^1,2,3Debra H. Needham, ^1,2David A. Kring
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2017.09.002]
¹Center for Lunar Science and Exploration, Lunar and Planetary Institute, Houston, TX, United States
²NASA Solar System Exploration Virtual Institute
³NASA Marshall Space Flight Center, Huntsville, AL, United States
Copyright Elsevier

Studies of the lunar atmosphere have shown it to be a stable, low-density surface boundary exosphere for the last 3 billion years. However, substantial volcanic activity on the Moon prior to 3 Ga may have released sufficient volatiles to form a transient, more prominent atmosphere. Here, we calculate the volume of mare basalt emplaced as a function of time, then estimate the corresponding production of volatiles released during the mare basalt-forming eruptions. Results indicate that during peak mare emplacement and volatile release ∼3.5 Ga, the maximum atmospheric pressure at the lunar surface could have reached ∼1 kPa, or ∼1.5 times higher than Mars’ current atmospheric surface pressure. This lunar atmosphere may have taken ∼70 million years to fully dissipate. Most of the volatiles released by mare basalts would have been lost to space, but some may have been sequestered in permanently shadowed regions on the lunar surface. If only 0.1% of the mare water vented during these eruptions remains in the polar regions of the Moon, volcanically-derived volatiles could account for all hydrogen deposits – suspected to be water – currently observed in the Moon’s permanently shadowed regions. Future missions to such locations may encounter evidence of not only asteroidal, cometary, and solar wind-derived volatiles, but also volatiles vented from the interior of the Moon.

^1,2Martin D.Suttle, ^1,2Matthew J.Genge
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2017.07.052]
¹Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, South Kensington, London, SW7 2AZ, UK
²Department of Earth Science, The Natural History Museum, Cromwell Rd, London SW7 5BD, UK
Copyright Elsevier

We report the discovery of fossil micrometeorites from Late Cretaceous chalk. Seventy-six cosmic spherules were recovered from Coniacian (87±1 Ma) sediments of the White Chalk Supergroup. Particles vary from pristine silicate and iron-type spherules to pseudomorphic spherules consisting of either single-phase recrystallized magnetite or Fe-silicide. Pristine spherules are readily identified as micrometeorites on the basis of their characteristic mineralogies, textures and compositions. Both magnetite and silicide spherules contain dendritic crystals and spherical morphologies, testifying to rapid crystallisation of high temperature iron-rich metallic and oxide liquids. These particles also contain spherical cavities, representing weathering and removal of metal beads and irregular cavities, representing vesicles formed by trapped gas during crystallization; both features commonly found among modern Antarctic Iron-type (I-type) cosmic spherules. On the basis of textural analysis, the magnetite and Fe-silicide spherules are shown to be I-type cosmic spherules that have experienced complete secondary replacement during diagenesis (fossilization). Our results demonstrate that micrometeorites, preserved in sedimentary rocks, are affected by a suite of complex diagenetic processes, which can result in disparate replacement minerals, even within the same sequence of sedimentary beds. As a result, the identification of fossil micrometeorites requires careful observation of particle textures and comparisons with modern Antarctic collections. Replaced micrometeorites imply that geochemical signatures the extraterrestrial dust are subject to diagenetic remobilisation that limits their stratigraphic resolution. However, this study demonstrates that fossil, pseudomorphic micrometeorites can be recognised and are likely common within the geological record.

Nan Liu¹, Thomas Stephan^2,3, Patrick Boehnke^2,3, Larry R. Nittler¹, Conel M. O’D. Alexander¹, Jianhua Wang¹, Andrew M. Davis^2,3,4, Reto Trappitsch^2,3,5, and Michael J. Pellin^2,3,4,6
The Astrophysical Journal Letters 844 L12 Link to Article [https://doi.org/10.3847/2041-8213/aa7d4c]
¹Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA
²Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
³Chicago Center for Cosmochemistry, Chicago, IL, USA
⁴The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
⁵Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
⁶Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA

