent body. Apatite grains in regions less than 100 mu m bordering the SMVs or in relatively "cold" regions within the SMVs were partly decomposed to tuite, driven by the localized transient heating within the SMVs. Their U-Pb systematics were disrupted to varying extents. Apatite in regions close to the center of the SMVs has been completely transformed to tuite, which yields an age of 4481 +/- 30 Ma (2 sigma), providing an upper limit to the impact event. This study clearly demonstrates that by integrating in situ U-Pb isotope analysis with detailed microstructural and compositional analysis of phosphates, it is possible to deduce the timing of early and small-scale celestial impact events, and hence create a more comprehensive understanding of the impact history of the solar system.

The proposed US Extremely Large Telescope (ELT) Program would secure national open access to at least 25% of the observing time on the Thirty Meter Telescope in the north and the Giant Magellan Telescope in the south. ELTs would advance solar system science via exceptional angular resolution, sensitivity, and advanced instrumentation. ELT contributions would include the study of interstellar objects, giant planet systems and ocean worlds, the formation of the solar system traced through small objects in the asteroid and Kuiper belts, and the active support of planetary missions. We recommend that (1) the US ELT Program be listed as critical infrastructure for solar system science, that (2) some support from NASA be provided to ensure mission support capabilities, and that (3) the US ELT Program expand solar-system community participation in development, planning, and operations.

The current composition of giant planet atmospheres provides information on how such planets formed, and on the origin of the solid building blocks that contributed to their formation. Noble gas abundances and their isotope ratios are among the most valuable pieces of evidence for tracing the origin of the materials from which the giant planets formed. In this review we first outline the current state of knowledge for heavy element abundances in the giant planets and explain what is currently understood about the reservoirs of icy building blocks that could have contributed to the formation of the Ice Giants. We then outline how noble gas isotope ratios have provided details on the original sources of noble gases in various materials throughout the solar system. We follow this with a discussion on how noble gases are trapped in ice and rock that later became the building blocks for the giant planets and how the heavy element abundances could have been locally enriched in the protosolar nebula. We then provide a review of the current state of knowledge of noble gas abundances and isotope ratios in various solar system reservoirs, and discuss measurements needed to understand the origin of the ice giants. Finally, we outline how formation and interior evolution will influence the noble gas abundances and isotope ratios observed in the ice giants today. Measurements that a future atmospheric probe will need to make include (1) the 3He/4He isotope ratio to help constrain the protosolar D/H and 3He/4He; (2) the 20Ne/22Ne and 21Ne/22Ne to separate primordial noble gas reservoirs similar to the approach used in studying meteorites; (3) the Kr/Ar and Xe/Ar to determine if the building blocks were Jupiter-like or similar to 67P/C-G and Chondrites; (4) the krypton isotope ratios for the first giant planet observations of these isotopes; and (5) the xenon isotopes for comparison with the wide range of values represented by solar system reservoirs.Mandt, K. E., Mousis, O., Lunine, J., Marty, B., Smith, T., Luspay-Kuti, A., & Aguichine, A. (2020). Tracing the origins of the ice giants through noble gas isotopic composition. Space Science Reviews, 216(5), 1-37.

Geochemical and astronomical evidence demonstrate that planet formation occurred in two spatially and temporally separated reservoirs. The origin of this dichotomy is unknown. We use numerical models to investigate how the evolution of the solar protoplanetary disk influenced the timing of protoplanet formation and their internal evolution. Migration of the water snow line can generate two distinct bursts of planetesimal formation that sample different source regions. These reservoirs evolve in divergent geophysical modes and develop distinct volatile contents, consistent with constraints from accretion chronology, thermo-chemistry, and the mass divergence of inner and outer Solar System. Our simulations suggest that the compositional fractionation and isotopic dichotomy of the Solar System was initiated by the interplay between disk dynamics, heterogeneous accretion, and internal evolution of forming protoplanets., Comment: Published 21 January 2021; authors' version; 30 pages, 18 figures; summary available at http://bit.ly/BifurcationBlog (blog) and https://bit.ly/BifurcationVideo (video)

