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.