^1,2,3Birger Schmitz, ¹Samuele Boschi, ¹Anders Cronholm, ^2,4Philipp R. Heck, ⁵Simonetta Monechi, ⁶Alessandro Montanari, ¹Fredrik Terfelt
¹Astrogeobiology Laboratory, Department of Physics, Lund University, Sweden
²Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, Chicago, IL, USA
³Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, Honolulu, HI, USA
⁴Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL, USA
⁵Department of Earth Sciences, Florence University, Florence, Italy
⁶Geological Observatory of Coldigioco, Frontale di Apiro, Macerata, Italy

The onset of Earth’s present icehouse climate in the Late Eocene coincides with astronomical events of enigmatic causation. At ∼36 Ma ago the 90–100 km large Popigai and Chesapeake Bay impact structures formed within ∼10–20 ka∼10–20 ka. Enrichments of 3He in coeval sediments also indicate high fluxes of interplanetary dust to Earth for ∼2 Ma∼2 Ma. Additionally, several medium-sized impact structures are known from the Late Eocene. Here we report from sediments in Italy the presence of abundant ordinary chondritic chromite grains (63–250 μm) associated with the ejecta from the Popigai impactor. The grains occur in the ∼40 cm∼40 cm interval immediately above the ejecta layer. Element analyses show that grains in the lower half of this interval have an apparent H-chondritic composition, whereas grains in the upper half are of L-chondritic origin. The grains most likely originate from the regoliths of the Popigai and the Chesapeake Bay impactors, respectively. These asteroids may have approached Earth at comparatively low speeds, and regolith was shed off from their surfaces after they passed the Roche limit. The regolith grains then settled on Earth some 100 to 1000 yrs after the respective impacts. Further neon and oxygen isotopic analyses of the grains can be used to test this hypothesis.
If the Popigai and Chesapeake Bay impactors represent two different types of asteroids one can rule out previous explanations of the Late Eocene extraterrestrial signatures invoking an asteroid shower from a single parent-body breakup. Instead a multi-type asteroid shower may have been triggered by changes of planetary orbital elements. This could have happened due to chaos-related transitions in motions of the inner planets or through the interplay of chaos between the outer and inner planets. Asteroids in a region of the asteroid belt where many ordinary chondritic bodies reside, were rapidly perturbed into orbital resonances. This led to an increase in small to medium-sized collisional breakup events over a 2–5 Ma period. This would explain the simultaneous delivery of excess dust and asteroids to the inner solar system. Independent evidence for our scenario are the common cosmic-ray exposure ages in the range of ca. 33–40 Ma for recently fallen H and L chondrites.
The temporal coincidence of gravity disturbances in the asteroid belt with the termination of ice-free conditions on Earth after 250 Ma is compelling. We speculate that this coincidence and a general correlation during the past 2 Ga between K–Ar breakup ages of parent bodies of the ordinary chondrites and ice ages on Earth suggest that there may exist an astronomical process that disturbs both regions of the inner asteroid belt and Earth’s orbit with a potential impact on Earth’s climate.

Reference
Schmitz B, Boschi S, Cronholm A, Heck PR, Monechi S, Montanari A, Terfelt F (2015) Fragments of Late Eocene Earth-impacting asteroids linked to disturbance of asteroid belt. Earth and Planetary Science Letters 425, 77–83.
Link to Article [doi:10.1016/j.epsl.2015.05.041]

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¹Yang Chen, ¹Yang Liu, ²Yunbin Guan, ²John M. Eiler, ²Chi Ma, ²George R. Rossman, ³Lawrence A. Taylor
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
³Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA

We report unambiguous chemical evidence for subsurface water activity in the martian crust at <600 Ma based on the data from Tissint, a fresh martian meteorite fall with minimal terrestrial weathering. The impact-melt pockets in Tissint contain abundant volatiles (H₂O, CO₂, F, and Cl), and their concentrations are positively correlated with each other. Higher H₂O concentrations also accompany higher deuterium contents. These correlations suggest mixing between two volatile sources. The first source is H₂O in the precursor basalt inherited from martian magma. Magmatic H₂O in the basalt had low deuterium concentration and was likely stored in the nominally anhydrous minerals. This source contributed little CO₂ or halogens to the impact melts. The second source is inferred to be aqueous-alteration products introduced to the basalt by water activity after the basalt erupted. These alteration materials contributed more volatiles to the impact melts than the magmatic source, and had high deuterium abundance, reflecting isotope equilibrium with recent martian atmosphere. The water activities occurred beneath the martian surface after ∼600 Ma (crystallization age), but before ∼1 Ma (ejection age). The chemical and isotopic signatures of the alteration products in Tissint resemble previously known martian samples associated with old water activities on Mars, which can be traced back to ∼4.2 billion years ago (e.g., the mudstone at Gale Crater). This similarity in chemistry and the wide age-span indicate that such water activities were common on Mars throughout its history, which had the potential to form habitable environment. However, the rarity of the volatile-rich zone in Tissint suggests that Martian crustal aqueous processes, where they have occurred are generally limited in their extent of water–rock reaction.

