On the source of diogenites and olivine diogenites: Compositional diversity from variable fO2

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.

Noble Gas Abundance Ratios Indicate the Agglomeration of 67P/Churyumov–Gerasimenko from Warmed-up Ice

O. Mousiset 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.

Sodium, Potassium, and Calcium in Europa: An Atomic Journey through Water Ice

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.

Alumina Polymorphism in the Circumstellar Dust Shells of Asymptotic Giant Branch Stars

Benjamin A. Sargent1,2
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.

Energy Dissipation in Large Collisions—No Change in Planet Formation Outcomes

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.

Excitation of Planetary Obliquities through Planet–Disk Interactions

Sarah Millholland1,3 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 lesssim a lesssim 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.

First Abundance Measurement of Organic Molecules in the Atmosphere of HH 212 Protostellar Disk

Chin-Fei Lee1,2, Claudio Codella3,4, 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.