Calcium-aluminum-rich inclusions (CAIs) are the first solids to form in the early Solar System, and they exhibit nucleosynthetic anomalies in many isotope systems. The overwhelming majority of isotopic data for CAIs has been limited to inclusions from the CV chondrite Allende and a select few other CV, CO, CM, and ordinary chondrites. It is therefore important to ascertain whether previously reported values for CAIs are representative of the broader CAI-forming region and to make a more rigorous assessment of the extent and implications of isotopic heterogeneity in the early Solar System. Here, we report the mass-independent Ti isotopic compositions of a suite of 23 CAIs of diverse petrologic and geochemical types, including 11 from Allende and 12 from seven other CV3 and CK3 chondrites; the data for CAIs from CK chondrites represent the first reported measurements of Ti isotope compositions of refractory inclusions from this meteorite class. The resolved variation in the mass-independent Ti isotopic compositions of these CAIs indicates that the CAI-forming region of the early Solar System preserved isotopic variability at their time of formation. Nevertheless, the range of Ti isotope compositions reported here for CAIs from CV and CK chondrites falls within the range observed in previously analyzed CAIs from CV, CO, CM, and ordinary chondrites. This implies that CAIs from CV, CK, CO, CM, and ordinary chondrites originated from a common nebular source reservoir characterized by mass-independent isotopic variability in Ti (and other select elements). We further interpret these data to indicate that the Ti isotope anomalies in CAIs represent the isotopic signatures of supernova components in presolar grains that were incorporated into the Solar System in an initially poorly mixed reservoir that was progressively homogenized over time. We conclude that the differing degrees of isotopic variability observed for different elements in normal CAIs are the result of distinct carrier phases and that these CAIs were likely formed towards the final stages of homogenization of the large-scale isotopic heterogeneity that initially existed in the solar nebula.

The compositions of the first solar system solids were shaped by nonequilibrium condensation and evaporation., Equilibrium condensation of solar gas is often invoked to explain the abundance of refractory elements in planets and meteorites. This is partly motivated, by the observation that the depletions in both the least and most refractory rare earth elements (REEs) in meteoritic group II calcium-aluminum–rich inclusions (CAIs) can be reproduced by thermodynamic models of solar nebula condensation. We measured the isotopic compositions of Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb in eight CAIs to test this scenario. Contrary to expectation for equilibrium condensation, we find light isotope enrichment for the most refractory REEs and more subdued isotopic variations for the least refractory REEs. This suggests that group II CAIs formed by a two-stage process involving fast evaporation of preexisting materials, followed by near-equilibrium recondensation. The calculated time scales are consistent with heating in events akin to FU Orionis– or EX Lupi–type outbursts of eruptive pre–main-sequence stars.

Historically the information for observational astronomers has largely been provided by electromagnetic radiation, and the arrival of astronomy via neutrinos and gravitational waves has led to the term “multimessenger astronomy”. But we have already seen that cosmic rays, which have been known for over a century, are a different messenger, and in this chapter we deal with another messenger, which has been somewhat overlooked in the excitement. This is material from outside the Earth: mostly meteorites, but also rocks from the Moon, more recently from comets, asteroids, and even Mars. This material can be analysed in the laboratory by chemical and radiochemical techniques. The basic composition of meteorites includes chondrules, which crystallised in space, refractory inclusions, which incorporate particles similar to that of the gas forming the solar nebula, and a matrix including pre-solar dust grains. Isotope ratios and the composition of the radioactive nuclides can be used to infer developments in the history and pre-history of the solar system. Specific large meteorites provide a particularly rich field of exploration. The composition of different types of meteorites is checked against the predictions of stellar “dust factories”, notably stars in the “asymptotic giant branch (AGB) phase of evolution. In this chapter I discuss the results of the examination of meteorites, but also outline specific missions to bring back material from comets and asteroids, and some key results of the analysis of the Moon rocks brought to Earth by the Apollo astronauts, and results from the penultimate NASA mission to Mars.

