T. Tsujimoto¹ and T. Shigeyama²

¹National Astronomical Observatory of Japan, Mitaka-shi, 181-8588 Tokyo Japan
²Research Center for the Early Universe, Graduate School of Science, University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

The origin of r-process elements remains unidentified and still puzzles us. The recent discovery of evidence for the ejection of r-process elements from a short-duration γ-ray burst singled out neutron star mergers (NSMs) as their origin. In contrast, core-collapse supernovae are ruled out as the main origin of heavy r-process elements (A > 110) by recent numerical simulations. However, the properties characterizing NSM events – their rarity and high yield of r-process elements per event – have been claimed to be incompatible with the observed stellar records on r-process elements in the Galaxy. We add to this picture with our results, which show that the observed constant [r-process/H] ratio in faint dwarf galaxies and one star unusually rich in r-process in the Sculptor galaxy agree well with this rarity of NSM events. Furthermore, we found that a large scatter in the abundance ratios of r-process elements to iron in the Galactic halo can be reproduced by a scheme that incorporates an assembly of various protogalactic fragments, in each of which r-process elements supplied by NSMs pervade the whole fragment while supernovae distribute heavy elements only inside the regions swept up by the blast waves. Our results demonstrate that NSMs occurring at Galactic rate of 12–23 Myr^-1 are the main site of r-process elements, and we predict the detection of gravitational waves from NSMs at a high rate with upcoming advanced detectors.

Reference
Tsujimoto T and Shigeyama T (2014) Enrichment history of r-process elements shaped by a merger of neutron star pairs.  Astronomy & Astrophysics 565:AL5.
[doi:10.1051/0004-6361/201322175]
Reproduced with permission © ESO

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Thomas M. McCollom¹, Bethany L. Ehlmann^3,4, Alian Wang⁵, Brian M. Hynek^1,2, Bruce Moskowitz^6,7 and Thelma S. Berquó⁸

¹Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80309, U.S.A.
²Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, U.S.A.
³California Institute of Technology, Pasadena, California 91125, U.S.A.
⁴Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, U.S.A.
⁵Deptartment of Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University, St. Louis, Missouri 63130, U.S.A.
⁶Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.
⁷Institute for Rock Magnetism, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.
⁸Department of Physics, Concordia College, Moorhead, Minnesota 56562, U.S.A.

Natroalunite containing substantial amounts of Fe occurs as a prominent secondary phase during acid-sulfate alteration of pyroclastic basalts in volcanic fumaroles in Nicaragua and elsewhere, and has been observed in laboratory simulations of acid-sulfate alteration as well. Reaction path models constrained by field and experimental observations predict that Fe-rich natroalunite should also form as a major secondary phase during alteration of martian basalt under similar circumstances. Here, we evaluate the potential to use spectroscopic methods to identify minerals from the alunite group with chemical compositions intermediate between natroalunite and natrojarosite on the surface of Mars, and to remotely infer their Fe contents. X-ray diffraction and spectroscopic measurements (Raman, visible/near infrared, mid-infrared, Mössbauer) were obtained for a suite of synthetic solid solutions with a range of Fe contents ranging from natroalunite to natrojarosite. In the visible/near infrared, minerals with intermediate compositions display several spectral features not evident in end-member spectra that could be used to remotely identify these minerals and infer their composition. In addition, Raman spectra, mid-infrared spectra, and X-ray diffraction peaks all show systematic variation with changing Fe content, indicating that these methods could potentially be used to infer mineral compositions as well. The results suggest that alunite group minerals with intermediate Fe compositions may be able to account for some visible/near-infrared and Mössbauer spectral features from Mars that had previously been unidentified or attributed to other phases. Overall, our findings indicate that consideration of solid solutions may lead to new identifications of alunite group minerals on the surface of Mars, and raise the possibility that minerals with compositions intermediate between natroalunite and natrojarosite may be widely distributed on the planet.

