¹A. Ciaravella, ¹C. Cecchi-Pestellini, ²Y.-J. Chen, ³G. M. Muñoz Caro, ²C.-H. Huang, ¹A. Jiménez-Escobar, ⁴A. M. Venezia
Astrophysical Journal 828, 29 Link to Article [http://dx.doi.org/10.3847/0004-637X/828/1/29]
¹INAF—Osservatorio Astronomico di Palermo, P.za Parlamento 1, I-90134 Palermo, Italy
²Department of Physics, National Central University, Jhongli City, Taoyuan County 32054, Taiwan
³Centro de Astrobiología (INTA-CSIC), Carretera de Ajalvir, km 4, Torrejón de Ardoz, E-28850 Madrid, Spain
⁴ISMN—CNR, Via Ugo La Malfa 153, I-90146 Palermo, Italy

The processing of energetic photons on bare silicate grains was simulated experimentally on silicate films submitted to soft X-rays of energies up to 1.25 keV. The silicate material was prepared by means of a microwave assisted sol–gel technique. Its chemical composition reflects the Mg2SiO4 stoichiometry with residual impurities due to the synthesis method. The experiments were performed using the spherical grating monochromator beamline at the National Synchrotron Radiation Research Center in Taiwan. We found that soft X-ray irradiation induces structural changes that can be interpreted as an amorphization of the processed silicate material. The present results may have relevant implications in the evolution of silicate materials in X-ray-irradiated protoplanetary disks.

¹Brian J. Fry, ¹Brian D. Fields, ²John R. Ellis
Astrophysical Journal 827, 48 Link to Article [http://dx.doi.org/10.3847/0004-637X/827/1/48]
¹Department of Astronomy, University of Illinois, Urbana, IL 61801, USA
²Theoretical Physics and Cosmology Group, Department of Physics, King’s College London, London WC2R 2LS, UK; Theory Department, CERN, CH-1211 Geneva 23, Switzerland

Several searches have found evidence of ${}^{60}{\rm{Fe}}$ deposition, presumably from a near-Earth supernova (SN), with concentrations that vary in different locations on Earth. This paper examines various influences on the path of interstellar dust carrying ${}^{60}{\rm{Fe}}$ from an SN through the heliosphere, with the aim of estimating the final global distribution on the ocean floor. We study the influences of magnetic fields, angle of arrival, wind, and ocean cycling of SN material on the concentrations at different locations. We find that the passage of SN material through the mesosphere/lower thermosphere has the greatest influence on the final global distribution, with ocean cycling causing lesser alteration as the SN material sinks to the ocean floor. SN distance estimates in previous works that assumed a uniform distribution are a good approximation. Including the effects on surface distributions, we estimate a distance of ${46}_{-6}^{+10}$ pc for an $8\mbox{–}10\ {M}_{\odot }$ SN progenitor. This is consistent with an SN occurring within the Tuc-Hor stellar group ~2.8 Myr ago, with SN material arriving on Earth ~2.2 Myr ago. We note that the SN dust retains directional information to within 1◦ through its arrival in the inner solar system, so that SN debris deposition on inert bodies such as the Moon will be anisotropic, and thus could in principle be used to infer directional information. In particular, we predict that existing lunar samples should show measurable ${}^{60}{\rm{Fe}}$ differences.

^1,2Rhian H. Jones, ^2,3,4Francis M. Mccubbin, ⁵Yunbin Guan
American Mineralogist 101, 2452-2467 Link to Article [DOI: 10.2138/am-2016-5728DOI: 10.2138/am-2016-5728]
¹School of Earth and Environmental Sciences, The University of Manchester, Manchester M13 9PL, U.K.
²Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
³Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
⁴NASA Johnson Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, Texas 77058, U.S.A.
⁵Division of Geological and Planetary Sciences, Caltech, Pasadena, California 91125, U.S.A.
Copyright: The Mineralogical Society of America

Phosphate minerals in ordinary chondrites provide a record of fluids that were present during metamorphic heating of the chondrite parent asteroids. We have carried out a petrographic study of the phosphate minerals, merrillite and apatite, in metamorphosed H group ordinary chondrites of petrologic type 4–6, to understand development of phosphate minerals and associated fluid evolution during metamorphism. In unbrecciated chondrites, apatite is Cl rich and shows textural evolution from fine-grained apatite-merrillite assemblages in type 4 toward larger, uniform grains in type 6. The Cl/F ratio in apatite shows a similar degree of heterogeneity in all petrologic types, and no systematic change in compositions with metamorphic grade, which suggests that compositions in each meteorite are dictated by localized conditions, possibly because of a limited fluid/rock ratio. The development of phosphate minerals in H chondrites is similar to that of L and LL chondrites, despite the fact that feldspar equilibration resulting from albitization is complete in H4 chondrites but not in L4 or LL4 chondrites. This suggests that albitization took place during an earlier period of the metamorphic history than that recorded by preserved apatite compositions, and chemical equilibrium was not achieved throughout the H chondrite parent body or bodies during the late stages of metamorphism. A relict igneous clast in the H5 chondrite, Oro Grande has apatite rims on relict phenocrysts of (possibly) diopside that have equilibrated with the host chondrite. Apatite in the Zag H3–6 regolith breccia records a complex fluid history, which is likely related to the presence of halite in this meteorite. The porous dark H4 matrix of Zag, where halite is observed, has a high apatite/merrillite ratio, and apatite is extremely Cl rich. One light H6 clast contains similarly Cl-rich apatite. In a second light H6 clast, apatite compositions are very heterogeneous and more F-rich. Apatites in both H4 matrix and H6 clasts have very low H2O contents. Heterogeneous apatite compositions in Zag record multiple stages of regolith processing and shock at the surface of the H chondrite parent body, and apatite records either the passage of fluids of variable compositions resulting from different impact-related processes, or the passage of a single fluid whose composition evolved as it interacted with the chondrite regolith. Unraveling the history of apatite can potentially help to interpret the internal structure of chondrite parent bodies, with implications for physical and mechanical properties of chondritic asteroids. The behavior of halogens recorded by apatite is important for understanding the behavior of volatile elements in general: if impact-melt materials close to the surface of a chondritic asteroid are readily degassed, the volatile inventories of terrestrial planets could be considerably more depleted than the CI carbonaceous chondrite abundances that are commonly assumed.