Recent observations of the long-period comet C/2016 R2 (PanSTARRS; hereafter R2) indicate an unusually high N2/CO abundance ratio, typically larger than ∼0.05, and at least 2–3 times higher than the one measured in 67P/Churyumov–Gerasimenko. Another striking compositional feature of this comet is its heavy depletion in H2O (H2O/CO ∼ 0.32%), compared to other comets. Here we investigate the formation circumstances of a generic comet whose composition reproduces these two key features. We first envisage the possibility that this comet agglomerated from clathrates, but we find that such a scenario does not explain the observed low water abundance. We then alternatively investigate the possibility that the building blocks of R2 agglomerated from grains and pebbles made of pure condensates via the use of a disk model describing the radial transport of volatiles. We show that N2/CO ratios reproducing the value estimated in this comet can be found in grains condensed in the vicinity of the CO and N2 ice lines. Moreover, high CO/H2O ratios (>100 times the initial gas-phase value) can be found in grains condensed in the vicinity of the CO ice line. If the building blocks of a comet assembled from such grains, they should present N2/CO and CO/H2O ratios consistent with the measurements made in R2’s coma. Our scenario indicates that R2 formed in a colder environment than the other comets that share more usual compositions. Our model also explains the unusual composition of the interstellar comet 2l/Borisov.

Molecular D/H ratios are frequently used to probe the chemical past of Solar System volatiles. Yet it is unclear which parts of the Solar Nebula hosted an active deuterium fractionation chemistry. To address this question, we present 0".2-0".4 ALMA observations of DCO+ and DCN 2-1, 3-2 and 4-3 towards the nearby protoplanetary disk around TW Hya, taken as part of the TW Hya Rosetta Stone project, augmented with archival data. DCO+ is characterized by an excitation temperature of ~40 K across the 70 au radius pebble disk, indicative of emission from a warm, elevated molecular layer. Tentatively, DCN is present at even higher temperatures. Both DCO+ and DCN present substantial emission cavities in the inner disk, while in the outer disk the DCO+ and DCN morphologies diverge: most DCN emission originates from a narrow ring peaking around 30~au, with some additional diffuse DCN emission present at larger radii, while DCO+ is present in a broad structured ring that extends past the pebble disk. Based on parametric disk abundance models, these emission patterns can be explained by a near-constant DCN abundance exterior to the cavity, and an increasing DCO+ abundance with radius. There appears to be an active deuterium fractionation chemistry in multiple disk regions around TW Hya, but not in the cold planetesimal-forming midplane and in the inner disk. More observations are needed to explore whether deuterium fractionation is actually absent in these latter regions, and if its absence is a common feature, or something peculiar to the old TW Hya disk., Comment: Accepted for publication in The Astronomical Journal

We review recent advances in our understanding of magnetism in the solar nebular and protoplanetary disks (PPDs). We discuss the implications of theory, meteorite measurements, and astronomical observations for planetary formation and nebular evolution. Paleomagnetic measurements indicate the presence of fields of 0.54$\pm$0.21 G at $\sim$1 to 3 astronomical units (AU) from the Sun and $\gtrsim$0.06 G at 3 to 7 AU until >1.22 and >2.51 million years (Ma) after solar system formation, respectively. These intensities are consistent with those predicted to enable typical astronomically-observed protostellar accretion rates of $\sim$10$^{-8}$ M$_\odot$ yr$^{-1}$, suggesting that magnetism played a central role in mass and angular momentum transport in PPDs. Paleomagnetic studies also indicate fields, Revised version published in Science Advances

Chapter 9 sketched the labours of elite mathematicians across a century and a half from 1846 to about 1990, discarding Bode’s Law and seeking to refine celestial mechanics in order to narrow the search area and discover further planets in the Outer Solar System. All, including the long-held Laplacian Theory for formation of the Solar System, would be swept aside, as Solar System studies based on Isaac Newton’s most exacting science have been rejuvenated by supercomputers.

This white paper advocates for the inclusion of small, captured Outer Solar system objects, found in the Ice Giant region in the next Decadal Survey. These objects include the Trojans and Irregular satellite populations of Uranus and Neptune. The captured small bodies provide vital clues as to the formation of our Solar system. They have unique dynamical situations, which any model of Solar system formation needs to explain. The major issue is that so few of these objects have been discovered, with very little information known about them. The purpose of this document is to prioritize further discovery and characterization of these objects. This will require the use of NASA and NSF facilities over the 2023 2032 decade, including additional support for analysis. This is in preparation for potential future insitu missions in the following decades., Comment: Community Science White Paper for the Planetary and Astrobiology Decadal Survey, 2023-2032