Reference
McCollom TM, Ehlmann BL, Wang A, Hynek BM, Moskowitz B and Berquó TS (2014) Detection of iron substitution in natroalunite-natrojarosite solid solutions and potential implications for Mars. American Mineralogist 99:948-964.
[doi:10.2138/am.2014.4617]
Copyright: The Mineralogical Society of America

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C. T. Pillinger¹, J. M. Pillinger¹, D. Johnson¹, R. C. Greenwood¹, A. G. Tindle², A. J. T. Jull³, D. H. Allen⁴ and B. Cunliffe⁵

¹Planetary and Space Sciences, The Open University, Milton Keynes, UK
²CEPSAR, The Open University, Milton Keynes, UK
³NSF Arizona AMS Laboratory and Department of Geosciences, The University of Arizona, Tucson, Arizona, USA
⁴Arts and Museums Service, Hampshire County Council, Winchester, UK
⁵Institute of Archaeology, University of Oxford, Oxford, UK

What remains of a 30 g sample, first recognized as a meteorite in 1989 during characterization of metalworking debris from Danebury, an Iron Age hillfort, in Hampshire, England, has been classified as an H5 ordinary chondrite. Its arrival on Earth has been dated as 2350 ± 120 yr BP, making it contemporary with the period of maximum human activity at the recovery site. Despite its considerable terrestrial residence age, the interior of the specimen is remarkably fresh with a weathering index of W1/2. There is, however, no evidence of human intervention in its preservation. Its near-pristine state is explained in terms of its serendipitous burial during the back-fill of a pit dug into chalk by prehistoric people for the storage of grain. This chance discovery has interesting ramifications for the survival of meteorites in areas having a high pH because of a natural lime content arising as a result of the local geology.

Reference
Pillinger CT, Pillinger JM, Johnson D, Greenwood RC, Tindle AG, Jull AJT, Allen DH and Cunliffe B (in press) The Danebury Iron Age meteorite—An H5 ordinary chondrite “find” from Hampshire, England. Meteoritics & Planetary Science
[doi:10.1111/maps.12301]
Published by arrangement with John Wiley & Sons

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Shoko Oshigami¹, Shiho Watanabe², Yasushi Yamaguchi², Atsushi Yamaji³, Takao Kobayashi⁴, Atsushi Kumamoto⁵, Ken Ishiyama⁵ and Takayuki Ono⁵

¹National Astronomical Observatory of Japan, Oshu, Japan
²Graduate School of Environmental Studies, Nagoya University, Chikusa-ku, Japan
³Graduate School of Science, Kyoto University, Kyoto, Japan
⁴Geological Research Division, Korean Institute of Geoscience and Mineral Resources, Daejeon, South Korea
⁵Graduate School of Science, Tohoku University, Sendai, Japan

The Lunar Radar Sounder (LRS) onboard Kaguya (SELENE) detected widespread horizontal reflectors under some nearside maria. Previous studies estimated that the depths of the subsurface reflectors were up to several hundreds of meters and suggested that the reflectors were interfaces between mare basalt units. The comparison between the reflectors detected in the LRS data and surface age maps indicating the formation age of each basalt unit allows us to discuss the lower limit volume of each basalt unit and its space and time variation. We estimated volumes of basalt units in the ages of 2.7 Ga to 3.8 Ga in the nearside maria including Mare Crisium, Mare Humorum, Mare Imbrium, Mare Nectaris, Mare Serenitatis, Mare Smythii, and Oceanus Procellarum. The lower limit volumes of the geologic units estimated in this study were on the order of 10³ to 10⁴ km³. This volume range is consistent with the total amount of erupted lava flows derived from numerical simulations of thermal erosion models of lunar sinuous rille formation and is also comparable to the average flow volumes of continental flood basalt units formed after the Paleozoic and calculated flow volumes of Archean komatiite flows on the Earth. The lower limits of average eruption rates estimated from the unit volumes were on the order of 10⁻⁵ to 10⁻³ km³/yr. The estimated volumes of the geologic mare units and average eruption rate showed clear positive correlations with their ages within the same mare basin, while they vary among different maria compared within the same age range.

