Francesco C. Pignatale^1,2, Sébastien Charnoz¹, Marc Chaussidon¹, and Emmanuel Jacquet²
Astrophysical Journal Letters 867, L23 Link to Article [DOI: 10.3847/2041-8213/aaeb22]
¹Institut de Physique du Globe de Paris (IPGP) 1 rue Jussieu, F-75005, Paris, France
²Muséum national d’Histoire naturelle, UMR 7590, CP52 57 rue Cuvier, F-75005, Paris, France

Chondritic meteorites, the building blocks of terrestrial planets, are made of an out-of-equilibrium assemblage of solids formed at high and low temperatures, either in our Solar system or previous generations of stars. For decades this was considered to result from large-scale transport processes in the Sun’s isolated accretion disk. However, mounting evidence suggests that refractory inclusions in chondrites formed contemporaneously with the disk building. Here we numerically investigate, using a 1D model and several physical and chemical processes, the formation and transport of rocky materials during the collapse of the Sun’s parent cloud and the consequent assembling of the Solar Nebula. The interplay between the cloud collapse, the dynamics of gas and dust, vaporization, recondensation, and thermal processing of different species in the disk results in a local mixing of solids with different thermal histories. Moreover, our results also explain the overabundance of refractory materials far from the Sun and their short-formation timescales, during the first tens of kyr of the Sun, corresponding to class 0-I, opening new windows into the origin of the compositional diversity of chondrites.

Judit Szulágyi^1,2, Marco Cilibrasi¹, and Lucio Mayer¹
Astrophysical Journal Letters 868, L13 Link to Article [DOI: 10.3847/2041-8213/aaeed6]
¹Center for Theoretical Astrophysics and Cosmology, Institute for Computational Science, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
²Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093, Zurich, Switzerland

Satellites of giant planets have been thought to form in gaseous circumplanetary disks (CPDs) during the late planet-formation phase, but it was unknown whether or not smaller-mass planets such as the ice giants could form such disks, and thus moons, there. We combined radiative hydrodynamical simulations with satellite population synthesis to investigate the question in the case of Uranus and Neptune. For both ice giants we found that a gaseous CPD is created at the end of their formation. The population synthesis confirmed that Uranian-like, icy, prograde satellite system could form in these CPDs within a couple of 10⁵ yr. This means that Neptune could have a Uranian-like moon system originally that was wiped away by the capture of Triton. Furthermore, the current moons of Uranus can be reproduced by our model without the need for planet–planet impact to create a debris disk for the moons to grow. These results highlight that even ice giants—among the most common mass category of exoplanets—can also form satellites, opening a way to a potentially much larger population of exomoons than previously thought.

Nienke van der Marel¹, Jonathan P. Williams², and Simon Bruderer³
Astrophysical Journal Letters 867, L14 Link to Article [DOI: 10.3847/2041-8213/aae88e]
¹Herzberg Astronomy & Astrophysics Programs, National Research Council of Canada, 5071 West Saanich Road, Victoria BC V9E 2E7, Canada
²Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, 96822 Honolulu, HI, USA
³Max-Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 2, D-85741 Garching bei München, Germany

High-resolution Atacama Large Millimeter/submillimeter Array observations of protoplanetary disks have revealed that many, if not all, primordial disks consist of ring-like dust structures. The origin of these dust rings remains unclear, but a common explanation is the presence of planetary companions that have cleared gaps along their orbit and trapped the dust at the gap edge. A signature of this scenario is a decrease of gas density inside these gaps. In a recent work, Isella et al. derived drops in gas density that are consistent with Saturn-mass planets inside the gaps in the HD 163296 disk through spatially resolved CO isotopologue observations. However, as CO abundance and temperature depends on a large range of factors, the interpretation of CO emission is non-trivial. We use the physical–chemical code DALI to show that the gas temperature increases inside dust density gaps, implying that any gaps in the gas, if present, would have to be much deeper, consistent with planet masses >M _Jup. Furthermore, we show that a model with increased grain growth at certain radii, as expected at a snowline, can reproduce the dust rings in HD 163296 equally well without the need for companions. This scenario can explain both younger and older disks with observed gaps, as gaps have been seen in systems as young <1 Myr. While the origin of the rings in HD 163296 remains unclear, these modeling results demonstrate that care has to be taken when interpreting CO emission in protoplanetary disk observations.

