Establishment Of LIBS Spectral Library And Major Elemental Model Available For Tianwen 1 Spectral Data Interpretation

Tianwen-1 is China’s first Mars exploration mission,which successfully landed and released the Zhurong rover in southern Utopia Planitia on the northern hemisphere of Mars on 15 May 2021.The landing site is within the Vastitas Borealis Formation(VBF),which is one of the largest sedimentary units on Mars.VBF covers most of the northern plains of Mars,which is proposed to be deposited from a potential ancient ocean,outflow channels from southern highlands,or ice-related processes.The geological origin of VBF is still unknown.The in-situ detections of VBF materials by the Zhurong rover in a new region could provide new constraints for the geological origin of VBF.The Mars Surface Composition Detector(MarSCoDe)instrument suite is the only payload onboard the rover to obtain chemical information about VBF materials(Si,Al,Fe,Ca,Mg,K,Na,O,H,etc.).MarSCoDe consists of a Laser-Induced Breakdown Spectrometer(LIBS),a Short-Wave Infrared Spectrometer(SWIR),and a Micro-Imaging Camera(MI),with the intent to investigate the elemental compositions,mineralogy,and morphology of encountered rocks and soils.However,in terms of the interpretation of MarSCoDe-LIBS spectra,the plasma generation and evolution processes will be significantly affected by the diverse rock/mineral types and complex environment on Mars,which will affect the LIBS spectra and quantitative results about elements.Therefore,it is one of the highlighted problems that how to obtain accurate and reliable chemical compositions on Mars based on MarSCoDe-LIBS spectra.Moreover,it is one of the most effective solutions to establish multivariate models based on LIBS standard library and LIBS spectral database.Several works were conducted in our thesis as follows:(1)A laboratory SDU-LIBS system was established to provide a platform for LIBS spectral studies in the laboratory.(2)We established a geological standard library and a LIBS spectral library.(3)We built several qualitative and quantitative models based on SDU-LIBS spectra,and evaluated the influence of laser energy and pressure on the accuracy of LIBS quantitative analysis.(4)MarSCoDe spectra were preliminarily interpreted to acquire chemical and mineral information about VBF-rocks and soils,with the intent to further constrain the origin of the VBF as well as its hydrological evolution history.The main conclusions of this thesis are as follows:(1)We finished establishing a geological standard library and a LIBS spectral library in a Mars-like atmosphere.In this thesis,we selected 316 terrestrial rocks based on the types and chemical compositions of typical Martian minerals/rocks and the potential material types at the Tianwen-1 landing site.These rocks were crushed,ground,and sieved into particles less than 38 μm.Finally,27.7 kg powder samples were obtained.Subsequently,the abundance of major elements(Si.Al,Fe,Ca,Mg,K,Na,Ti,etc.)and minor elements(Mn,Cr,Sr,Zn,Cu,S,etc.)was obtained by X-Ray Fluorescence Spectrometer(XRF).Finally,we finished the establishment of the LIBS standard library,which contains the standard name,pictures,rock type,geological origin,quality,particle size(<38 μm),source,and chemical compositions.Especially,the particle size of the standards is less than 38μm,which can determine their homogeneity at the scale of laser spot size(submillimeter scale).In addition,the standard library covers the typical rock/mineral types on Mars and has a similar compositional region to ChemCam standard library(a widely used standard library for LIBS),suggesting the standard library is suitable for LIBS spectral interpretation.Subsequently,a LIBS spectral library was built based on the LIBS standard library using the SDU-LIBS system in a Mars-like atmosphere(700 Pa.CO2).The LIBS spectral library contains 948 LIBS spectra(each spectrum is the average of 10 plasma emission spectra and thus spectra from 9480 plasmas are collected),environment(700 Pa,CO2,temperature),instrument parameters(laser wavelength,1064 nm;laser pulse width.6.06 ns;laser energy,200 mJ;laser energy on target surface,66.8 mJ;frequency,5 Hz;integration time,1 ms),and spectral range(230 nm-850 nm).