Reference
Oshigami S, Watanabe S, Yamaguchi Y, Yamaji A, Kobayashi T, Kumamoto A, Ishiyama K, and Ono T (in press) Mare volcanism: Reinterpretation based on Kaguya Lunar Radar Sounder data. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004568]
Published by arrangement with John Wiley & Sons

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Alan P. Boss and Sandra A. Keiser

Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015-1305, USA

A key test of the supernova triggering and injection hypothesis for the origin of the solar system’s short-lived radioisotopes is to reproduce the inferred initial abundances of these isotopes. We present here the most detailed models to date of the shock wave triggering and injection process, where shock waves with varied properties strike fully three-dimensional, rotating, dense cloud cores. The models are calculated with the FLASH adaptive mesh hydrodynamics code. Three different outcomes can result: triggered collapse leading to fragmentation into a multiple protostar system; triggered collapse leading to a single protostar embedded in a protostellar disk; or failure to undergo dynamic collapse. Shock wave material is injected into the collapsing clouds through Rayleigh–Taylor fingers, resulting in initially inhomogeneous distributions in the protostars and protostellar disks. Cloud rotation about an axis aligned with the shock propagation direction does not increase the injection efficiency appreciably, as the shock parameters were chosen to be optimal for injection even in the absence of rotation. For a shock wave from a core-collapse supernova, the dilution factors for supernova material are in the range of ~10⁻⁴ to ~3 × 10⁻⁴, in agreement with recent laboratory estimates of the required amount of dilution for ⁶⁰Fe and ²⁶Al. We conclude that a type II supernova remains as a promising candidate for synthesizing the solar system’s short-lived radioisotopes shortly before their injection into the presolar cloud core by the supernova’s remnant shock wave.

Reference
Boss AP and Keiser SA (2014) Triggering Collapse of the Presolar Dense Cloud Core and Injecting Short-lived Radioisotopes with a Shock Wave. III. Rotating Three-dimensional Cloud Cores. The Astrophysical Journal 788:20.
[doi:10.1088/0004-637X/788/1/20]

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P. Sánchez¹ and D. J. Scheeres²

¹Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, USA
²Engineering Sciences Colorado, The Center for Astrodynamics Research, The University of Colorado at Boulder, Boulder, Colorado, USA

We explore the hypothesis that, due to small van der Waals forces between constituent grains, small rubble pile asteroids have a small but nonzero cohesive strength. The nature of this model predicts that the cohesive strength should be constant independent of asteroid size, which creates a scale dependence with relative strength increasing as size decreases. This model counters classical theory that rubble pile asteroids should behave as scale-independent cohesionless collections of rocks. We explore a simple model for asteroid strength that is based on these weak forces, validate it through granular mechanics simulations and comparisons with properties of lunar regolith, and then explore its implications and ability to explain and predict observed properties of small asteroids in the NEA and Main Belt populations, and in particular of asteroid 2008 TC₃. One conclusion is that the population of rapidly rotating asteroids could consist of both distributions of smaller grains (i.e., rubble piles) and of monolithic boulders.

Reference
Sánchez P and Scheeres DJ (in press) The strength of regolith and rubble pile asteroids. Meteoritics & Planetary Science
[doi:10.1111/maps.12293]
Published by arrangement with John Wiley & Sons

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B.C. Johnson^a, T.J. Bowling^b and H.J. Melosh^b

^aDepartment of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139
^bDepartment of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907

The extreme pressures reached during jetting, a process by which material is squirted out from the contact point of two colliding objects, causes melting and vaporization at low impact velocities. Jetting is a major source of melting in shocked porous material, a potential source of tektites, a possible origin of chondrules, and even a conceivable origin of the moon. Here, in an attempt to quantify the importance of jetting, we present numerical simulation of jetting during the vertical impacts of spherical projectiles on both flat and curved targets. We find that impacts on curved targets result in more jetted material but that higher impact velocities result in less jetted material. For an Aluminum impactor striking a flat Al target at 2 km/s we find that 3.4% of a projectile mass is jetted while 8.3% is jetted for an impact between two equal sized Al spheres. Our results indicate that the theory of jetting during the collision of thin plates can be used to predict the conditions when jetting will occur. However, we find current analytic models do not make accurate predictions of the amount of jetted mass. Our work indicates that the amount of jetted mass is independent of model resolution as long as some jetted material is resolved. This is the result of lower velocity material dominating the mass of the jet.