We report Mo isotopic data of 27 new presolar SiC grains, including 12 ¹⁴N-rich AB (¹⁴N/¹⁵N > 440, AB2) and 15 mainstream (MS) grains, and their correlated Sr and Ba isotope ratios when available. Direct comparison of the data for the MS grains, which came from low-mass asymptotic giant branch (AGB) stars with large s-process isotope enhancements, with the AB2 grain data demonstrates that AB2 grains show near-solar isotopic compositions and lack s-process enhancements. The near-normal Sr, Mo, and Ba isotopic compositions of AB2 grains clearly exclude born-again AGB stars, where the intermediate neutron-capture process (i-process) takes place, as their stellar source. On the other hand, low-mass CO novae and early R- and J-type carbon stars show ¹³C and ¹⁴N excesses but no s-process enhancements and are thus potential stellar sources of AB2 grains. Because both early R-type carbon stars and CO novae are rare objects, the abundant J-type carbon stars (10%–15% of all carbon stars) are thus likely to be a dominant source of AB2 grains.

Sam M. Austin^1,2, Christopher West^2,3,4, and Alexander Heger^2,3,5,6
The Astrophysical Journal Letters 839 L9 Link to Article [https://doi.org/10.3847/2041-8213/aa68e7]
¹National Superconducting Cyclotron Laboratory, Michigan State University, 640 South Shaw Lane, East Lansing, MI 48824-1321, USA
²Joint Institute for Nuclear Astrophysics—Center for the Evolution of the Elements, Michigan State University, East Lansing, MI 48824-1321, USA
³Minnesota Institute for Astronomy, School of Physics and Astronomy, University of Minnesota, Twin Cities, Minneapolis, MN 55455-0149, USA
⁴Center for Academic Excellence, Metropolitan State University, St. Paul, MN, 55106, USA
⁵Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, VIC 3800, Australia
⁶Center for Nuclear Astrophysics, Department of Physics and Astronomy, Shanghai Jiao-Tong University, Shanghai 200240, P. R. China

We have used effective reaction rates (ERRs) for the helium burning reactions to predict the yield of the gamma-emitting nuclei ²⁶Al, ⁴⁴Ti, and ⁶⁰Fe in core-collapse supernovae (SNe). The variations in the predicted yields for values of the reaction rates allowed by the ERR are much smaller than obtained previously, and smaller than other uncertainties. A “filter” for SN nucleosynthesis yields based on pre-SN structure was used to estimate the effect of failed SNe on the initial mass function averaged yields; this substantially reduced the yields of all these isotopes, but the predicted yield ratio ⁶⁰Fe/²⁶Al was little affected. The robustness of this ratio is promising for comparison with data, but it is larger than observed in nature; possible causes for this discrepancy are discussed.

David Nesvorný¹, David Vokrouhlický², Luke Dones¹, Harold F. Levison¹, Nathan Kaib³, and Alessandro Morbidelli⁴
Astrophysical Journal 843, 120 Link to Article [https://doi.org/10.3847/1538-4357/aa7cf6]
¹Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, USA
²Institute of Astronomy, Charles University, V Holešovičkách 2, CZ-18000 Prague 8, Czech Republic
³H.L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019, USA
⁴Département Cassiopée, University of Nice, CNRS, Observatoire de la Côte d’Azur, Nice, F-06304, France

Comets are icy objects that orbitally evolve from the trans-Neptunian region into the inner solar system, where they are heated by solar radiation and become active due to the sublimation of water ice. Here we perform simulations in which cometary reservoirs are formed in the early solar system and evolved over 4.5 Gyr. The gravitational effects of Planet 9 (P9) are included in some simulations. Different models are considered for comets to be active, including a simple assumption that comets remain active for perihelion passages with perihelion distance . The orbital distribution and number of active comets produced in our model is compared to observations. The orbital distribution of ecliptic comets (ECs) is well reproduced in models with and without P9. With P9, the inclination distribution of model ECs is wider than the observed one. We find that the known Halley-type comets (HTCs) have a nearly isotropic inclination distribution. The HTCs appear to be an extension of the population of returning Oort-cloud comets (OCCs) to shorter orbital periods. The inclination distribution of model HTCs becomes broader with increasing , but the existing data are not good enough to constrain from orbital fits. is required to obtain a steady-state population of large active HTCs that is consistent with observations. To fit the ratio of the returning-to-new OCCs, by contrast, our model implies that , possibly because the detected long-period comets are smaller and much easier to disrupt than observed HTCs.