Understanding the origin of life-essential volatiles like N in the Solar System and beyond is critical to evaluate the potential habitability of rocky planets. Whether the inner Solar System planets accreted these volatiles from their inception or had an exogenous delivery from the outer Solar System is, however, not well understood. Using previously published data of nucleosynthetic anomalies of Ni, Mo, W and Ru in iron meteorites along with their 15N-14N ratios, here we show that the earliest formed protoplanets in the inner and outer protoplanetary disk accreted isotopically distinct N. While the Sun and Jupiter captured N from nebular gas, concomitantly growing protoplanets in the inner and outer disk possibly sourced their N from organics and/or dust - with each reservoir having a different N isotopic composition. A distinct N isotopic signature of the inner Solar System protoplanets coupled with their rapid accretion suggests that non-nebular, isotopically processed N was ubiquitous in their growth zone at 0-0.3 Myr after the formation of CAIs. Because 15N-14N ratio of the bulk silicate Earth falls between that of inner and outer Solar System reservoirs, we infer that N in the present-day rocky planets represents a mixture of both inner and outer Solar System material., Comment: 29 pgaes, 4 figures, 3 extended data figures

The loss and gain of volatile elements during planet formation is key for setting their subsequent climate, geodynamics, and habitability. Two broad regimes of volatile element transport in and out of planetary building blocks have been identified: that occurring when the nebula is still present, and that occurring after it has dissipated. Evidence for volatile element loss in planetary bodies after the dissipation of the solar nebula is found in the high Mn to Na abundance ratio of Mars, the Moon, and many of the solar system's minor bodies. This volatile loss is expected to occur when the bodies are heated by planetary collisions and short-lived radionuclides, and enter a global magma ocean stage early in their history. The bulk composition of exo-planetary bodies can be determined by observing white dwarfs which have accreted planetary material. The abundances of Na, Mn, and Mg have been measured for the accreting material in four polluted white dwarf systems. Whilst the Mn/Na abundances of three white dwarf systems are consistent with the fractionations expected during nebula condensation, the high Mn/Na abundance ratio of GD362 means that it is not ( > 3 σ ). We find that heating of the planetary system orbiting GD362 during the star's giant branch evolution is insufficient to produce such a high Mn/Na. We, therefore, propose that volatile loss occurred in a manner analogous to that of the solar system bodies, either due to impacts shortly after their formation or from heating by short-lived radionuclides. We present potential evidence for a magma ocean stage on the exo-planetary body which currently pollutes the atmosphere of GD362.

Exocomets are small bodies releasing gas and dust which orbit stars other than the Sun. Their existence was first inferred from the detection of variable absorption features in stellar spectra in the late 1980s using spectroscopy. More recently, they have been detected through photometric transits from space, and through far-IR/mm gas emission within debris disks. As (exo)comets are considered to contain the most pristine material accessible in stellar systems, they hold the potential to give us information about early stage formation and evolution conditions of extra Solar Systems. In the Solar System, comets carry the physical and chemical memory of the protoplanetary disk environment where they formed, providing relevant information on processes in the primordial solar nebula. The aim of this paper is to compare essential compositional properties between Solar System comets and exocomets. The paper aims to highlight commonalities and to discuss differences which may aid the communication between the involved research communities and perhaps also avoid misconceptions. Exocomets likely vary in their composition depending on their formation environment like Solar System comets do, and since exocomets are not resolved spatially, they pose a challenge when comparing them to high fidelity observations of Solar System comets. Observations of gas around main sequence stars, spectroscopic observations of "polluted" white dwarf atmospheres and spectroscopic observations of transiting exocomets suggest that exocomets may show compositional similarities with Solar System comets. The recent interstellar visitor 2I/Borisov showed gas, dust and nuclear properties similar to that of Solar System comets. This raises the tantalising prospect that observations of interstellar comets may help bridge the fields of exocomet and Solar System comets., Comment: 25 pages, 3 figures. To be published in PASP. This paper is the product of a workshop at the Lorentz Centre in Leiden, the Netherlands

