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.

Rogerio Deienno, Kevin J. Walsh, Katherine A. Kretke, and Harold F. Levison
Astrophysical Journal 876, 103 Link to Article [DOI: 10.3847/1538-4357/ab16e1]
Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Boulder, CO 80302, USA

It is often asserted that more accurate treatment of large collisions in planet formation simulations will lead to vastly different results—in particular a lower final angular momentum deficit (AMD—commonly used to measure orbital excitement). As nearly all simulations to date consider perfect merging (100% energy dissipation) during embryo–embryo collisions, and typically end up with an overexcited final terrestrial planetary system, it has been suggested that a better treatment of energy dissipation during large collisions could decrease the final dynamical excitation (or AMD). Although some work related to energy dissipation has been done (mostly during the runaway growth phase when planetesimals grow into protoplanets), this had never been fully tested in the post-runaway phase, where protoplanets (embryos) grow chaotically into planets via large collisions among themselves. In this work, we test varying amounts of energy dissipation within embryo–embryo collisions, by assuming a given coefficient of restitution for collisions. Our results show that varying the level of energy dissipated within embryo–embryo collisions do not play any important role in the final terrestrial planetary system. We have found a strong linear correlation in our results related to the final number of planets formed and the final AMD. Additionally, reproducing the current radial mass concentration of the terrestrial planets, even when starting from an annulus of material, is challenging when modeling growth from planetesimals to planets.

Sarah Millholland^1,3 and Konstantin Batygin²
Astrophysical Journal 876, 119 Link to Article [DOI: 10.3847/1538-4357/ab19be]
¹Department of Astronomy, Yale University, New Haven, CT 06511, USA
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
³NSF Graduate Research Fellow.

The tilt of a planet’s spin axis off its orbital axis (“obliquity”) is a basic physical characteristic that plays a central role in determining the planet’s global circulation and energy redistribution. Moreover, recent studies have also highlighted the importance of obliquities in sculpting not only the physical features of exoplanets but also their orbital architectures. It is therefore of key importance to identify and characterize the dominant processes of excitation of nonzero axial tilts. Here we highlight a simple mechanism that operates early on and is likely fundamental for many extrasolar planets and perhaps even solar system planets. While planets are still forming in the protoplanetary disk, the gravitational potential of the disk induces nodal recession of the orbits. The frequency of this recession decreases as the disk dissipates, and when it crosses the frequency of a planet’s spin axis precession, large planetary obliquities may be excited through capture into a secular spin–orbit resonance. We study the conditions for encountering this resonance and calculate the resulting obliquity excitation over a wide range of parameter space. Planets with semimajor axes in the range 0.3 au  a  2 au are the most readily affected, but large-a planets can also be impacted. We present a case study of Uranus and Neptune, and show that this mechanism likely cannot help explain their high obliquities. While it could have played a role if finely tuned and envisioned to operate in isolation, large-scale obliquity excitation was likely inhibited by gravitational planet–planet perturbations.

Chin-Fei Lee^1,2, Claudio Codella^3,4, Zhi-Yun Li⁵, and Sheng-Yuan Liu¹
Astrophysical Journal 876, 63 Link to Article [DOI: 10.3847/1538-4357/ab15db ]
¹Academia Sinica Institute of Astronomy and Astrophysics, P.O. Box 23-141, Taipei 106, Taiwan
²Graduate Institute of Astronomy and Astrophysics, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan
³INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
⁴Univ. Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), F-38000 Grenoble, France
⁵Astronomy Department, University of Virginia, Charlottesville, VA 22904, USA

HH 212 is one of the well-studied protostellar systems, showing the first vertically resolved disk with a warm atmosphere around the central protostar. Here we report a detection of nine organic molecules (including newly detected ketene, formic acid, deuterated acetonitrile, methyl formate, and ethanol) in the disk atmosphere, confirming that the disk atmosphere is, for HH 212, the chemically rich component, identified before at a lower resolution as a “hot corino.” More importantly, we report the first systematic survey and abundance measurement of organic molecules in the disk atmosphere within ~40 au of the central protostar. The relative abundances of these molecules are similar to those in the hot corinos around other protostars and in Comet Lovejoy. These molecules can be either (i) originally formed on icy grains and then desorbed into gas phase or (ii) quickly formed in the gas phase using simpler species ejected from the dust mantles. The abundances and spatial distributions of the molecules provide strong constraints on models of their formation and transport in star formation. These molecules are expected to form even more complex organic molecules needed for life and deeper observations are needed to find them.