Nan Liu¹, Larry R. Nittler¹, Marco Pignatari^2,3, Conel M. O’D. Alexander¹, and Jianhua Wang¹
The Astrophysical Journal Letters 842 L1 Link to Article [https://doi.org/10.3847/2041-8213/aa74e5]
¹Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA
²E. A. Milne Centre for Astrophysics, Department of Physics & Mathematics, University of Hull, Hull HU6 7RX, UK
³NuGrid collaboration, http://www.nugridstars.org.

We report C, N, and Si isotopic data for 59 highly ¹³C-enriched presolar submicron- to micron-sized SiC grains from the Murchison meteorite, including eight putative nova grains (PNGs) and 29 ¹⁵N-rich (¹⁴N/¹⁵N ≤ solar) AB grains, and their Mg–Al, S, and Ca–Ti isotope data when available. These 37 grains are enriched in ¹³C, ¹⁵N, and ²⁶Al with the PNGs showing more extreme enhancements. The ¹⁵N-rich AB grains show systematically higher ²⁶Al and ³⁰Si excesses than the ¹⁴N-rich AB grains. Thus, we propose to divide the AB grains into groups 1 (¹⁴N/¹⁵N < solar) and 2 (¹⁴N/¹⁵N ≥ solar). For the first time, we have obtained both S and Ti isotopic data for five AB1 grains and one PNG and found ³²S and/or ⁵⁰Ti enhancements. Interestingly, one AB1 grain had the largest ³²S and ⁵⁰Ti excesses, strongly suggesting a neutron-capture nucleosynthetic origin of the ³²S excess and thus the initial presence of radiogenic ³²Si (t _1/2 = 153 years). More importantly, we found that the ¹⁵N and ²⁶Al excesses of AB1 grains form a trend that extends to the region in the N–Al isotope plot occupied by C2 grains, strongly indicating a common stellar origin for both AB1 and C2 grains. Comparison of supernova models with the AB1 and C2 grain data indicates that these grains came from supernovae that experienced H ingestion into the He/C zones of their progenitors.

O. Muñoz¹, F. Moreno¹, F. Vargas-Martín², D. Guirado¹, J. Escobar-Cerezo¹, M. Min^3,4, and J. W. Hovenier⁴
Astrophysical Journal 846, 85 Link to Article [https://doi.org/10.3847/1538-4357/aa7ff2]
¹Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
²Department of Electromagnetism and Electronics, University of Murcia, E-30100 Murcia, Spain
³SRON Netherlands Institute for Space Research, Sobornnelaan 2, 3584 CA Utrecht, The Netherlands
⁴Astronomical Institute “Anton Pannekoek,” University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands

We present the experimental phase functions of three types of millimeter-sized dust grains consisting of enstatite, quartz, and volcanic material from Mount Etna, respectively. The three grains present similar sizes but different absorbing properties. The measurements are performed at 527 nm covering the scattering angle range from 3° to 170°. The measured phase functions show two well-defined regions: (i) soft forward peaks and (ii) a continuous increase with the scattering angle at side- and back-scattering regions. This behavior at side- and back-scattering regions is in agreement with the observed phase functions of the Fomalhaut and HR 4796A dust rings. Further computations and measurements (including polarization) for millimeter-sized grains are needed to draw some conclusions about the fluffy or compact structure of the dust grains.