N. Mari^a, A.J.V. Riches^b, L.J. Hallis^a, Y. Marrocchi^c, J. Villeneuve^c, P. Gleissner^d, H. Becker^d M.R. Lee^a
Geochimica et Cosmochimcia Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.05.025]
^aSchool of Geographical and Earth Sciences, University of Glasgow, University Avenue, Glasgow, G12 8QQ, UK
^bDepartment of Earth Sciences, Durham University, Stockton Road, Durham, DH1 3LE, UK
^cCentre de Recherches Pétrographiques et Géochimiques, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy, 54501, France
^dFreie Universität Berlin, Institut für Geologische Wissenschaften, Kaiserswerther Str. 16-18, 14195, Berlin, Germany
Copyright Elsevier

Martian lava flows likely acquired S-rich material from the regolith during their emplacement on the planet’s surface. We investigated five of the twenty known nakhlites (Nakhla, Lafayette, Miller Range (MIL) 090032, Yamato 000593, and Yamato 000749) to determine whether these lavas show evidence of regolith assimilation, and to constrain the potential implications that this process has on chemical tracing of martian mantle source(s). To establish the proportionate influence of atmospheric, hydrothermal, and volcanic processes on nakhlite isotopic systematics we obtained in situ sulphur isotope data (Δ³³S and δ³⁴S) for sulphide grains (pyrrhotite and pyrite) in all five nakhlite samples. For Nakhla, Lafayette, and MIL 090032, these data are integrated with highly siderophile element (HSE) abundances and Os-isotope compositions, as well as textural information constrained prior to isotopic analysis. This work thereby provides the first Re-Os isotope systematics for two different nakhlites, and also the first Re-Os isotope data for martian sample for which detailed petrographic information was constrained prior to digestion.

We report the largest variation in δ³⁴S yet found in martian meteorites (-13.20 ‰ to +15.16 ‰). The relatively positive Δ³³S and δ³⁴S values of MIL 090032 (δ³⁴S = +10.54 ± 0.09 ‰; Δ³³S = -0.67 ± 0.10 ‰) indicate this meteorite assimilated sulphur affected by UV-photochemistry. In contrast, the strongly negative values of Lafayette (δ³⁴S = -10.76 ± 0.14 ‰; Δ³³S = -0.09 ± 0.12 ‰) are indicative of hydrothermal processes on Mars. Nakhla, Yamato 000593, and Yamato 000749 sulphides have a narrower range of sulphur isotope compositions (Δ³³S and δ³⁴S ∼ 0) that is consistent with no assimilation of martian surface materials during lava flow emplacement. Consequently we used this second group of Δ³³S values to approximate the Δ³³S of the nakhlite source, yielding a Δ³³S value of -0.1 ‰.

Nakhlite HSE patterns result from a sulphide-saturated melt where Ru-Os-Ir alloys/sulphide were likely crystallized during earlier phases of magmatic processing in Mars to result in the fractionated HSE patterns of the nakhlites. Our data, alongside a synthesis of previously published data, suggest assimilation of an enriched component to the primary nakhlite melt, potentially a late-stage crystallization cumulate from the martian magma ocean stage. In the context of this model, and within large uncertainties, our data hint at perturbation and potential decoupling of nakhlite Re-Os isotope systematics from other isotopic systems as a result of small degrees of assimilation of a regolith component with highly radiogenic ¹⁸⁷Os/¹⁸⁸Os.