We infer the crater chronologies of Ceres and Vesta from a self-consistent dynamical model of asteroid impactors. The model accounts for planetary migration/instability early in the solar system history and tracks asteroid orbits over 4.56 Gy. It is calibrated on the current population of the asteroid belt. The model provides the number of asteroid impacts on different worlds at any time throughout the solar system history. We combine the results with an impactor-crater scaling relationship to determine the crater distribution of Ceres and Vesta and compare these theoretical predictions with observations. We find that: (i) The Ceres and Vesta chronologies are similar, whereas they significantly differ from the lunar chronology. Therefore, using the lunar chronology for main belt asteroids, as often done in previous publications, is incorrect. (ii) The model results match the number and size distribution of large (diameter $>90$ km) craters observed on Vesta, but overestimate the number of large craters on Ceres. This implies that large crater erasure is required for Ceres. (iii) In a model where planetary migration/instability happens early, the probability to form the Rheasilvia basin on Vesta during the last 1 Gy is 10\%, a factor of $\sim1.5$ higher than for the late instability case and $\sim2.5$ times higher than found in previous studies. Thus, while the formation of the Rheasilvia at $\sim1$ Gy ago (Ga) would be somewhat unusual, it cannot be ruled out at more than $\simeq1.5\sigma$. In broader context, our work provides a self-consistent framework for modeling asteroid crater records.

The close encounters of the Pluto–Charon system and the Kuiper Belt object Arrokoth (formerly 2014 MU69) by NASA’s New Horizons spacecraft in 2015 and 2019, respectively, have given new perspectives on the most distant planetary bodies yet explored. These bodies are key indicators of the composition, chemistry, and dynamics of the outer regions of the Solar System’s nascent environment. Pluto and Charon reveal characteristics of the largest Kuiper Belt objects formed in the dynamically evolving solar nebula inward of ~30 AU, while the much smaller Arrokoth is a largely undisturbed relic of accretion at ~45 AU. The surfaces of Pluto and Charon are covered with volatile and refractory ices and organic components, and have been shaped by geological activity. On Pluto, N2, CO and CH4 are exchanged between the atmosphere and surface as gaseous and condensed phases on diurnal, seasonal and longer timescales, while Charon’s surface is primarily inert H2O ice with an ammoniated component and a polar region colored with a macromolecular organic deposit. Arrokoth is revealed as a fused binary body in a relatively benign space environment where it originated and has remained for the age of the Solar System. Its surface is a mix of CH3OH ice, a red-orange pigment of presumed complex organic material, and possibly other undetected components.

The isotopic dichotomy between non-carbonaceous (NC) and carbonaceous (CC) meteorites indicates that meteorite parent bodies derive from two genetically distinct reservoirs, which presumably were located inside (NC) and outside (CC) the orbit of Jupiter and remained isolated from each other for the first few million years of the solar system. Here we review the discovery of the NC–CC dichotomy and its implications for understanding the early history of the solar system, including the formation of Jupiter, the dynamics of terrestrial planet formation, and the origin and nature of Earth’s building blocks. The isotopic difference between the NC and CC reservoirs is probably inherited from the solar system’s parental molecular cloud and has been maintained through the rapid formation of Jupiter that prevented significant exchange of material from inside (NC) and outside (CC) its orbit. The growth and/or migration of Jupiter resulted in inward scattering of CC bodies, which accounts for the co-occurrence of NC and CC bodies in the present-day asteroid belt and the delivery of presumably volatile-rich CC bodies to the growing terrestrial planets. Earth’s primitive mantle, at least for siderophile elements like Mo, has a mixed NC–CC composition, indicating that Earth accreted CC bodies during the final stages of its growth, perhaps through the Moon-forming giant impactor. The late-stage accretion of CC bodies to Earth is sufficient to account for the entire budget of Earth’s water and highly volatile species.

The solar system's terrestrial planets are thought to have accreted over millions of years out of a sea of smaller embryos and planetesimals. Because it is impossible to know the surface density profile for solids and size frequency distribution in the primordial solar nebula, distinguishing between the various proposed evolutionary schemes has been historically difficult. Nearly all previous simulations of terrestrial planet formation assume that Moon to Mars massed embryos formed throughout the inner solar system during the primordial gas-disk phase. However, validating this assumption through models of embryo accretion is computationally challenging because of the large number of bodies required. Here, we reevaluate this problem with GPU-accelerated, direct N-body simulations of embryo growth starting from r~100 km planetesimals. We find that embryos emerging from the primordial gas phase at a given radial distance already have masses similar to the largest objects at the same semi-major axis in the modern solar system. Thus, Earth and Venus attain ~50% of their modern mass, Mars-massed embryos form in the Mars region, and Ceres-massed objects are prevalent throughout asteroid belt. Consistent with other recent work, our new initial conditions for terrestrial accretion models produce markedly improved solar system analogs when evolved through the giant impact phase of planet formation. However, we still conclude that an additional dynamical mechanism such as giant planet migration is required to prevent Earth-massed Mars analogs from growing., 16 pages, 13 figures, 4 tables, accepted for publication in The Planetary Science Journal