Reference
Chen Y, Liu Y, Guan Y, Eiler JM, Ma C, Rossman GR, Taylor LC (2015) Evidence in Tissint for recent subsurface water on Mars. Earth and Planetary Science Letters 425, 55–63
Link to Article [doi:10.1016/j.epsl.2015.05.004]

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¹Kevin M. Cannon, ¹John F. Mustard
¹Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island 02912, USA

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Reference
Cannon KM, Mustard JF (2015) Preserved glass-rich impactites on Mars. Geology (in Press)
Link to Article [10.1130/G36953.1]

¹Mai Yia Xiong, ¹Evgenya S. Shelobolina, ¹Eric E. Roden
¹Department of Geoscience, University of Wisconsin, and NASA Astrobiology Institute, University of Wisconsin, Madison, Wisconsin.

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Reference
Xiong MY, Shelobolina ES, Roden EE (2015) Potential for Microbial Oxidation of Ferrous Iron in Basaltic Glass.
Astrobiology 15(5), 331-340.
Link to Article [doi:10.1089/ast.2014.1233]

¹G. J. Wasserburg, ²O. Trippella, ³M. Busso
¹Lunatic Asylum, Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
²Department of Physics & Geology, University of Perugia, and INFN, Section of Perugia, via A. Pascoli, Perugia, I-06123, Italy

We study the effects of neutron captures in AGB stars on “Fe-group” elements, with an emphasis on Cr, Fe, and Ni. These elements show anomalies in 54Cr, 58Fe, and 64Ni in solar system materials, which are commonly attributed to supernovae (SNe). However, as large fractions of the interstellar medium (ISM) were reprocessed in AGB stars, these elements were reprocessed, too. We calculate the effects of such reprocessing on Cr, Fe, and Ni through 1.5 ${{M}_{\odot }}$ and 3 ${{M}_{\odot }}$ AGB models, adopting solar and 1/3 solar metallicities. All cases produce excesses of 54Cr, 58Fe, and 64Ni, while the other isotopes are little altered; hence, the observations may be explained by AGB processing. The results are robust and not dependent on the detailed initial isotopic composition. Consequences for other “Fe group” elements are then explored. They include 50Ti excesses and some production of $^{46,47,49}$Ti. In many circumstellar condensates, Ti quantitatively reflects these effects of AGB neutron captures. Scatter in the data results from small variations (granularity) in the isotopic composition of the local ISM. For Si, the main effects are instead due to variations in the local ISM from different SN sources. The problem of Ca is discussed, particularly with regard to 48Ca. The measured data are usually represented assuming terrestrial values for 42Ca/44Ca. Materials processed in AGB stars or sources with variable initial 42Ca/44Ca ratios can give apparent 48Ca excesses/deficiencies, attributed to SNe. The broader issue of galactic chemical evolution is also discussed in view of the isotopic granularity in the ISM.

Reference
Wasserburg GJ, Tripella O, Busso M (2015) Isotope Anomalies in the Fe-group Elements in Meteorites and Connections to Nucleosynthesis in AGB Stars. Astrophysical Journal 805, 7.

Link to Article [doi:10.1088/0004-637X/805/1/7]

¹R. Trappitsch, ¹F. J. Ciesla
¹Department of the Geophysical Sciences and Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL 60637

Solar cosmic-ray (SCR) interactions with a protoplanetary disk have been invoked to explain several observations of primitive planetary materials. In our own Solar System, the presence of short-lived radionuclides (SLRs) in the oldest materials has been attributed to spallation reactions induced in phases that were irradiated by energetic particles in the solar nebula. Furthermore, observations of other protoplanetary disks show a mixture of crystalline and amorphous grains, though no correlation between grain crystallinity and disk or stellar properties have been identified. As most models for the origin of crystalline grains would predict such correlations, it was suggested that amorphization by stellar cosmic-rays may be masking or erasing such correlations. Here we quantitatively investigate these possibilities by modeling the interaction of energetic particles emitted by a young star with the surrounding protoplanetary disk. We do this by tracing the energy evolution of SCRs emitted from the young star through the disk and model the amount of time that dust grains would spend in regions where they would be exposed to these particles. We find that this irradiation scenario cannot explain the total SLR content of the solar nebula; however, this scenario could play a role in the amorphization of crystalline material at different locations or epochs of the disk over the course of its evolution.