John M. Brewer^1,2, Songhu Wang¹, Debra A. Fischer¹, and Daniel Foreman-Mackey³
Astrophysical Journal Letters 867, L3 Link to Article [DOI: 10.3847/2041-8213/aae710]
¹Department of Astronomy, Yale University, 52 Hillhouse Avenue, New Haven, CT 06511, USA
²Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027, USA
³Flatiron Institute, 162 5th Avenue, New York, NY 10010, USA

In systems with detected planets, hot Jupiters and compact systems of multiple planets are nearly mutually exclusive. We compare the relative occurrence of these two architectures as a fraction of detected planetary systems to determine the role that metallicity plays in planet formation. We show that compact multi-planet systems occur more frequently around stars of increasingly lower metallicities using spectroscopically derived abundances for more than 700 planet hosts. At higher metallicities, compact multi-planet systems comprise a nearly constant fraction of the planet hosts despite the steep rise in the fraction of hosts containing hot and cool Jupiters. Since metal-poor stars have been underrepresented in planet searches, this implies that the occurrence rate of compact multis is higher than previously reported. Due to observational limits, radial velocity planet searches have focused mainly on high-metallicity stars, where they have a higher chance of finding giant planets. New extreme-precision radial velocity instruments coming online that can detect these compact multi-planet systems can target lower-metallicity stars to find them.

C. J. Clarke¹ et al. (>10)
Astrophysical Journal Letters 866, L6 Link to Article [DOI: 10.3847/2041-8213/aae36b]
¹Institute of Astronomy, Madingley Road, Cambridge CB3 OHA, UK

We present high-resolution millimeter continuum imaging of the disk surrounding the young star CI Tau, a system hosting the first hot Jupiter candidate in a protoplanetary disk system. The system has extended mm emission on which are superposed three prominent annular gaps at radii ~13, 39, and 100 au. We argue that these gaps are most likely to be generated by massive planets so that, including the hot Jupiter, the system contains four gas giant planets at an age of only 2 Myr. Two of the new planets are similarly located to those inferred in the famous HL Tau protoplanetary disk; in CI Tau, additional observational data enables a more complete analysis of the system properties than was possible for HL Tau. Our dust and gas dynamical modeling satisfies every available observational constraint and points to the most massive ensemble of exoplanets ever detected at this age, with its four planets spanning a factor 1000 in orbital radius. Our results show that the association between hot Jupiters and gas giants on wider orbits, observed in older stars, is apparently in place at an early evolutionary stage.

Jennifer T. Mitchell and Andrew G. Tomkins
Geochimica et Cosmochimcia Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.05.021]
School of Earth Atmosphere & Environment, Monash University, Clayton, VIC 3800, Australia
Copyright Elsevier

The genesis of diogenites and olivine-bearing diogenites has been debated for decades, with current models favouring formation via either mineral settling in a homogeneous magma ocean, or as late stage intrusions into the crust of asteroid 4 Vesta. Using pMELTS, both equilibrium and fractional crystallisation modelling was conducted on a large range of melt compositions generated by varied extent of batch melt extraction from 11 bulk Vesta starting compositions at a range of fO₂conditions to simulate the magma ocean concept. The resulting mineral compositions were compared with those of 200 diogenite meteorites in an attempt to resolve this debate. Models that involve < 20% initial partial melt cannot produce orthopyroxenites. Orthopyroxenitic diogenites have compositional ranges from En₅₃-En₈₂, whereas ‘olivine diogenites’ show less compositional diversity with orthopyroxenes ranging from En₇₁-En₇₆. Olivine-bearing diogenites are therefore not the most magnesian samples, which contradicts expected crystallisation trends expected from a single homogeneous source. The orthopyroxene compositions produced by models that use fO₂ previously suggested for Vesta of ΔIW -2.05 are too magnesian, and the extent of source partial melting used in the models has negligible effect on this result. Modelling using different initial oxygen fugacity conditions produces a large range of pyroxene compositions that better match the range seen in diogenites, with models ranging from ΔIW fO₂ -1.6 to -1.2 producing the best fit. These results thus imply that the diogenites crystallised from a variety of magmas sourced from a region of heterogeneous oxygen fugacity. This variation can be explained by metasomatism of a homogenous source region by fO₂-modifying sulfidation reactions. The model orthopyroxene compositions are displaced with regards to Wo from natural diogenites; this can be explained by a delayed genesis model whereby a Ca-poor diogenite source developed in response to the melt extraction necessary for formation of a eucritic crust. Our models suggest that diogenites were derived from a series of magma chambers in the Vestan crust.