We investigate the possible origins of real high-inclination Centaurs and trans-neptunian objects using a high-resolution statistical search for stable orbits that simulates their evolution back in time to the epoch when planet formation ended 4.5 billion years in the past. The simulation is a precise orbit determination method that does not involve ad hoc initial conditions or assumptions such as those found in planetesimal disk relaxation models upon which their conclusions depend. It can therefore be used to independently test origin theories based on relaxation models by examining the past orbits of specific real objects. Here, we examined 17 multiple-opposition high-inclination Centaurs and the two polar trans-neptunian objects 2008 KV42 and (471325) 2011 KT19. The statistical distributions show that their orbits were nearly polar 4.5 Gyr in the past, and were located in the scattered disk and inner Oort cloud regions. Early polar inclinations cannot be accounted for by current Solar system formation theory as the early planetesimal system must have been nearly flat in order to explain the low-inclination asteroid and Kuiper belts. Furthermore, the early scattered disk and inner Oort cloud regions are believed to have been devoid of Solar system material as the planetesimal disk could not have extended far beyond Neptune's current orbit in order to halt the planet's outward migration. The nearly polar orbits of high-inclination Centaurs 4.5 Gyr in the past therefore indicate their probable early capture from the interstellar medium., 10 pages, 4 figures

Each type of asteroids and comets are important, serving as the unique puzzle pieces of the solar system. The countless number of small bodies spread vastly from the near-Earth orbits to the main belt and beyond Jupiter. Thus, in order to complete the whole puzzle, and hence requires a well-designed roadmap of sample return (SR) missions and international coordination. The main consideration is the accreting locations of various types of asteroids, which may be referred through their redox status and abundances of water and volatile components. C-complex asteroids are water and volatile-rich, likely accreted in the outer solar system. Two C-complex asteroids are being explored by Hayabusa-2 and OSIRIS-REx missions, respectively. In contrast, enstatite chondrite-like asteroids formed under extremely reducing conditions in the inner solar system. The samples returned from enstatite chondrite-like asteroids will clarify the nebular processes in the zone closest to the Sun, and reveal fractionation of the solar nebula along the radial direction, via comparing with those of the C-complex asteroids. The exploration will also shed light on the bulk compositions of the Earth and terrestrial planets accreted in the same region of the inner solar system. The SR missions will focus on the first 10 Ma history of the solar system, including the initial condition, the nebular processes, and the accretion of planetesimals. Because the secondary processes that took place in planetesimals, such as thermal metamorphism, aqueous alteration, melting and differentiation, could largely erase the records of the nebular events, it is critically important to choose the primordial asteroid targets. Although C-complex asteroids accreted at low temperature in the outer solar nebula, they, especially those hydrated, could have suffered severe aqueous alteration as observed in CM chondrites. Other preferred targets are L-type asteroids, which probably contain abundant Ca-, Al-rich inclusions, the first solid assemblages of the solar system. Based on the roadmap of SR missions, we propose to return samples first from enstatite chondrite-like or L-type Near-Earth asteroids.

Examining Arrokoth The New Horizons spacecraft flew past the Kuiper Belt object (486958) Arrokoth (also known as 2014 MU 69 ) in January 2019. Because of the great distance to the outer Solar System and limited bandwidth, it will take until late 2020 to downlink all the spacecraft's observations back to Earth. Three papers in this issue analyze recently downlinked data, including the highest-resolution images taken during the encounter (see the Perspective by Jewitt). Spencer et al. examined Arrokoth's geology and geophysics using stereo imaging, dated the surface using impact craters, and produced a geomorphological map. Grundy et al. investigated the composition of the surface using color imaging and spectroscopic data and assessed Arrokoth's thermal emission using microwave radiometry. McKinnon et al. used simulations to determine how Arrokoth formed: Two gravitationally bound objects gently spiraled together during the formation of the Solar System. Together, these papers determine the age, composition, and formation process of the most pristine object yet visited by a spacecraft. Science , this issue p. eaay3999 , p. eaay3705 , p. eaay6620 ; see also p. 980

