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Simulation of space weathering on asteroid spectra through hydrogen ion and laser irradiation of meteorites

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Title: Simulation of space weathering on asteroid spectra through hydrogen ion and laser irradiation of meteorites
Author(s): Lakshika, Palamakumbure
Contributor: University of Helsinki, Faculty of Science
Degree program: Master's Programme in Geology and Geophysics
Specialisation: Solid Earth Geophysics
Language: English
Acceptance year: 2022
Space weathering can be defined as the combination of physical and chemical changes that occur in material exposed to an interplanetary environment on the surface of airless bodies. This process produces amorphous surface layers often containing small opaque particles such as nanophase metallic iron (npFe0). This darkens the topmost layer resulting in alterations in material spectroscopic features.Eventually it can lead to misinterpretation of remotely sensed data in the visible- near-infrared (VIS-NIR) spectrum. The goal of this research is to simulate solar wind effects on asteroid spectra through low energy 1 keV hydrogen ion irradiation of meteorite powder samples and measure the changes in their reflectance spectra. This allows to understand how space weathering depends on the mineralogy of the material. We used Bjurböle (L/LL4), Avanhandava (H6) and Luotolax (Howardite) meteorites. H+ ion irradiation was carried out on powdered samples compressed into pellets. The pellets were placed into a vacuum chamber with pressure between 1.2 x 10 -7-2.4 x 10 -7 mbar for the whole experiment. To simulate solar wind irradiation, H+ ions were used with 1 keV under three fluences; 1 x 1017, 2 x 1017 and 5 x 1017 ions/cm2. Subsequently reflectance spectra of the samples were measured and processes using Modified Gaussian Model (MGM) to derive key spectral parameters. Both chondrites show significant reddening in the VIS region. Bjurböle being an LL, it is more oxidized than Avanhandava. The reddening in the NIR region is more significant in Avanhandava than in Bjurböle. My work indicates that even for low-energy solar wind conditions, the chondritic materials (Q/S-type asteroids) with high olivine content and/or higher fayalite (Fa) compositions are more susceptible to silicate absorption bands reduction. Luotolax meteorite being howardite rich in orthopyroxene and clinopyroxene, shows VIS reddening but not observable band depth changes with increasing exposure to H+ ion irradiation. The smaller change in Luotolax may be due to higher pyroxene resistance to low-energy ion irradiation. Overall, at short timescales and typical solar wind energies, VIS slope reddening is the most dominant factor in all three material compositions.
Keyword(s): Space weathering low energy hydrogen ion irradiation howardite ordinary chondrites

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