(2)We established several qualitative and quantitative models,and evaluated the influence of laser energy and pressure on the LIBS spectra.We built a method to evaluate the chemical weathering degree of geological samples using the SDU-LIBS spectral database and the Chemical Index of Alteration(CIA)of LIBS standards.We found that the scatter plot of(Al+Fe+Mg+Ca+Na)/Ti vs Si/Ti and Al vs Al+Na+K+Ca could constrain the weathering trend of geological samples(i.e.,the CIA increasing trend).This method could rapidly evaluate the chemical weathering degree of rocks based on their LIBS peak intensities.Subsequently,we built several multivariate models based on the LIBS spectral library and chemical compositions of standards using Partial Least Squares(PLS),Least Absolute Shrinkage and Selection Operator(LASSO),and Artificial Neural Network(ANN)regression algorithms to quantitatively obtain the abundance of SiO2,Al2O3,Fe2O3T,CaO,MgO,K2O,Na2O,and TiO2.The model performance(Root Mean Square Error,RMSE)is comparable to that of ChemCam(Chemistry and Camera)and SuperCam models,suggesting the quantitative models are reliable to acquire chemical compositions of geological standards.These models can obtain chemical compositions of unknown geological samples based on their SDU-LIBS spectra.Moreover,we analyzed the effects of laser energy and pressure on the generation and evolution of laser-induced plasma,with the intent to evaluate their effects on MarSCoDe-LIBS spectral interpretation.We found that laser energy and pressure could affect the particle number density,temperature,and ionization degree in plasma,which would generate the non-uniform variations in LIBS peak intensities(e.g.,the changes of Ca(Ⅱ)and Ca(Ⅰ)emission lines are different)and could not be completely restrained by traditional normalization methods.However,small variations of RMSE values were observed from LIBS spectra at different laser energy and pressure,suggesting that laser energy and pressure had a small influence on the accuracy of the quantitative investigation of LIBS spectra.(3)We obtained chemical and mineral information about VBF-rocks and soils at the Tianwen-1 landing site.We preliminarily interpreted the MarSCoDe spectral of six selected targets in VBF(VBF-ZR targets),including indurated soil(VBF-ZR-01),coarse-grained soils(VBF-ZR-09 and VBF-ZR-17),fine-grained soil(VBF-ZR-26),a layered rock(VBF-ZR-21)and a coherent rock(VBF-ZR-28).The indurated soil is exposed by the turbulent airflow during landing and indurated by wheel tracks,which has less covered dust compared to other VBF-ZR targets.The main findings of chemical and mineral characteristics of these targets are as follows:1)VBF-ZR soils.The indurated soil(VBF-ZR-01)is rich in Si and Al,and contains more amorphous aluminosilicate(allophane and imogolite/opal)than other VBF targets.The coarse-grained soil(VBF-ZR-17)and fine-grained soil(VBF-ZR-26)have similar basaltic compositions,suggesting the coarse soils could be aggregates or precursors of fine particles.In addition,the fine-grained soil has a stronger H emission line.While another coarse-grained soil(VBF-ZR-09)is rich in Mg and poor in Si,Fe，K,and Na,which is similar to the noritic cal-target.The different compositions between VBF-soils may reflect their different proportions of silicic and mafic minerals at the submillimeter scale,which may be caused by sorting processes of aeolian sand along a transport pathway.2)VBF-ZR rocks.The layered rock(VBF-ZR-21)is rich in Si,Al,K,and Na with noritic characteristics,suggesting more alkaline and felsic minerals compared to the coherent rock(VBF-ZR-28).While the coherent rock(VBF-ZR-28)exhibits basaltic composition with more Fe content,indicating this rock is richer in mafic compositions relative to another rock.Moreover,the compositions of the coherent rock are similar to the coarse-grained soil(VBF-ZR-17)and the fine-grained soil(VBF-ZR-26),suggesting that this rock may be a potential parent rock of VBF-ZR soils.3)Origin of VBF.The indurated soil(VBF-ZR-01)contains more amorphous aluminosilicate(allophane and imogolite/opal)but less dust than other VBF-ZR targets,suggesting the amorphous aluminosilicate are contained in VBF-soils instead of dust.In addition,the aluminosilicate is widely distributed along the traverse path of the Zhurong rover,which is mainly associated with highly leached and weathered volcanic glass in cold and slightly acidic conditions.Therefore,a limited water-rock interaction can be proposed in the absence of large amounts of phyllosilicates(e.g.,montmorillonite and nontronite)and sulfates(e.g.,gypsum),as well as the mixture of pyroxene and amorphous aluminosilicate in VBF-ZR targets.The limited water-rock interaction may be generated from limited water(e.g.,rainy or melting from frost/snow/ice)or a short-lived standing body of water(e.g.,melting of glaciers or sourced from southern highlands).