Reference
Trappitsch R, Ciesla FJ (2015) Solar Cosmic-ray Interaction with Protoplanetary Disks: Production of Short-lived Radionuclides and Amorphization of Crystalline Material. Astrophysical Journal 805, 5
Link to Article [doi:10.1088/0004-637X/805/1/5]

¹P. Beck, ¹B. Schmitt, ²E.A. Cloutis, ³P. Vernazza
¹Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France, CNRS, IPAG, F-38000 Grenoble, France
²Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, 6 Manitoba, Canada R3B 2E9
³Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France

The surface of Ceres, the most massive asteroid, presents a peculiar absorption band at 3.06 μm. This feature has been attributed to a number of candidate phases, including a magnesium hydroxide, brucite (Mg(OH)2). In order to gain insights into this possibility we have investigated the evolution of brucite reflectance spectrum under decreasing temperature (down to 93 K). Following early observation of brucite infrared spectra in transmission, a strong evolution of the reflectance spectrum of brucite is found under decreasing temperature. Small shifts of the band positions are found in particular at 1.36 and 1.39 μm, while the most important evolution is a decrease in intensity of the features at 2.82 and 3.06 μm. These observations can be seen in the light of the nature of these modes, which are transitions from excited states (difference bands) that are less populated under low-temperature. Such results provide a major test for the presence of brucite on Ceres from forthcoming DAWN observations, by searching for a possible evolution of the band with local time and then surface temperature. Based on the fact that the equivalent summation bands are not observed in Ceres spectra, brucite is not favored as the major constituent to the 3.06 μm feature. The possibility that this feature rather corresponds to ammoniated minerals (phyllosilicates or salts) is discussed.

Reference
Beck P, Schmitt B, Cloutis EA, Vernazza P (2015) Low-Temperature reflectance spectra of brucite and the primitive surface of 1-Ceres? Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.05.031]

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^1,2 D. Dai, ¹ C. Zhou, ^1,3 X. Chen
¹Institute of Geology, Hunan University of Science and Technology, Xiangtan, China
²Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China
³Hunan Provincial Key Laboratory of Shale Gas Resource Utilization, Xiangtan, China

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Reference
Dai D, Zhou C, Chen X (2015) Ca-, Al-Rich Inclusions in Two New Carbonaceous Chondrites from Grove Mountains, Antarctica. Earth, Moon , and Planets (in Press)
Link to Article [DOI 10.1007/s11038-015-9470-1]

¹Ramakant R. Mahajan
¹Physical Research Laboratory, Ahmedabad-380009, India

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Reference
Mahajan RR (2015) Lunar meteorite Yamato-983885: Noble gases, nitrogen and cosmic ray exposure history. Planetary and Space Science (in Press)
Link to Article [doi:10.1016/j.pss.2015.05.003]

¹F. Zambona et al. (>10)*
¹INAF-IAPS Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100, 00133 Rome, Italy
*Find the extensive, full author and affiliation list on the publishers website

The Av-13 (Tuccia) and Av-14 (Urbinia) quadrangles are located in the south-west region of Vesta. They are characterized by a large topographic variability, from the highest (Vestalia terra highlands) to the lowest (Rheasilvia basin). Many geological units in these quadrangle are not associated with mineralogical variability, as shown by the color-composite maps. Maps of mafic absorption band-center position reveal that the principal lithology is eucrite-rich howardite, but diogenite-rich howardite areas also are present, corresponding to particular features such as Antonia and Justina craters, which are also characterized by strong mafic absorptions. These quadrangles, especially Urbinia, are characterized by many bright ejecta, such as those of Tuccia crater, which are the highest reflectance materials on Vesta (Zambon et al., 2014). Dark areas are also present and correspond with regions with deeper OH-signature. The two quadrangle also contain many vertical ridge crests associated with the Rheasilvia impact. These ridges do not show mineralogical differences with respect to their surroundings, but have a distinctive appearance in color-ratio composite images.

Reference
Zambona F et al. (2015) Spectral Analysis of the Quadrangles Av-13 and Av-14 on Vesta. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.05.015]
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