O. Mousis¹ et al. (10)
Astrophysical Journal Letters 865, L11 Link to Article [DOI: 10.3847/2041-8213/aadf89]
¹Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France

The origin of cometary volatiles remains a major open question in planetary science. Comets may have either agglomerated from crystalline ices condensed in the protosolar nebula (PSN) or from amorphous ice originating from the molecular cloud and interstellar medium. Here, based on the recent argon, krypton, and xenon measurements performed by the ROSINA mass spectrometer on board the European Space Agency’s Rosetta spacecraft in the coma of 67P/Churyumov–Gerasimenko, we show that these noble gas relative abundances can be explained if the comet’s building blocks formed from a mixture of gas and H₂O grains resulting from the annealing of pristine amorphous ice (i.e., originating from the presolar cloud) in the PSN. In this scenario, the different volatiles released during the amorphous-to-crystalline ice phase transition would have been subsequently trapped at lower temperatures in stoichiometric hydrate or clathrate hydrate forms by the crystalline water ice generated by the transition. Once crystalline water was completely consumed by clathration in the ~25–80 K temperature range, the volatile species remaining in the gas phase would have formed pure condensates at lower temperatures. The formation of clathrates hydrates and pure condensates to explain the noble gas relative abundances is consistent with a proposed interstellar origin of molecular oxygen detected in 67P/Churyumov–Gerasimenko, and with the measured molecular nitrogen depletion in comets.

Ozge Ozgurel¹, Olivier Mousis², Françoise Pauzat¹, Yves Ellinger¹, Alexis Markovits¹, Steven Vance³, and François Leblanc⁴
Astrophysical Journal Letters 865, L16 Link to Article [DOI: 10.3847/2041-8213/aae091]
¹Sorbonne Université, CNRS, Laboratoire de Chimie Théorique, LCT, F-75005 Paris, France
²Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
⁴Sorbonne Université, UVSQ, CNRS, LATMOS/IPSL, Paris F-75005, France

Sodium and potassium are known to be present as neutral elements in the exosphere of Europa. The question of the origin of these alkalis—endogenous or exogenous—remains open. They have been ascribed to exogenous contamination due to volcanism from nearby Io, or the accretion of meteorites and dust. However, these mechanisms fail to fit the observed sodium-to-potassium ratio. Sodium and potassium have also been considered to originate from Europa’s putative subsurface ocean, generated by past rock-water leaching. The latter scenario implies a journey of the ions and atoms throughout the ice covering Europa. This raises questions about their stability into the bulk as well as on top of ice. These questions are addressed with first principle periodic solid-state density functional theory simulations describing the relative propensities of sodium, potassium, and calcium for being trapped in the bulk. The evolution of the ionic character of these atoms is followed by means of a topological analysis as they come up to the surface of the ice crust. We find that the metals, almost totally ionized in the ice bulk (net charge ~+0.8) where they are stabilized by ~1 eV or more, recover a quasi-neutrality (net charge ~+0.2) when weakly adsorbed at the surface by ~0.15 eV. Our results are consistent with the assumption that sodium and potassium observed in Europa exosphere come from the sputtering of materials issued from the underlying ocean and exposed by resurfacing events. They also suggest that calcium should be searched for by future missions.

Benjamin A. Sargent^1,2
Astrophysical Journal Letters 866, L1 Link to Article [DOI: 10.3847/2041-8213/aae085]
¹Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²Center for Imaging Science and Laboratory for Multiwavelength Astrophysics, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester NY 14623, USA

Many emission features remain unidentified in the infrared spectra of asymptotic giant branch (AGB) stars. In particular, features at ~11, 20, 28, and 32 μm have been noted in mid-infrared spectra of oxygen-rich AGB stars. Here, I present models of dust excess emission in 36 spectra of 24 AGB stars from the Short Wavelength Spectrometer on board the Infrared Space Observatory and the Infrared Spectrograph on the Spitzer Space Telescope. The models include opacities of grains composed of mixtures of various polymorphs of alumina obtained by preparing bayerite and boehmite at high temperatures, and these dust components provide satisfactory fits to the 11, 20, 28, and 32 μm features. Though not a direct conclusion from this study, the presence of grains of the various polymorphs of aluminas in circumstellar dust shells around AGB stars suggests that corundum may have a role in giving rise to the 13 μm feature.