The New Horizons spacecraft's encounter with the cold classical Kuiper belt object (486958) Arrokoth (formerly 2014 MU69) revealed a contact-binary planetesimal. We investigate how it formed, finding it is the product of a gentle, low-speed merger in the early Solar System. Its two lenticular lobes suggest low-velocity accumulation of numerous smaller planetesimals within a gravitationally collapsing, solid particle cloud. The geometric alignment of the lobes indicates the lobes were a co-orbiting binary that experienced angular momentum loss and subsequent merger, possibly due to dynamical friction and collisions within the cloud or later gas drag. Arrokoth's contact-binary shape was preserved by the benign dynamical and collisional environment of the cold classical Kuiper belt, and so informs the accretion processes that operated in the early Solar System., Published in Science 28 Feb 2020 (First release 13 Feb 2020)

The New Horizons flyby of the cold classical Kuiper Belt object MU69 showed it to be a contact binary. The existence of other contact binaries in the 1-10km range raises the question of how common these bodies are and how they evolved into contact. Here we consider that the lobes of MU69 formed as a binary in the Solar nebula, and calculate its orbital evolution in the presence of gas. We find that the sub-Keplerian wind of the disk brings the drag timescales for 10km bodies under 1 Myr for quadratic-velocity drag. In the Kuiper belt, however, the drag is linear with velocity and the effect of the wind cancels out as the angular momentum gained in half an orbit is lost in the other half: the drag timescales for 10 km bodies remain over 10 Myr. In this situation we find that a combination of drag and Kozai-Lidov oscillations is a promising channel for collapse. We analytically solve the hierarchical three-body problem with drag and implement it into a Kozai plus tidal friction model. The permanent quadrupoles of the lobes make the Kozai oscillations stochastic, and as drag shrinks the semimajor axis it more easily allows the fluctuations to bring the system into contact. Evolution to contact happens very rapidly (within $10^4$ yr) in the pure, double-average quadrupole, Kozai region between $\approx 85-95^\circ$, and within 3 Myr in the drag-assisted region beyond it. The synergy between $J_2$ and gas drag widens the window of contact to 80-100$^\circ$ initial inclination, over a larger range of semimajor axes. As such, the model predicts an initial contact binary fraction of about 10\% for the cold classicals in the Kuiper belt. The speed at contact deviates from the escape velocity only because of the oblateness. For MU69, the contact velocity should be 3.3-4.2 m/s, in line with the observational evidence from the lack of deformation features and estimate of the tensile strength., 13 pages (main article) + 15 pg appendix; 14 figures. Accepted by Icarus. To appear in Icarus' special issue on Arrokoth. Proof published online on May 22nd, 2020, at https://www.sciencedirect.com/science/article/abs/pii/S001910352030213X?via%3Dihub

International audience; Carbonaceous chondrites are often considered potential contributors of water and other volatiles to terrestrial planets as most of them contain significant amounts of hydrous mineral phases. As such, carbonaceous chondrites are candidate building blocks for Earth, and elucidating their thermal histories is of direct importance for understanding the volatile element history of Earth and the terrestrial planets. A significant fraction of CM type carbonaceous chondrites are thermally metamorphosed or "heated" and have lost part of their water content. The origin and the timing of such heating events are still debated, as they could have occurred either in the first Myrs of the Solar System via short-lived radioactive heating, or later by impact induced heating and/or solar radiation. Since Rb is more volatile than Sr, and some heated CM chondrites are highly depleted in Rb, a dating system based on the radioactive decay of Rb-87 to( 87)Sr(lambda(87) Rb = 1.393 x 10(-11) yr(-1)) could be used to date the heating event relating to the fractionation of Rb and Sr. Here, we have leveraged the Rb-87/Sr-87 system to date the heating of five CM chondrites (PCA 02012, PCA 02010, PCA 91008, QUE 93005 and MIL 07675). We find that the heating events of all five meteorites occurred at least 3 Ga after the formation of the Solar System. Such timing excludes short-lived radioactive heating as the origin of thermal metamorphism in these meteorites, and relates such heating events to ages of collisional families of C-type asteroids.