Reference
Johnson BC, Bowling TJ and Melosh HJ (in press) Jetting during vertical impacts of spherical projectiles. Icarus
[doi:10.1016/j.icarus.2014.05.003]
Copyright Elsevier

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A. J. Evans^1,2, M. T. Zuber¹, B. P. Weiss¹ and S. M. Tikoo¹

¹Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
²Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA

While recent analyses of lunar samples indicate the Moon had a core dynamo from at least 4.2–3.56 Ga, mantle convection models of the Moon yield inadequate heat flux at the core-mantle boundary to sustain thermal core convection for such a long time. Past investigations of lunar dynamos have focused on a generally homogeneous, relatively dry Moon, while an initial compositionally stratified mantle is the expected consequence of a postaccretionary lunar magma ocean. Furthermore, recent re-examination of Apollo samples and geophysical data suggests that the Moon contains at least some regions with high water content. Using a finite element model, we investigate the possible consequences of a heterogeneously wet, compositionally stratified interior for the evolution of the Moon. We find that a postoverturn model of mantle cumulates could result in a core heat flux sufficiently high to sustain a dynamo through 2.5 Ga and a maximum surface, dipolar magnetic field strength of less than 1 μT for a 350-km core and near ∼2 μT for a 450-km core. We find that if water was transported or retained preferentially in the deep interior, it would have played a significant role in transporting heat out of the deep interior and reducing the lower mantle temperature. Thus, water, if enriched in the lower mantle, could have influenced core dynamo timing by over 1.0 Gyr and enhanced the vigor of a lunar core dynamo. Our results demonstrate the plausibility of a convective lunar core dynamo even beyond the period currently indicated by the Apollo samples.

Reference
Evans AJ, Zuber MT, Weiss BP and Tikoo SM (in press) A wet, heterogeneous lunar interior: Lower mantle and core dynamo evolution. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004494]
Published by arrangement with John Wiley & Sons

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Alexander N. Krot, Changkun Park and Kazuhide Nagashima

Hawai‘i Institute for Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI, 96822, USA

