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Space weathering of silicate regoliths with various FeO contents: New insights from laser irradiation experiments and theoretical spectral simulations

Moroz, L.V. and Starukhina, Larissa V. and Rout, Surya Snata and Sasaki, S. and Helbert, Jörn and Baither, Dietmar and Bischoff, Addi and Hiesinger, Harald (2014) Space weathering of silicate regoliths with various FeO contents: New insights from laser irradiation experiments and theoretical spectral simulations. Icarus, 235, pp. 187-206. Elsevier. DOI: j.icarus.2014.03.021 ISSN 0019-1035

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Official URL: http://dx.doi.org/10.1016/j.icarus.2014.03.021

Abstract

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.

Item URL in elib:https://elib.dlr.de/94338/
Document Type:Article
Title:Space weathering of silicate regoliths with various FeO contents: New insights from laser irradiation experiments and theoretical spectral simulations
Authors:
AuthorsInstitution or Email of AuthorsAuthors ORCID iD
Moroz, L.V.Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, GermanyUNSPECIFIED
Starukhina, Larissa V.Astronomical Institute of Kharkov National University, Sumska 35, 61022 Kharkov, UkraineUNSPECIFIED
Rout, Surya SnataInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, GermanyUNSPECIFIED
Sasaki, S.National Astronomical Observatory of JapanUNSPECIFIED
Helbert, Jörnjoern.helbert (at) dlr.deUNSPECIFIED
Baither, DietmarInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, GermanyUNSPECIFIED
Bischoff, AddiInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, GermanyUNSPECIFIED
Hiesinger, HaraldInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, GermanyUNSPECIFIED
Date:June 2014
Journal or Publication Title:Icarus
Refereed publication:Yes
Open Access:No
Gold Open Access:No
In SCOPUS:Yes
In ISI Web of Science:Yes
Volume:235
DOI :j.icarus.2014.03.021
Page Range:pp. 187-206
Publisher:Elsevier
ISSN:0019-1035
Status:Published
Keywords:Mercury, iron, space weathering, spectroscopy
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Space
HGF - Program Themes:Space Science and Exploration
DLR - Research area:Raumfahrt
DLR - Program:R EW - Erforschung des Weltraums
DLR - Research theme (Project):R - Vorhaben BepiColombo (old)
Location: Berlin-Adlershof
Institutes and Institutions:Institute of Planetary Research > Experimentelle Planetenphysik
Deposited By: Helbert, Dr.rer.nat. Jörn
Deposited On:08 Jan 2015 13:52
Last Modified:06 Sep 2019 15:26

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