David H. Weinberg
Astrophysical Journal 851, 25 Link to Article [DOI: 10.3847/1538-4357/aa96b2]
Department of Astronomy and Center for Cosmology and AstroParticle Physics, The Ohio State University, Columbus, OH 43210, USA

Observational studies show that the global deuterium-to-hydrogen ratio in the local interstellar medium (ISM) is about 90% of the primordial ratio predicted by Big Bang nucleosynthesis. The high implies that only a small fraction of interstellar gas has been processed through stars, which destroy any deuterium they are born with. Using analytic arguments for one-zone chemical evolution models that include accretion and outflow, I show that the deuterium abundance is tightly coupled to the abundance of core collapse supernova (CCSN) elements, such as oxygen. These models predict that the ratio of the ISM deuterium abundance to the primordial abundance is , where r is the recycling fraction, is the ISM oxygen mass fraction, and is the population-averaged CCSN yield of oxygen. Using values r = 0.4 and appropriate to a Kroupa initial mass function and recent CCSN yield calculations, solar oxygen abundance corresponds to , consistent with the observations. This approximation is accurate for a wide range of parameter values, and physical arguments and numerical tests suggest that it should remain accurate for more complex chemical evolution models. The good agreement with the upper range of observed values supports the long-standing suggestion that sightline-to-sightline variations of deuterium are a consequence of dust depletion, rather than a low global enhanced by localized accretion of primordial composition gas. This agreement limits deviations from conventional yield and recycling values, including models in which most high-mass stars collapse to form black holes without expelling their oxygen in supernovae, and it implies that Galactic outflows eject ISM hydrogen as efficiently as they eject CCSN metals.

Hiroaki Katsuragi^1,2 and Jürgen Blum¹
Astrophysical Journal 851, 23 Link to Article [DOI: 10.3847/1538-4357/aa970d]
¹Institut für Geophysik und extraterrestrische Physik, Technische Universität zu Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany
²Department of Earth and Environmental Sciences, Nagoya University, Furocho, Chikusa, Nagoya, Aichi 464-8601, Japan

Dynamic characterization of mechanical properties of dust aggregates has been one of the most important problems to quantitatively discuss the dust growth in protoplanetary disks. We experimentally investigate the dynamic properties of dust aggregates by low-speed (3.2 m s⁻¹) impacts of solid projectiles. Spherical impactors made of glass, steel, or lead are dropped onto a dust aggregate with a packing fraction of  = 0.35 under vacuum conditions. The impact results in cratering or fragmentation of the dust aggregate, depending on the impact energy. The crater shape can be approximated by a spherical segment and no ejecta are observed. To understand the underlying physics of impacts into dust aggregates, the motion of the solid projectile is acquired by a high-speed camera. Using the obtained position data of the impactor, we analyze the drag-force law and dynamic pressure induced by the impact. We find that there are two characteristic strengths. One is defined by the ratio between impact energy and crater volume and is 120 kPa. The other strength indicates the fragmentation threshold of dynamic pressure and is 10 kPa. The former characterizes the apparent plastic deformation and is consistent with the drag force responsible for impactor deceleration. The latter corresponds to the dynamic tensile strength to create cracks. Using these results, a simple model for the compaction and fragmentation threshold of dust aggregates is proposed. In addition, the comparison of drag-force laws for dust aggregates and loose granular matter reveals the similarities and differences between the two materials.

Althea V. Moorhead

Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13066]
NASA Meteoroid Environment Office, Marshall Space Flight Center, Huntsville, Alabama, USA
Published by arrangement with John Wiley & Sons

Debiasing the velocity distribution of meteors observed by the Canadian Meteor Orbit Radar (CMOR) yields a distribution with large numbers of slow meteors. The distribution also contains significant numbers of hyperbolic meteors, in conflict with the expectation that interstellar meteors should be rare. In Moorhead et al. (2017a), we noted that measurement uncertainties were possibly smoothing the speed distribution and redistributing meteors to the extreme ends of the speed distribution. In this report, we use techniques analogous to image sharpening to remove the blurring caused by measurement uncertainties. The deconvolved speed distribution appears to have no meteors slower than 14 km s⁻¹ and none faster than 74 km s⁻¹. The result is to substantially raise the characteristic velocity of incoming meteoroids from 12.9 to 20.0 km s⁻¹.

Jens Hopp^1,2,*, Natalie Schröter¹, Olga Pravdivtseva³, Hans-Peter Meyer¹, Mario Trieloff^1,2 and Ulrich Ott^1,4,5

Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13062]
¹Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany
²Klaus-Tschira-Labor für Kosmochemie, Heidelberg, Germany
³McDonnell Center for the Space Sciences and Physics Department of Washington University, Saint Louis, Missouri, USA
⁴MTA Atomki, Debrecen, Hungary
⁵Max-Planck-Institut für Chemie, Mainz, Germany
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 7325 is an anomalous achondrite that experienced episodes of large-degree melt extraction and interaction with melt under reducing conditions. Its composition led to speculations about a Mercurian origin and provoked a series of studies of this meteorite. We present the noble gas composition, and results of ⁴⁰Ar/³⁹Ar and ¹²⁹I-¹²⁹Xe studies of whole rock splits of NWA 7325. The light noble gases are dominated by cosmogenic isotopes. ²¹Ne and ³⁸Ar cosmic-ray exposure ages are 25.6 and 18.9 Ma, respectively, when calculated with a nominal whole rock composition. This ³⁸Ar age is in reasonable agreement with a cosmic-ray exposure age of 17.5 Ma derived in our ⁴⁰Ar/³⁹Ar dating study. Due to the low K-content of 19 ± 1 ppm and high Ca-content of approximately 12.40 ± 0.15 wt%, no reliable ⁴⁰Ar/³⁹Ar age could be determined. The integrated age strongly depends on the choice of an initial ⁴⁰Ar/³⁶Ar ratio. An air-like component is dominant in lower temperature extractions and assuming air ⁴⁰Ar/³⁶Ar for the trapped component results in a calculated integrated age of 3200 ± 260 (1σ) Ma. This may represent the upper age limit for a major reheating event affecting the K-Ar system. Results of ¹²⁹I-¹²⁹Xe dating give no useful chronological information, i.e., no isochron is observed. Considering the highest ¹²⁹Xe*/¹²⁸Xe_I ratio as equivalent to a lower age limit, we calculate an I-Xe age of about 4536 Ma. In addition, elevated ¹²⁹Xe/¹³²Xe ratios of up to 1.65 ± 0.18 in higher temperature extractions indicate an early formation of NWA 7325, with subsequent disturbance of the I-Xe system.

E. B. Bierhaus^1,*, A. S. McEwen², S. J. Robbins³, K. N. Singer³, L. Dones³, M. R. Kirchoff³ and J.-P. Williams⁴

Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13057]
¹Lockheed Martin Space, Denver, Colorado, USA
²University of Arizona, Tucson, Arizona, USA
³Southwest Research Institute, Boulder, Colorado, USA
⁴University of California, Los Angeles, California, USA
Published by arrangement with John Wiley & Sons

We review the secondary-crater research over the past decade, and provide new analyses and simulations that are the first to model an accumulation of a combined primary-plus-secondary crater population as discrete cratering events. We develop the secondary populations by using scaling laws to generate ejecta fragments, integrating the trajectories of individual ejecta fragments, noting the location and velocity at impact, and using scaling laws to estimate secondary-crater diameters given the impact conditions. We also explore the relationship between the impactor size–frequency distribution (SFD) and the resulting secondary-crater SFD. Our results from these analyses indicate that the “secondary effect” varies from surface to surface and that no single conclusion applies across the solar system nor at any given moment in time—rather, there is a spectrum of outcomes both spatially and temporally, dependent upon target parameters and the impacting population. Surface gravity and escape speed define the spatial distribution of secondaries. A shallow-sloped impactor SFD will cause proportionally more secondaries than a steeper-sloped SFD. Accounting for the driving factors that define the magnitude and spatial distribution of secondaries is essential to determine the relative population of secondary craters, and their effect on derived surface ages.

Following several recent suggestions, Cosmochemistry Letters will now also cover (accepted) PhD and doctoral theses. So if you just had your thesis accepted, please send us a message with the details (Title, Author, Institution etc.) If available, please include a link.

We will also provide a separate roundup/overview for PhD theses, in order to make them more accessible.

In addition, we have also updated the layout of the page a little bit – the site pages have been updated  and compacted into Introduction and Coverage.

¹Allison M McGraw, ¹Vishnu Reddy, ²Juan A Sanchez
Monthly Notices of the Royal Astronomical Society 476, 630–634, Link to Article [https://doi.org/10.1093/mnras/sty250]
¹Lunar and Planetary Lab, The University of Arizona, 1629 E University Blvd, Tucson, AZ 85721, USA
²Planetary Science Institute, 1700 E Ft. Lowell, Tucson, AZ 85719, USA

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¹Lucille Le Corre, ¹Juan A Sanchez, ²Vishnu Reddy, ^3,4Driss Takir, ⁵Edward A Cloutis, ⁶Audrey Thirouin, ⁷Kris J Becker, ¹Jian-Yang Li, ⁸Seiji Sugita, ⁸Eri Tatsumi
Monthly Notices of the Royal Astronomical Society 475,614–623, Link to Article [https://doi.org/10.1093/mnras/stx3236]
¹Planetary Science Institute, 1700 E Fort Lowell Road, Tucson, AZ 85719, USA
²Lunar and Planetary Laboratory, University of Arizona, 1629 E University Blvd, Tucson, AZ 85721, USA
³SETI Institute, 89 Bernardo Ave, Suite 200, Mountain View, CA 94043, USA
⁴Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration
⁵Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba R3B 2E9, Canada
⁶Lowell Observatory, 1400 W Mars Hill Rd, Flagstaff, AZ 86001, USA
⁷USGS Astrogeology Science Center, 2255 N. Gemini Drive, Flagstaff, AZ 86001, USA
⁸Department of Earth and Planetary Science, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

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¹James R. Lyons, ²Ehsan Gharib-Nezhad, ¹Thomas R. Ayres
Nature Communications 9, 908 Link to Article [doi:10.1038/s41467-018-03093-3doi:10.1038/s41467-018-03093-3]
¹School of Earth and Space Exploration, Arizona State University, 781 S. Terrace Rd, Tempe, AZ, 85281, USA
²School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287, USA
³Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO, 80309, USA

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¹Alessandra Migliorini,¹M. C. De Sanctis,²D. Lazzaro,³E. Ammannito
Monthly Notices of the Royal Astronomical Society, 475, 353–358 Link to Article [https://doi.org/10.1093/mnras/stx3193]
¹Institute of Space Astrophysics and Planetology, IAPS-INAF, I-00133 Rome, Italy
²Observatório Nacional, COAA, 20921-400 Rio de Janeiro, Brazil
³Italian Space Agency, ASI, I-00133 Rome, Italy

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