The Psyche Mission presents the first opportunity to explore the largest metal asteroid in the solar system, (16) Psyche, which is believed to be the exposed core of a larger planetesimal that was stripped of its rocky mantle through multiple collisions during early solar system formation. The mission was selected in January 2017 for a 2022 launch as part of NASA's Discovery Program and is uniquely enabled by the integration of a Solar Electric Propulsion (SEP) Chassis delivered by Maxar Space Solutions with JPL's core deep space avionics, flight software, and fault management architectures. One of the key design tasks is the development of a fault management system capable of being responsive to the unique elements of the combined JPL and Maxar spacecraft architecture. This new design leverages the strengths of each organization, with Maxar delivering its well-proven highvoltage power bus and low-thrust electric propulsion subsystem from its GEO communications satellite product line, and JPL delivering its deep space mission expertise and the hardware and software most critical to deep space mission design. The development of a robust low-thrust mission and the integration of design philosophies and hardware from two organizations is not without its challenges though. A key challenge in the development of the Psyche fault management architecture and design is in the integration of design philosophies and hardware from JPL and Maxar. At the architecture level, Maxar GEO communications satellites are developed under the premise of highly responsive ground in the loop for the resolution of anomalies, and the implementation takes a fail-operational approach to minimize down time for its customers. In contrast, a deep space mission must be able to maintain safety with long periods of ground communication outage. Additionally, with no time-critical events after launch, the Psyche spacecraft will generally failsafe in the presence of anomalous conditions; special consideration is being given to this approach, however, to minimize the loss of electric propulsion thrust time, which is critical to low-thrust missions. At the hardware level, the detailed definition of interfaces between JPL and Maxar hardware presents a unique challenge in the development and flowdown of fault management requirements, the development and implementation of fault monitors and responses, and the development and verification of fault containment boundaries. This paper describes the evolution of the Psyche fault management architecture and design from the concept study into the preliminary design phase, with a focus on the unique challenges associated with flying GEO communications satellite hardware in deep space, implementing a robust low-thrust mission, and the integration of design philosophies and hardware from JPL and Maxar. Details regarding how these challenges are addressed in the fault management design in order to maximize heritage, leverage the strengths of each organization, and minimize risk across the design are also discussed.

We report the structure, chemical composition, O, Al-Mg, He, and Ne isotope systematics of an interplanetary dust particle, "Manchanito". These analyses indicate that Manchanito solidified as refractory glass (with oxidized Fe but reduced Ti) in a chondrule-like formation environment more than 3.2 Myr after CAIs, after which it was exposed to Q-like noble gases in the dissipating solar nebula. Manchanito's He and Ne isotopic composition and concentrations are similar to those measured in samples of comet Wild 2, from which we infer that Manchanito's parent body was a comet. We propose that after formation and exposure to Q-like gases, Manchanito was transported to the outer Solar System where it came into contact with organics and volatile ices on its cometary parent body. Manchanito provides additional evidence that cometary solids have been subjected to energetic processing and large-scale transport in a wide range of environments in the Solar System.

One of the key goals of the Rosetta mission was to understand how, where and when comets formed in our solar system. There are two major hypotheses for the origin of comets, both pre-Rosetta: (1) hierarchical accretion of dust and ice grains in the Solar Nebula and (2) the growth of pebbles, which are then brought together by streaming instabilities in the Solar Nebula to form larger bodies. Rosetta provided a wealth of new information on comet nuclei and confirmed many past ideas on comets, e.g., high volatile content, lack of aqueous alteration of grains, and the low bulk density of the nucleus. Rosetta also provided new data on the nature of cometary activity, the active geology on the nucleus surface and the interior structure and bulk density of the nucleus. Supporters of the above-mentioned origin hypotheses each find confirmation of their ideas in the Rosetta results. But the question of which hypothesis is preferred, or if there are other, better hypotheses that could be invoked, could not be answered. Theoretical studies suggest that comet nuclei were collisionally processed in the Primordial Disk though it is not clear that the nuclei we see today display the effects of that process. Both theoretical and observational studies suggest that the major end-states for cometary nuclei are dynamical ejection, random disruption and disintegration, and/or evolution of nuclei to inactive, asteroidal-appearing objects. Rosetta has provided us with many new insights that will help to guide future cometary missions, observations, experiments and theoretical investigations that will lead to answers to the fundamental questions with regard to cometary origin.