Amoeboid olivine aggregates (AOAs) in CH carbonaceous chondrites are texturally and mineralogically similar to those in other carbonaceous chondrite groups. They show no evidence for alteration and thermal metamorphism in an asteroidal setting and consist of nearly pure forsterite (Fa_❤; in wt%, CaO = 0.10.8, Cr₂O₃ = 0.040.48; MnO <0.5), anorthite, Al-diopside (in wt%, Al₂O₃ = 0.78.1; TiO₂ <1), Fe,Ni-metal, spinel, and, occasionally, low-Ca pyroxene (Fs₁Wo₂₃), and calcium-aluminum-rich inclusions (CAIs). The CAIs inside AOAs are composed of hibonite, grossite, melilite (Åk₁₃₄₄), spinel, perovskite, Al,Ti-diopside (in wt%, Al₂O₃ up to 19.6; TiO₂ up to 13.9), and anorthite. The CH AOAs, including CAIs within AOAs, have isotopically uniform ¹⁶O-rich compositions (average Δ¹⁷O = 23.4±2.3‰, 2SD) and on a three-isotope oxygen diagram plot along ∼slope-1 line. The only exception is a low-Ca pyroxene-bearing AOA 1-103 that shows a range of Δ¹⁷O values, from 24‰ to 13‰. Melilite, grossite, and hibonite in four CAIs within AOAs show no evidence for radiogenic ²⁶Mg excess (δ²⁶Mg). In contrast, anorthite in five out of six AOAs measured has δ²⁶Mg corresponding to the inferred initial ²⁶Al/²⁷Al ratio of (4.3±0.7)10⁵, (4.2±0.6)10⁵, (4.0±0.3)10⁵, (1.7±0.2)10⁵, and (3.0±2.6)10⁶. Anorthite in another AOA shows no resolvable δ²⁶Mg excess; an upper limit on the initial ²⁶Al/²⁷Al ratio is 510⁶. We infer that CH AOAs formed by gas-solid condensation and aggregation of the solar nebula condensates (forsterite and Fe,Ni-metal) mixed with the previously formed CAIs. Subsequently they experienced thermal annealing and possibly melting to a small degree in a ¹⁶O-rich gaseous reservoir during a brief epoch of CAI formation. The low-Ca pyroxene-bearing AOA 1-103 may have experienced incomplete melting and isotope exchange in an ¹⁶O-poor gaseous reservoir. The lack of resolvable δ²⁶Mg excess in melilite, grossite, and hibonite in CAIs within AOAs reflects heterogeneous distribution of ²⁶Al in the solar nebula during this epoch. The observed variations of the inferred initial ²⁶Al/²⁷Al ratios in anorthite of the mineralogically pristine and uniformly ¹⁶O-rich CH AOAs could have recorded (i) admixing of ²⁶Al in the protoplanetary disk during the earliest stages of its evolution and/or (ii) closed-system Mg-isotope exchange between anorthite and Mg-rich minerals (spinel, forsterite, and Al-diopside) during subsequent prolonged (days-to-weeks) thermal annealing at high temperature (∼1100°C) and slow cooling rates (~0.01 K hr¹) that has not affected their O-isotope systematics. The proposed thermal annealing may have occurred in an impact-generated plume invoked for the origin of non-porphyritic magnesian chondrules and Fe,Ni-metal grains in CH and CB carbonaceous chondrites about 5 Myr after formation of CV CAIs.

Reference
Krot AN, Park C and Nagashima K (in press) Amoeboid olivine aggregates from CH carbonaceous chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.050]
Copyright Elsevier

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F. Zambon^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aINAF-IAPS Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100, 00133 Rome, Italy

Vesta spectra have prominent near-infrared absorption bands characteristic of pyroxenes, indicating a direct link to the howardite, eucrite and diogenite meteorites. Many localized dark and bright materials are present on Vesta’s surface. Here we focus on the bright material (BM) units to determine their spectral properties, their origin, the presence of mineralogical phases different from pyroxenes, and whether different bright units share a common lithology. VIR, the Visible and Infrared spectrometer onboard Dawn, allows us to first do a detailed analysis of the spectral properties of a large number of bright material units on Vesta including examples of the different morphological classes. The spectral parameters used are band centers, band depths, and Band Area Ratio (BAR) for the pyroxene bands at ∼0.9 and ∼1.9 μm. The mineralogies of most bright regions are consistent with those of the howardite, eucrite and diogenite meteorites typical of Vesta’s surface. We find that bright material units exhibit the full range of HED pyroxene composition, from eucrites to diogenites. Large part of the bright materials are eucrite-rich, according with the Vesta’s mineralogy. In most cases, the bright materials have the same mineralogy of the surrounding terrain, but have larger band depth values. The band depths can be related to the abundance of the absorbing minerals, the abundance of Fe²⁺, grain size, and/or to the abundance of opaque materials. We found a positive correlation between albedo and band depth, which suggests that the grain size is not the main factor responsible for the higher albedo. The analysis of the band parameters indicates that most of the bright materials, excluding the few olivine-rich units, represent fresh uncontaminated Vestan pyroxenes from a variety of lithologies exposed from beneath the surface by impacts.

Reference
Zambon et al. (in press) Spectral Analysis of the Bright Materials on the Asteroid Vesta. Icarus
[doi:10.1016/j.icarus.2014.04.037]
Copyright Elsevier

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