Dana E. Anderson¹, Edwin A. Bergin², Geoffrey A. Blake¹, Fred J. Ciesla³, Ruud Visser⁴, and Jeong-Eun Lee⁵
Astrophysical Journal 845, 13 Link to Article [https://doi.org/10.3847/1538-4357/aa7da1]
¹Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
²Department of Astronomy, University of Michigan, 1085 S. University, Ann Arbor, MI 48109-1107, USA
³Department of Geophysical Sciences, The University of Chicago, 5734 South Ellis Ave., Chicago, IL 60637, USA
⁴European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748, Garching, Germany
⁵School of Space Research, Kyung Hee University, 1732, Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Korea

The Earth and other rocky bodies in the inner solar system contain significantly less carbon than the primordial materials that seeded their formation. These carbon-poor objects include the parent bodies of primitive meteorites, suggesting that at least one process responsible for solid-phase carbon depletion was active prior to the early stages of planet formation. Potential mechanisms include the erosion of carbonaceous materials by photons or atomic oxygen in the surface layers of the protoplanetary disk. Under photochemically generated favorable conditions, these reactions can deplete the near-surface abundance of carbon grains and polycyclic aromatic hydrocarbons by several orders of magnitude on short timescales relative to the lifetime of the disk out to radii of ~20–100+ au from the central star depending on the form of refractory carbon present. Due to the reliance of destruction mechanisms on a high influx of photons, the extent of refractory carbon depletion is quite sensitive to the disk’s internal radiation field. Dust transport within the disk is required to affect the composition of the midplane. In our current model of a passive, constant-αdisk, where α = 0.01, carbon grains can be turbulently lofted into the destructive surface layers and depleted out to radii of ~3–10 au for 0.1–1 μm grains. Smaller grains can be cleared out of the planet-forming region completely. Destruction may be more effective in an actively accreting disk or when considering individual grain trajectories in non-idealized disks.

Alexey Potapov¹, Cornelia Jäger¹, Thomas Henning², Mindaugas Jonusas^3,4, and Lahouari Krim^3,4
Astrophysical Journal 846, 131 Link to Article [https://doi.org/10.3847/1538-4357/aa85e8]
¹Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, D-07743 Jena, Germany
²Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
³Department of Chemistry, Sorbonne Universités, UPMC Univ Paris 06, UMR 8233, MONARIS, Paris F-75005, France
⁴CNRS, UMR 8233, MONARIS, Paris F-75005, France

An understanding of possible scenarios for the formation of astrophysically relevant molecules, particularly complex organic molecules, will bring us one step closer to the understanding of our astrochemical heritage. In this context, formaldehyde is an important molecule as a precursor of methanol, which in turn is a starting point for the formation of more complex organic species. In the present experiments, for the first time, following the synthesis of CO, formaldehyde has been produced on the surface of interstellar grain analogs, hydrogenated fullerene-like carbon grains, by O and H atom bombardment. The formation of H₂CO is an indication for a possible methanol formation route in such systems.

O. Mousis¹ et al. (>10)
The Astrophysical Journal Letters 839 L4 Link to Article [https://doi.org/10.3847/2041-8213/aa6839]
¹Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, F-13388, Marseille, France

Because of the high fraction of refractory material present in comets, the heat produced by the radiogenic decay of elements such as aluminum and iron can be high enough to induce the loss of ultravolatile species such as nitrogen, argon, or carbon monoxide during their accretion phase in the protosolar nebula (PSN). Here, we investigate how heat generated by the radioactive decay of ²⁶Al and ⁶⁰Fe influences the formation of comet 67P/Churyumov–Gerasimenko, as a function of its accretion time and the size of its parent body. We use an existing thermal evolution model that includes various phase transitions, heat transfer in the ice-dust matrix, and gas diffusion throughout the porous material, based on thermodynamic parameters derived from Rosetta observations. Two possibilities are considered: either, to account for its bilobate shape, 67P/Churyumov–Gerasimenko was assembled from two primordial ~2 km sized planetesimals, or it resulted from the disruption of a larger parent body with a size corresponding to that of comet Hale–Bopp (~70 km). To fully preserve its volatile content, we find that either 67P/Churyumov–Gerasimenko’s formation was delayed between ~2.2 and 7.7 Myr after that of Ca–Al-rich Inclusions in the PSN or the comet’s accretion phase took place over the entire time interval, depending on the primordial size of its parent body and the composition of the icy material considered. Our calculations suggest that the formation of 67P/Churyumov–Gerasimenko is consistent with both its accretion from primordial building blocks formed in the nebula or from debris issued from the disruption of a Hale–Bopp-like body.