Experimental Studies On Space Weathering Simulations And Spectro-Photometric Properties Of Major Lunar-Type Minerals

Optical remote sensing is one of the main methods of planetary science detections.Among them,visible-near infrared?VNIR?reflectance spectral data is relatively easy to acquire,hence it is widely used in various planetary exploration missions.The accuracy of interpretation of remote-sensed spectral data is affected by a variety of factors,including space weathering,different illumination and viewing geometries,and so on.In this dissertation,we tried to analyze and/or reduce the effects of the above two factors by carrying out simulation experiments of space weathering and spectrophotometric measurements in a controlled laboratory setting.Space weathering,including micro-meteorite impact and solar wind particle implantation and some other processes,changes the VNIR reflectance spectra of the surface material,darkens and reddens the original spectra,and weakens its characteristic absorption bands.Such spectral changes are mainly caused by nanophase metallic iron particles?npFe⁰?produced during space weathering.The diagnostic absorption features of silicate minerals?i.e.,olivine and pyroxene?in the VNIR band are mainly caused by the transition of outer electron of Fe²⁺in the crystal field which absorbs the energy of photons with the same frequency,therefore they are greatly affected by the production of npFe⁰.In contrast,in the mid-infrared?MIR?,the spectral characteristics of minerals are mainly caused by the vibration of the lattice.Therefore,the effects of space weathering might be different in the MIR.The Hapke model is widely used to quantitatively invert the mineral abundances from the VNIR reflectance spectra.The phase function is a very important parameter in the model and is used to characterize the angular distribution characteristics of the scattered light of the particle.Theoretically,the phase function is dependent on both the mineral type and wavelength,but in actual spectral modeling,the phase functions of each mineral endmember and mixture are often assumed to be a constant value independent of endmember and wavelength.Ilmenite is an opaque mineral with strong absorption,which is likely to be very different compared to transparent or semitransparent silicate minerals occurred on the lunar surface such as olivine,pyroxene,and plagioclase.Therefore,using only a single phase function value to characterize lunar soil particles may cause large deviations in the inversion results of the spectra,especially the abundance of ilmenite.In response to the above problems,we carried out studies from the following aspects:1.Space weathering simulation experiment using pulse laserIn this study,a pulsed laser was used to irradiate olivine powder samples placed in a vacuum chamber,and simulated products with different weathering degrees were produced by changing the pulse energy and irradiation time.We measured the VNIR and MIR reflectance spectra of samples before and after irradiations.The VNIR spectra show obvious reddening,darkening,and weakening in the absorption band which are typical effects of space weathering.In contrast,the MIR spectra only experienced limited changes,most of the vibration modes of olivine remained even after intense irradiations.The microstructure changes of these samples before and after irradiations were analyzed using transmission electron microscopy and Raman spectroscopy.The results show that the pulsed laser irradiation has successfully simulated the typical products of actual space weathering:nano-phase iron particles and amorphous coatings.2.Numerical spectral modeling of the effects of space weatheringIn this study,the spectral effects of npFe⁰ and amorphous coatings in the VNIR and MIR bands were predicated by numerical simulation.We employed the approximate analytical solution of the radiative transfer equation?Hapke model?and a numerical algorithm?DISORT?.Although the spectral effects of npFe⁰ have been studied by many researchers using the Hapke model,most of them focus on the VNIR band and assume that the npFe⁰ is evenly distributed inside the hose particles,the influences of the npFe⁰ size in spectral modeling are not well understood yet.In this study,we incorporated the effective medium theory and an algorithm specific for coated particles to predict the spectral effects of npFe⁰ with different contents and distributions.The spectral effects of amorphous coating with varied compositions and thickness were also studied.The results show that npFe⁰ has a significant effect on the VNIR reflectance spectra,but the effect on the MIR is very limited.For the amorphous coating,only when it is thick enough the MIR spectra will be affected.3.Single scattering phase functions of typical lunar-type mineralsIn this study,we employed the bidirectional reflectance spectrometer at Brown University's REflecatnce LABoratory?RELAB?to measure the reflectance spectra of the four typical lunar minerals,and based on these new spectrophotometric data,the parameters of two different forms of phase functions of these four minerals were inverted.Then these new phase function parameters were used for spectral unmixing of mineral mixtures prepared in the laboratory with the Hapke model.The effects of different forms of phase functions on the spectral unmixing results were analyzed.The results show that the accurate end-member and mixture phase functions are beneficial to improve the spectral quantitative inversion accuracy of ilmenite-containing mixtures.In summary,the results of the above experiments and modeling have demonstrated that:?1?compared with the VNIR reflectance spectra,the MIR bands are less affected by laser irradiations.Under real weathering conditions,the MIR spectral features of materials on the surface of airless bodies might be preserved more intact,and hence the MIR spectroscopy may be one of the development directions of future remote sensing;?2?To improve the accuracy of mineral abundance inversion using the Hapke model,the effort can be made on studies of the phase functions of the endmembers and mixtures.