Lyuba V. Moroz^a,b, Larissa V. Starukhina^c, Surya Snata Rout^a,1, Sho Sasaki^d,2, Jörn Helbert^b, Dietmar Baither^e, Addi Bischoff^a and Harald Hiesinger^a

^aWestfälische Wilhelms-Universität Münster, Institut für Planetologie, Wilhelm-Klemm Str. 10, D-48149 Münster, Germany
^bDeutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), Institut für Planetenforschung, Rutherfordstr. 2, D-12489 Berlin, Germany
^cAstronomical Institute of Kharkov National University, Sumska 35, 61022 Kharkov, Ukraine
^dRISE Project, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizugawa, Oshu 023-0861, Iwate, Japan
^eWestfälische Wilhelms-Universität Münster, Institut für Materialphysik, Wilhelm-Klemm Str. 10, D-48149 Münster, Germany
¹Present address: Center for Meteoritics and Polar Studies, The Field Museum of Natural History, 1400 S Lake Shore Dr., Chicago, IL 60605-2496, USA.
²Present address: Osaka University, Toyonaka, Japan.

To investigate effects of micrometeorite bombardment on optical spectra and composition of planetary and asteroid regoliths with low Fe contents, we irradiated samples of a Fe-poor plagioclase feldspar (andesine–labradorite) using a nanosecond pulsed laser. We measured reflectance spectra of irradiated and non-irradiated areas of the samples (pressed pellets) between 0.5 and 18 μm and performed SEM/EDS and TEM studies of the samples. Bulk FeO content of 0.72 wt.% in the samples is comparable, for example, to FeO content in silicates on the surface of Mercury, that was recently mapped by NASA’s MESSENGER mission and will be spectrally mapped by remote sensing instruments MERTIS and SYMBIO-SYS on board the ESA BepiColombo spacecraft. We also employed theoretical spectral modeling to characterize optical alteration caused by formation of nano- and submicrometer Fe⁰ inclusions within space-weathered surface layers and grain rims of a Fe-poor regolith. The laser-irradiated surface layer of plagioclase reveals significant melting, while reflectance spectra show mild darkening and reddening in the visible and near-infrared (VNIR). Our spectral modeling indicates that the optical changes observed in the visible require reduction of bulk FeO (including Fe from mineral impurities found in the sample) and formation of nanophase (np) Fe⁰ within the glassy surface layer. A vapor deposit, if present, is optically too thin to contribute to optical modification of the investigated samples or to cause space weathering-induced optical alteration of Fe-poor regoliths in general. Due to low thickness of vapor deposits, npFe⁰ formation in the latter can cause darkening and reddening only for a regolith with rather high bulk Fe content. Our calculations show that only a fraction of bulk Fe is likely to be converted to npFe⁰ in nanosecond laser irradiation experiments and probably in natural regolith layers modified by space weathering. The previously reported differences in response of different minerals to laser irradiation, and probably to space weathering-induced heating are likely controlled by their differences in electrical conductivities and melting points. For a given mineral grain, its susceptibility to melting/vaporization is also affected by electric conductivities of adjacent grains of other minerals in the regolith. Published nanosecond laser irradiation experiments simulate optical alteration of immature regoliths, since only the uppermost surface layer of an irradiated pellet is subject to heating. According to our calculations, if regolith particles due to impact-induced turnover are mantled with npFe⁰-bearing rims of the same thickness, then even low contents of Fe similar to our sample or Mercury’ surface can cause significant darkening and reddening, provided a melt layer, rather than a thin vapor deposit is involved into npFe⁰ formation. All spectral effects observed in the thermal infrared (TIR) after irradiation of our feldspar sample are likely to be associated with textural changes. We expect that mineralogical interpretation of the BepiColombo MERTIS infrared spectra of Mercury between 7 and 17 μm will be influenced mostly by textural effects (porosity, comminution) and impact glass formation rather than formation of npFe⁰ inclusions.

Reference
Moroz LV, Starukhina LV, Rout SS, Sasaki S, Jörn Helbert J, Baither D, Bischoff B and Hiesinger H (2014) Space weathering of silicate regoliths with various FeO contents: New insights from laser irradiation experiments and theoretical spectral simulations. Icarus 235:187.
[doi:10.1016/j.icarus.2014.03.021]
Copyright Elsevier

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S. Bisterzo^1,2, C. Travaglio^1,5, R. Gallino^2,5, M. Wiescher³ and F. Käppeler⁴

¹INAF-Astrophysical Observatory Turin, Turin, Italy
²Department of Physics, University of Turin, Turin, Italy
³Joint Institute for Nuclear Astrophysics (JINA), Department of Physics, University of Notre Dame, Notre Dame IN, USA
⁴Karlsruhe Institute of Technology, Campus Nord, Institut für Kernphysik, Karlsruhe, Germany
⁵B2FH Association-c/o Strada Osservatorio 20, I-10023 Turin, Italy.

We study the s-process abundances (A  90) at the epoch of the solar system formation. Asymptotic giant branch yields are computed with an updated neutron capture network and updated initial solar abundances. We confirm our previous results obtained with a Galactic chemical evolution (GCE) model: (1) as suggested by the s-process spread observed in disk stars and in presolar meteoritic SiC grains, a weighted average of s-process strengths is needed to reproduce the solar s distribution of isotopes with A > 130; and (2) an additional contribution (of about 25%) is required in order to represent the solar s-process abundances of isotopes from A = 90 to 130. Furthermore, we investigate the effect of different internal structures of the ¹³C pocket, which may affect the efficiency of the ¹³C(α, n)¹⁶O reaction, the major neutron source of the s process. First, keeping the same ¹³C profile adopted so far, we modify by a factor of two the mass involved in the pocket; second, we assume a flat ¹³C profile in the pocket, and we test again the effects of the variation of the mass of the pocket. We find that GCE s predictions at the epoch of the solar system formation marginally depend on the size and shape of the ¹³C pocket once a different weighted range of ¹³C-pocket strengths is assumed. We obtain that, independently of the internal structure of the ¹³C pocket, the missing solar system s-process contribution in the range from A = 90 to 130 remains essentially the same.

Reference
Bisterzo S. Travaglio C, Gallino R, Wiescher M and Käppeler F (2014) Galactic Chemical Evolution and Solar s-process Abundances: Dependence on the 13C-pocket Structure. The Astrophysical Journal 787:10.
[doi:10.1088/0004-637X/787/1/10]

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